Brody's Human Pharmacology: With STUDENT CONSULT

Chapter 37 Drugs and Substance Abuse, Addiction, and Treatment

MAJOR DRUG CLASSES 

Drugs of abuse

Cannabinoids

Depressants

Dissociative compounds

Hallucinogens

Opioids

Stimulants

Drugs to treat opioid dependence

Opioid receptor agonists

Drugs to treat nicotine dependence

Nicotine products for replacement

Nicotinic receptor partial agonist

Therapeutic Overview

The nonmedical use of drugs and other substances affecting the central nervous system (CNS) has increased dramatically. Alcohol, hallucinogens, caffeine, nicotine, and other compounds were used historically to alter mood and behavior and are still in common use. In addition, many stimulants, antianxiety agents, and drugs to alleviate pain, all of which are intended for medical use, are obtained illicitly and used for their mood-altering effects. Although the short-term effects of these drugs may be exciting or pleasurable, excessive use often leads to abuse.

Substance abuse is defined as a destructive pattern of drug use leading to significant social, occupational, or medical impairment. Abused substances may be categorized according to pharmacological class or may be defined by their use or source. Thus club drugs, which are taken at rave and trance events, include the depressants γ-hydroxybutyrate (GHB) and flunitrazepam, the dissociative compound ketamine, and the stimulant/hallucinogen methylenedioxymethamphetamine (MDMA). Prescription drug abuse typically involves illicit use of the opioids prescribed for pain, the benzodiazepines prescribed for anxiety and sleep disorders, and the stimulants prescribed for attention deficit disorder and narcolepsy. Designer drugs are chemical modifications to currently abused drugs and often become available before they are subject to legal control. These include heroin-like fentanyl derivatives (e.g., “China White”) and analogs of the dissociative anesthetic phencyclidine (PCP). The major drugs and substances abused, grouped by pharmacological class, are presented in the Therapeutic Overview Box.

In a 2006 survey conducted by the U.S. Substance Abuse and Mental Health Services Administration, more than 20 million Americans aged 12 and older had used illicit drugs in the month before the survey, representing more than 8% of the population. The most commonly used

Abbreviations

AIDS

Acquired immunodeficiency syndrome

CNS

Central nervous system

DA

Dopamine

GABA

γ-Aminobutyric acid

GHB

γ-Hydroxybutyrate

5-HT

Serotonin

IV

Intravenous

LSD

d-Lysergic acid diethylamide

MDMA

Methylenedioxymethamphetamine

NAcc

Nucleus accumbens

NE

Norepinephrine

NMDA

N-methyl-D-aspartate

NRT

Nicotine replacement therapy

PCP

Phencyclidine

THC

Tetrahydrocannabinol

VTA

Ventral tegmental area

illicit drug was marijuana, followed by prescription pain relievers, cocaine, stimulants, and the hallucinogens (Table 37-1). In addition, more than 50% of Americans used ethanol, with 23% reporting binge drinking (five or more drinks on one occasion) and nearly 7% reporting heavy drinking (binge drinking on at least five occasions) during the past month. Nearly 73 million Americans use some form of tobacco.

TABLE 37–1 Prevalence of Illicit Drug Use in the Unites States in 2006*

Marijuana

14.8

Prescription pain relievers

5.2

Cocaine

2.4

Stimulants

1.2

Hallucinogens

1.0

Inhalants

0.76

* Data from the U.S. Substance Abuse and Mental Health Services Administration; numbers represent millions of persons.

Tolerance and Dependence

Drug abuse does not require development of dependence on the drug or tolerance to its effects, although these often occur. Tolerance refers to a reduced effect with repeated use and a need for higher doses to produce the same effect. Because tolerance does not occur to the same extent for all effects of a single drug, people who take increasing amounts of drug risk an increase in effects for which less tolerance develops. For example, chronic heroin abusers may die from respiratory depression.

Dependence is characterized by physiological or behavioral changes after discontinuation of drug use, effects that are reversible on resumption of drug administration. Psychological dependence is characterized by intense craving and compulsive drug-seeking behavior. Abused substances often possess reinforcing effects accompanied by intense euphoria and feelings of well-being that foster their continued use. This is particularly true of stimulants such as cocaine and amphetamine. Physical dependence is associated with characteristic withdrawal signs upon cessation of drug administration. Thewithdrawal syndrome is similar for drugs within a pharmacological class but differs between classes. Its time course varies according to the rate of elimination of the drugs or their active metabolites. Withdrawal from long-acting drugs has a delayed onset, is relatively mild, and occurs over many days or weeks (Fig. 37-1, A), whereas withdrawal from more rapidly inactivated or eliminated drugs is more intense but of shorter duration (Fig. 37-1, B). Typically, physical dependence occurs when substances are used over extended times—usually days, weeks, or months. With repeated use, dependence becomes increasingly severe. Normally, occasional drug use does not result in clinically significant withdrawal. Spontaneous withdrawal occurs on cessation of drug taking. Precipitated withdrawaloccurs when an antagonist is administered to displace the drug from its receptors, causing more rapid and severe effects (Fig. 37-1, C). An example is the administration of the opioid antagonist naltrexone to heroin-dependent individuals.

image

FIGURE 37–1 The course of severity of withdrawal effects from dependence on drugs with long (A) and short (B) durations of action or after administration of a specific receptor antagonist (C).

Different drugs in the same class often can maintain physical dependence produced by other drugs in the same class, termed cross-dependence. Thus heroin withdrawal can be prevented by administration of other opioids, part of the rationale for the use of methadone in treatment. Alcohol, barbiturates, and benzodiazepines show cross-dependence with each other but not with opioids; thus benzodiazepines are effective in suppressing symptoms of alcohol withdrawal. Cross-tolerance is similar to cross-dependence, in that people tolerant to a drug in one class will usually be tolerant to other drugs in the same class but not to drugs in other classes.

In 2006, 22.6 million Americans were classified with substance abuse or dependence disorder as per criteria in the Diagnostic and Statistical Manual of Mental Disorders, 4th edition (DSM-IV). Among these, 5.6 million exhibited comorbid serious psychological distress. In addition, 24% of Americans with a major depressive disorder were dependent on or abused illicit drugs or alcohol. Thus drug dependence and abuse are medical problems that require treatment, both for acute overdose and withdrawal and for consequent medical conditions. Although primary caregivers provide diagnoses, referral, and short-term treatment, long-term treatment of substance abuse is the province of specialized, multidisciplinary programs that use several strategies. Detoxification is used to treat physical dependence and consists of abruptly or gradually reducing drug doses, whereas maintenance therapy involves using drugs such as methadone to continue opioid dependence, with psychological, social, and vocational therapies used to help deal with craving.

Therapeutic Overview

Cannabinoids

Hashish, marijuana

Depressants

Barbiturates, benzodiazepines, ethanol, GHB, methaqualone

Dissociative compounds

PCP, ketamine

Hallucinogens

LSD, mescaline, psilocybin

Opioids

Codeine, fentanyl, heroin, hydrocodone, hydromorphone, meperidine, morphine, opium, oxycodone

Stimulants

Amphetamine, cocaine, MDMA, methamphetamine, methylphenidate, nicotine

Anabolic steroids

Nandrolone, oxandrolone, oxymetholone, stanozolol, testosterone

Inhalants

Volatile solvents (toluene), gases (N2O), nitrites (amyl nitrite)

Mechanisms of Action

Reinforcing Compounds

Several pharmacological classes of abused drugs including the cannabinoids, depressants, opioids, and the stimulants all share the ability to activate the mesolimbic dopamine (DA) “reward” pathway in the brain (see Fig. 27-8). These agents either directly activate these dopaminergic neurons or alter the activity of other neurotransmitters such as acetylcholine, γ-aminobutyric acid (GABA), glutamate, serotonin (5-HT), and norepinephrine (NE), which modulate the activity of these neurons. A diagram of ventral tegmental area (VTA)-nucleus accumbens (NAcc) dopamine neurons and pathways affecting the activity of these neurons is shown in Figure 37-2.

image

FIGURE 37–2 A simplified diagram depicting the mesolimbic DA pathway thought to underlie the reinforcing actions of dependence-producing drugs.

Cannabinoids

Marijuana (cannabis) is the dried leaf material, buds, and flowering tops from the common hemp plant Cannabis sativa. It has a variety of names including “pot,” “weed,” “grass,” and “maryjane.”Hashish is the dried resinous material exuded by mature plants. Cannabis is the most commonly abused illegal drug in the United States (see Table 37-1). Its major active chemical is Δ9-tetrahydrocannabinol (Δ9-THC; Fig. 37-3). Δ9-THC and related molecules are termed cannabinoids and exert their effects by binding to specific CB1 and CB2 cannabinoid G protein-coupled receptors (seeChapter 1). CB1 receptors are expressed in the brain and gastrointestinal tract and CB2 receptors in the periphery and immune system. The endogenous ligand for these receptors is anandamide, a long-chain arachidonic acid derivative.

image

FIGURE 37–3 Structures of some commonly abused drugs.

CB1 receptors are found in high levels in the cerebellum (coordination of movement), cerebral cortex (cognitive functions), hippocampus (learning and memory), VTA and NAcc (rewarding effects), hypothalamus (satiety/feeding area), and basal ganglia (control of movement). The cannabinoids activate CB1 receptors in both the VTA and NAcc and increase the firing rate of VTA-NAcc neurons. Cannabinoids also increase corticotropin-releasing factor in the amygdala, a common action of many rewarding drugs.

Depressants

CNS depressants such as barbiturates, non-barbiturate sedatives, and benzodiazepines enhance inhibitory GABA transmission by enhancing the action of endogenous GABA at GABAA receptors (seeChapter 31). Flunitrazepam is a particularly notoriously abused benzodiazepine known as “roofies” or “rophies.”

GABA interneurons in the VTA express GABAA receptors that normally function to tonically suppress the firing of DA neurons (see Fig. 37-2). GABAA receptors are also located on DA cell bodies in the VTA, but these receptors lack α1 subunits and thus are a different subunit composition than those expressed by the GABA neurons. Studies suggest that the depressants, by hyperpolarizing GABA interneurons in the VTA, produce a disinhibition of mesolimbic DA neurons, thereby increasing DA release.

GHB, which is produced endogenously at low levels during GABA metabolism, also belongs in this group (see Fig. 37-3) and is commonly called “liquid ecstasy,” “G,” “grievous bodily harm,” or “Georgia Home Boy.” Studies suggest that GHB interacts with specific binding sites on GABAB receptors, which are present on both GABA interneurons and DA neurons in the VTA, the former more sensitive to GHB than the latter. At low concentrations reflecting those used by the typical abuser, GHB is thought to activate only the receptors expressed by the GABA neurons, thereby inhibiting GABA activity, promoting DA release.

Alcohol is also a CNS depressant that activates the mesolimbic DA pathway. Although several mechanisms have been postulated to be involved in the actions of ethanol including effects of GABAAreceptors (see Chapter 32), recent studies suggest that the ability of ethanol to promote DA release in the NAcc may involve its ability to release endocannabinoids and activate CB1 receptors. Further studies are warranted to discern the specific cellular mechanisms mediating the rewarding effects of alcohol.

Opioids

Opiates are compounds isolated from the opium poppy that act at opioid receptors, whereas opioids are any synthetic or natural compounds that interact with opioid receptors; however, these terms are often used interchangeably. The two principal naturally occurring opiates are morphine and codeine. Heroin (“H” or “smack”), which is 3-,6-diacetylmorphine (Fig. 37-3), is the most commonly abused opioid and is approximately three times more potent than morphine, but the two have very similar effects. Heroin is metabolized to 6-acetylmorphine and morphine to exert its effects. Codeine, which is 3-methoxymorphine, is also demethylated to the more potent morphine by cytochrome P450 enzymes (see Chapter 2). The isoform involved is genetically polymorphic, and persons with mutated forms are unresponsive to codeine. Other synthetic opioids are widely available and abused, often as diverted prescription medications.

The reinforcing actions of the opioids are mediated by μ opioid receptors on GABA interneurons in the VTA, which inhibit GABAergic inhibitory activity (see Chapter 36).

Stimulants

The stimulant drugs are sympathomimetic amines that act on the CNS by enhancing NE and DA neurotransmission. Cocaine (“coke,” “snow,” “blow,” or “crack” for the free base; see Fig. 37-3) is the active ingredient of the South American coca bush. Amphetamines are structurally related to catecholamine neurotransmitters and ephedrine (see Chapter 11) and include amphetamine, N-methylamphetamine (methamphetamine, known as “speed,” “meth,” or “ice”), and others such as methylphenidate.

The reinforcing effects of stimulants arise from enhanced neurotransmission at dopaminergic synapses in the mesolimbic and mesocortical pathways (see Fig. 37-2). Cocaine binds to DA transporters and blocks the reuptake of DA, increasing its synaptic concentration. Amphetamines also act on DA transporters to enhance DA release and may increase its concentrations by inhibiting destruction by monoamine oxidase. Amphetamines may also directly activate postsynaptic receptors. Nicotine interacts directly with nicotinic receptors on DA neurons in the VTA to increase the firing of these neurons.

Non-Reinforcing Compounds

As mentioned previously, drug abuse does not require drug dependence, and several groups of drugs are abused despite evidence for a lack of reinforcing actions. These include the dissociative compounds, hallucinogens, and others including anabolic steroids and inhalants.

Dissociative Compounds

PCP (“angel dust”) and ketamine (“K” or “special K”) were originally developed as anesthetics (see Fig. 37-3). Ketamine is still used for changing burn dressings, for anesthesia in children (see Chapter 35), and for short-duration anesthesia in veterinary medicine. PCP is not used therapeutically because of the severity of emergence delirium in patients. Dextromethorphan (“DXM” or “robo”) is an over-the-counter cough suppressant and when taken in high doses, produces effects similar to those of PCP and ketamine.

PCP, ketamine, and high-dose dextromethorphan produce their effects through a use-dependent noncompetitive antagonism at excitatory glutamate N-methyl-D-aspartate (NMDA) receptors throughout the CNS (see Chapter 1).

Hallucinogens

Hallucinogens produce changes in mood, thought, and sensory perception such that colors, sounds, and smells are intensified. Because of the latter actions, these compounds are referred to as psychedelic orpsychotomimetic agents. The hallucinogens have no recognized medical uses. Abused hallucinogens fall into two chemical classes, the substituted phenethylamines, of which mescaline is the prototype, and the indoleamines, of which D-lysergic acid diethylamide (LSD) is the prototype (see Figure 37-3). Others hallucinogens include psilocybin (from mushrooms) and dimethyltryptamine.

The unique effects of hallucinogens appear to result from modulation of 5-HT neurotransmission, in particular activation of 5-HT2A receptors in the cerebral cortex, a region involved in mood, cognition, and perception, and in the locus coeruleus, an area concerned with response to external stimuli. The drugs also have sympathomimetic effects, producing tachycardia and increased blood pressure caused by enhanced catecholaminergic neurotransmission.

MDMA (“ecstasy” or “XTC”) is a substituted amphetamine (see Fig. 37-3) and has properties of both a hallucinogen and a stimulant. MDMA acts to increase the release of DA and 5-HT.Methylenedioxyamphetamine is a related compound.

Inhalants

The inhalants include the volatile solvents and aerosols such as toluene, gases including nitrous oxide (N2O), and aliphatic nitrites such as amyl nitrite known as “poppers” or “snappers.” Inhalants other than nitrites produce their effects in a manner similar to alcohol; they are CNS depressants, producing initial excitation, disinhibition, lightheadedness, and agitation. In sufficient amounts, they can produce anesthesia.

The nitrites are abused for their vasodilating properties (see Chapter 24).

Anabolic Steroids

Anabolic steroids are synthetic substances related to androgens, male sex hormones. These drugs promote growth of male sexual characteristics and have important clinical uses (see Chapter 41). However, steroid abuse is widespread in body-building and sports for enhancement of skeletal muscle growth. Abused steroids include testosterone itself as well as synthetic compounds, such as nandrolone, oxandrolone, oxymetholone, and stanozolol. The mechanism of action of these compounds is discussed in Chapter 41.

Pharmacokinetics

Abused substances represent many compounds whose pharmacokinetics depend on their structures and methods of use. A rapid effect is sought by opioid and stimulant abusers, and methods such as smoking or IV injection are preferred. Smoking allows the drug to pass rapidly from the lungs into the blood and brain. This is not true for slower-acting depressants such as alcohol or GHB or longer-lasting compounds such as LSD. The difference in degree of euphoria and the length of stimulant action for cocaine administered by different routes of administration is shown in Figure 37-4.

image

FIGURE 37–4 The intensity and time course (in minutes) of cocaine intoxication at equivalent doses by different routes of administration.

Reinforcing Compounds

Cannabinoids

Marijuana and hashish are usually smoked in cigarettes (“joints” or “reefers”) or in pipes or “bongs” (water pipes) but are also taken orally. Effects begin almost immediately, peak in 15 to 30 minutes, and last for 1 to 3 hours. If taken orally, lower blood levels of Δ9-THC are obtained because of a combination of extensive first-pass metabolism (which forms an active metabolite, 11-OH-Δ9-THC, as well as inactive metabolites) and high lipid solubility. When ingested orally, effects begin in 1 hour and last up to 4 hours. Because of their high lipophilicity, cannabinoids remain in the body for weeks and accumulate with repeated use. Therefore it is not possible to determine the recent use of cannabinoids based on urinary concentrations.

Depressants

CNS depressants are taken orally, with rapid-acting barbiturates, such as secobarbital and pentobarbital, more widely abused than those with a slower onset, such as phenobarbital and benzodiazepines. GHB is rapidly absorbed and readily penetrates the CNS, producing effects within 15 to 30 minutes and lasting 2 to 3 hours. Oral flunitrazepam shows effects after 30 minutes and lasts up to 8 hours. The pharmacokinetics of the benzodiazepines are discussed in Chapter 31.

Opioids

Heroin and morphine have poor oral bioavailability because of first-pass metabolism in the liver, although this is not true for codeine (see Chapter 36). Heroin is usually snorted, smoked (“chasing the dragon”), or injected and provides a rapid feeling of euphoria in 7 to 8 seconds when taken IV or 10 to 15 minutes when snorted or smoked. Its effects last approximately 4 to 6 hours, depending on dose. This very rapid euphoria contributes to its powerful addictive actions.

Stimulants

Pure cocaine is used as an H2O-soluble salt or as a free base. Cocaine salt is a bitter-tasting, white, crystalline material that is generally snorted or injected IV. Crack (free base cocaine), sold as small hard pieces or “rocks,” is volatilized and inhaled. It is rapidly absorbed and provides an almost instantaneous action. When administered IV, cocaine takes a few seconds to take effect, whereas snorted cocaine takes 5 to 10 minutes but lasts longer. Oral cocaine is slowly absorbed (Fig. 37-4) and produces a less intense effect. The effect of IV or inhaled cocaine generally lasts only 30 minutes, and redosing is common in an attempt to maintain intoxication. Cocaine is rapidly metabolized by blood and liver esterases, with urinary metabolites present for up to a week after use. Some N-demethylation occurs in the liver, with metabolites excreted in urine. Measurement of urinary cocaine metabolites is an important basis for establishing recent usage.

Amphetamines and similar stimulants such as methylphenidate are usually taken orally but may be snorted. Oral methamphetamine produces effects within 15 to 20 minutes, but when snorted, effects are apparent within 3 to 5 minutes. Effects last for several hours. A smoked form of methamphetamine (“ice”) is used in a manner similar to crack cocaine and is an increasing problem.

Nicotine is absorbed from the lungs in cigarette smoke and is readily absorbed through the mouth if tobacco is chewed. Greater than 90% of nicotine in lungs passes into blood. It is widely distributed and rapidly metabolized to inactive products. Serious smokers maintain nicotine in the body day and night.

Non-Reinforcing Compounds

Dissociative Compounds

For a rapid onset of action, PCP is generally mixed with plants (dried parsley, tobacco, or marijuana) and smoked, but it may also be snorted. However, effects of oral PCP occur within minutes and last for several hours. Ketamine has a shorter duration of action (30 to 60 minutes). For illicit use, injectable ketamine (often diverted from veterinarian’s offices) is dried, powdered, and snorted or converted into pills. Dextromethorphan is taken orally in cough suppressant preparations at 6 to 10 times the antitussive dose.

Hallucinogens

LSD is usually taken orally as capsules, tablets, or on small paper squares but can also be injected or smoked. Effects begin in 30 to 90 minutes and last up to 12 hours. MDMA is usually taken orally. Effects last up to 6 hours, but more drug is often taken when effects start to fade.

Inhalants

Inhalants are sniffed or snorted from containers or bags into which aerosols have been sprayed, are sprayed directly into the nose or mouth, or are inhaled from balloons as gases. They are quickly absorbed, producing intoxication within minutes. However, effects last only a few minutes, and repeated administration is needed for a prolonged high. Nitrous oxide produces a short-lived intoxication similar to early stages of anesthesia.

Anabolic Steroids

Anabolic steroids are taken orally, applied to the skin in gel formulations, or administered by intramuscular injections. Their pharmacokinetics are discussed in Chapter 41.

Relationship of Mechanisms of Action to Clinical Response

Reinforcing Compounds

Cannabinoids

Cannabinoids are effective antiemetics and appetite stimulants and have some analgesic actions. Dronabinol is a synthetic, orally active cannabinoid approved for the treatment of cachexia (see Chapter 33) in patients with cancer or acquired immunodeficiency syndrome (AIDS) and to treat emesis caused by cancer chemotherapy in patients who do not respond to conventional antiemetics. Many also argue for use of smoked marijuana in treating chronic pain, improving appetite in AIDS patients, and suppressing spasticity in multiple sclerosis and spinal injury.

Depressants

The effects of the benzodiazepines are discussed in Chapter 31 and those of ethanol in Chapter 32. GHB induces anterograde amnesia and with increased doses causes drowsiness and sleep, leading to general anesthesia, coma, respiratory and cardiac depression, seizures, and death.

Opioids

The effects of the opioids are discussed in Chapter 36. The biological mechanisms underlying physical dependence on opioids are poorly understood. The most consistent indication of dependence is the increased sensitivity to precipitated withdrawal by administration of an opioid antagonist. This may begin with the first opioid dose, because under laboratory conditions, high doses of an antagonist can precipitate a withdrawal syndrome within a few hours after a single dose of morphine.

Stimulants

The pathways activated by stimulant drugs reflect activation of both the CNS and autonomic nervous system. Several of these agents are used legitimately for the treatment of attention deficit disorders.

At levels produced by smoking tobacco, nicotine also acts at sympathetic and parasympathetic ganglia and the adrenal medulla to produce effects on the cardiovascular and gastrointestinal systems (seeChapter 10).

Non-Reinforcing Compounds

Dissociative Compounds

Ketamine and PCP produce a “dissociative” anesthesia—that is, amnesia and profound analgesia although the patient is not asleep and respiration and blood pressure are unaltered. This is different from anesthesia produced by inhalation and IV anesthetics (see Chapter 35).

Pharmacovigilance: Side Effects, Clinical Problems, and Toxicity

Problems associated with drug abuse are summarized in the Clinical Problems Box.

Reinforcing Compounds

Cannabinoids

Marijuana produces an initial euphoric feeling of well-being, followed by drowsiness, sedation, and increased appetite. Thought processes, judgment, and time estimation are altered, and there is heightened sensitivity to music and other activities. Users have a reduced ability to form memories, although recall of previously learned facts is unaltered. Marijuana has little effect on psychomotor coordination, although altered perception and judgment can impair performance, including driving. Because of the prevalence of its use, accidents and injury are important concerns. High doses can induce personality changes, while physiological effects include tachycardia and reddening of conjunctival vessels. Death from acute overdose is extremely rare. Heavy smokers of marijuana will experience respiratory problems including bronchitis and emphysema. Tolerance to marijuana develops after heavy use, and withdrawal causes irritability and restlessness. Addiction can develop with long-term use.

Depressants

Dose-response curves for abused CNS depressant drugs are essentially the same as those for ethanol, except that they usually have shallower slopes. If one considers that a blood ethanol concentration of 0.1% represents one quarter to one third of the lethal concentration (see Chapter 32), the therapeutic index for alcohol would be among the poorest of any legal drug. In contrast, gram quantities of benzodiazepines may not be lethal, resulting in much larger therapeutic indices. Like ethanol, barbiturates, non-barbiturate sedatives, and GHB have dose-response curves with slopes much steeper than slopes of dose-response curves for benzodiazepines (Fig. 37-5).

image

FIGURE 37–5 Comparison of dose-response relationships for the acute effects of ethanol, barbiturates, and benzodiazepines.

Barbiturates and non-barbiturate sedatives can produce an ethanol-like intoxication and are sometimes abused for this purpose. Flunitrazepam is a highly efficacious, high-potency benzodiazepine that causes sedation, psychomotor impairment, and amnesia. It is tasteless and odorless and has achieved notoriety as a “date rape” drug.

GHB is used for its ability to cause euphoria, relaxation, and lack of inhibition. Like flunitrazepam, it has been used as a “date-rape” drug because of its short-lived hypnotic effects. It is also abused by body-builders for its purported anabolic properties and is taken by alcoholics to reduce alcohol craving. γ-Butyrolactone and 1,4-butanedione are industrial solvents that are abused because they are metabolized to GHB in vivo.

After an overdose with depressant drugs, patients are unresponsive, pupils are sluggish and miotic, respiration is shallow and slow, and deep tendon reflexes are absent or attenuated. There are no known antagonists for barbiturates, non-barbiturate sedatives, or GHB, whereas the competitive benzodiazepine antagonist flumazenil can completely reverse benzodiazepine intoxication (see Chapter 31). Although benzodiazepines are rarely lethal when taken alone, they enhance the effects of other depressants taken concurrently, including alcohol.

Repeated use of depressants produces physical dependence, and cross-dependence occurs among barbiturates, non-barbiturate sedatives, benzodiazepines, and alcohol. Signs and symptoms of withdrawal are often opposite to their acute effects. Occasional convulsions and delirium make depressant withdrawal a medical emergency. Long-acting benzodiazepines or phenobarbital can be used as substitution therapy to treat alcohol and barbiturate withdrawal.

The withdrawal symptoms after abrupt discontinuation of depressants and a comparison with those associated with opioid withdrawal are summarized in Table 37-2.

TABLE 37–2 Comparison of Opioid and Depressant Withdrawal

Opioid Withdrawal

Depressant Withdrawal*

Anxiety and dysphoria

Anxiety and dysphoria

Craving and drug-seeking behavior

Craving and drug-seeking behavior

Sleep disturbance

Sleep disturbance

Nausea and vomiting

Nausea and vomiting

Lacrimation

Tremors

Rhinorrhea

Hyper-reflexia

Yawning

Hyperpyrexia

Piloerection and gooseflesh

Confusion and delirium

Sweating

Convulsions

Diarrhea

Life-threatening convulsions

Mydriasis

 

Abdominal cramping

 

Hyperpyrexia

 

Tachycardia and hypertension

 

* Alcohol, barbiturates, or benzodiazepines.

Opioids

Opioids are widely abused and represent serious consequences for the users, their families, and the community. The administration of heroin IV causes a surge of intense euphoria, a “rush” accompanied by a warming of the skin, dry mouth, and heavy feelings in arms and legs. The rush subsides after a few minutes, and the user feels relaxed, carefree, and somewhat dreamy but able to carry on many normal activities. Users who are not physically dependent recover readily. Unlike a person intoxicated with alcohol or other CNS depressants, opioid abusers are difficult to detect by observable behaviors.

Overdose leads to unconsciousness, respiratory depression, and extreme miosis, although the latter is not always apparent because asphyxia can result in pupillary dilation. Deaths are due to respiratory failure. The IV injection of an opioid antagonist, such as naloxone or nalmefene, can reverse all effects immediately and produce rapid recovery (see Chapter 36). However, it is important not to administer too large a dose, because severe withdrawal could be precipitated in a physically dependent patient. The duration of action of naloxone is shorter than that of opioid agonists, and patients should be observed to ensure severe intoxication does not re-emerge. Nalmefene has a longer duration of action.

Heroin users often begin by smoking, but many eventually progress to IV injection because of the increased rapidity of effect. This leads to many medical problems. Particularly dangerous is sharing of needles and other injection paraphernalia, which dramatically increases blood-borne infections, including HIV and hepatitis. Street heroin is not pure but is “cut” with several compounds including sugar, starch, powdered milk, quinine, and strychnine, which causes convulsions. These substances are also dangerous because they can block small arteries to vital organs. Long-term effects of IV heroin use include collapsed veins, infection of heart linings and valves, abscesses, and pulmonary complications.

Most abusers only gradually become physically dependent. Some take opioids for years at intervals insufficient to produce dependence. Initial users are generally confident that they can control their use and only dimly aware of their gradual dependence. Taking multiple daily doses of heroin or other opioids usually results in significant dependence within a few weeks.

The significance of physical dependence is in the inexorable appearance of the withdrawal syndrome, beginning approximately 6 hours after the last heroin injection. Many signs and symptoms are opposite to the effects of acute administration. In withdrawal, the positive reinforcing effects are amplified by instant alleviation of withdrawal sickness. Many dependent abusers are tolerant to the positive reinforcing effects, and continued drug use provides only relief from withdrawal. If a person is not treated, withdrawal reaches peak severity in approximately 24 hours and ceases in 7 to 10 days. This rarely constitutes a medical emergency and is considerably less dangerous than withdrawal from alcohol or barbiturates.

Opioids cross the placental barrier, and a newborn of an opioid-dependent mother will undergo withdrawal within 6 to 12 hours of birth. The long-term consequences of prenatal opioid dependence are poorly understood, but it may be best to maintain the mother on methadone and treat the infant with opioids rather than withdraw the mother before parturition. Otherwise, she may leave treatment and resume opioid abuse without adequate prenatal care.

Cross-dependence occurs among all full opioid agonists. Hydromorphone, meperidine, oxycodone, and others can reverse withdrawal and at appropriate doses can produce a heroin-like intoxication in addicts. Less-efficacious agonists such as codeine and dextropropoxyphene also show cross-dependence. A major problem is that heroin abusers often exhibit convincing symptoms that require prescription of analgesics, which are then used to treat withdrawal.

Mixed opioid agonist-antagonists and partial agonists such as pentazocine, butorphanol, nalbuphine, and buprenorphine (see Chapter 36) are less abused than full agonists, although each has a slightly different profile. Except for buprenorphine, they show little cross-dependence with heroin and can exacerbate withdrawal; thus they offer little attraction for addicts.

Stimulants

The IV administration or inhalation of cocaine produces a rapid-onset rush with intense positive reinforcing effects. Cocaine and other stimulants produce increased alertness, feelings of elation and well-being, increased energy, feelings of competence, and increased sexuality. Athletic performance has been reported to be enhanced in athletes who use stimulants, particularly in sports requiring sustained attention and endurance. Although these effects are small, they provide a significant advantage. Thus all sympathomimetic drugs, including over-the-counter medications such as pseudoephedrine, are banned by most athletic associations.

Stimulant overdose results in excessive activation of the sympathetic nervous system. The resulting tachycardia and hypertension may result in myocardial infarction and stroke. Cocaine can cause coronary vasospasm and cardiac dysrhythmias. CNS symptoms in cocaine users include anxiety, feelings of paranoia and impending doom, and restlessness. Users exhibit unpredictable behavior and sometimes become violent. Adrenergic receptor antagonists alleviate some of these symptoms, although they are often ineffective.

An important component of stimulant intoxication is the “crash” that occurs as drug effects subside. Dysphoria, tiredness, irritability, and mild depression often occur within hours after stimulant ingestion. Abuse of cocaine by snorting may lead to irritation of the nasal mucosa, sinusitis, and a perforated septum. Cocaine salts are often cut with inert substances, with other local anesthetics similar in appearance and taste, or with other stimulants. The IV administration of stimulants is associated with the same problems as IV heroin.

A dangerous pattern of stimulant abuse is the extended, uninterrupted sequences referred to as “runs.” Runs result from attempts to maintain a continuous state of intoxication, to extend the pleasurable feeling, and to postpone the postintoxication crash. Acute tolerance can occur, particularly in those taking the substance IV, resulting in a need for increasingly larger doses. This spiral of tolerance and increased dose is often continued until drug supplies are depleted or the person collapses from exhaustion. During runs, drug taking and drug-seeking behavior take on a compulsive character, making treatment intervention difficult.

Another typical abuse pattern begins with self-medication. Some individuals such as long-distance truck drivers or students use stimulants to achieve sustained attention, while often homemakers use these drugs to make tasks (housework) appear easier and complete them faster. These patterns lead to increased doses and frequency of use, producing tolerance and further dose increases. Alcohol or depressant drugs are frequently used to counteract the resultant anxiety and insomnia, establishing a cycle of “uppers and downers.”

Dependence on stimulants is characterized principally by uncontrolled compulsive episodes of use. Various sequences of mood and behavior changes have been observed after cessation of use. The most notable are fatigue and depression, which often result in drug craving and relapse. Sleep disturbances, hyperphagia, and brain abnormalities have also been noted. These psychological sequelae are important in fostering continued abuse and are important treatment targets.

Personality changes often occur in stimulant abusers and include delusions, preoccupation with self, hostility, and paranoia. A toxic psychosis can develop. Often difficult to distinguish from paranoid schizophrenia, severe amphetamine and cocaine psychoses require psychiatric management. Antipsychotic medication can be useful (see Chapter 29).

Cocaine use during pregnancy may be associated with complications including abruptio placentae, premature birth, lower birth weight, and neurobehavioral impairment of the newborn.

MDMA has cardiovascular risks similar to cocaine and amphetamine. In high doses it produces hyperthermia and liver, kidney, and cardiovascular failure. Another potential danger is neurodegeneration resulting from severe depletion of DA or 5-HT, although whether this occurs in humans is still unknown. Effects on fetal development are poorly understood.

Cigarette smoking is the preferred source for 98% of nicotine users. Nicotine is highly addictive and produces an intense and long-lasting craving. Tolerance to the effects of nicotine develops rapidly, and there is withdrawal on cessation, producing symptoms including anxiety, dysphoria, irritability, and insomnia. Tar from cigarettes is important in causing lung cancer, bronchitis, and emphysema. The CO produced leads to cardiovascular disorders.

Non-Reinforcing Compounds

Dissociative Compounds

PCP produces a unique profile of effects and combines aspects of the actions of stimulants, depressants, and hallucinogens. The subjective experience of PCP intoxication is unlike that of other hallucinogens. Perceptual effects are not as profound and relate more to somesthesia. Distortions of body image are common, and one of the motivations for PCP abuse is to enhance sexual experience. PCP users have impaired judgment and may behave in bizarre and violent ways. PCP intoxication often includes motor incoordination and cataleptic behavior accompanied by nystagmus; high doses may result in a blank stare. PCP intoxication after smoking typically lasts 4 to 6 hours. Behavior may be more disrupted when PCP is taken with depressant drugs, including alcohol. Major dangers are risk-taking behavior and development of progressive personality changes, culminating in a toxic psychosis. PCP overdose is rarely lethal but may require careful management because of severe incapacitation. No PCP antagonist is available, but an antibody is being developed.

Ketamine is basically a less-potent version of PCP. Ketamine is abused because of its ability to induce a dream-like state, vivid images, and hallucinations with possible delirium. The “K-hole” is a frightening, almost complete sensory detachment.

High-dose dextromethorphan produces dissociative effects similar to PCP and ketamine. It is often taken in combination cough medications that contain decongestants that increase its risks. Dextromethorphan is particularly abused by teenagers and young adults.

Hallucinogens

The unique psychological effects of hallucinogens include lability of mood, altered thought processes, altered visual, auditory, or somatosensory perception, the experience of having enhanced insights into events and ideas, and impaired judgment. Mood swings can range from profound euphoria to anxiety and terror. Panic states (“bad trips”) are symptoms that most commonly lead abusers to seek assistance. Generally, bad trips are not caused by overdoses, although they are more common with larger doses. Rather, they result from the propensity of the drug experience to be rapidly transformed, because it is highly dependent on environmental context. Unexpected or frightening events can transform it dramatically. People on bad trips usually respond to calm reassurance and removal from a threatening environment until the drug wears off. Medication is rarely needed, although antipsychotic treatment may be useful. Nearly all hallucinogens produce varying degrees of sympathomimetic effects, and psychomotor stimulation may be evident. These effects are more common in people who use substituted amphetamines such as MDMA, particularly at higher doses.

Although acute overdose is not a common problem with hallucinogens, there are other hazards. The most significant is the risk of injury stemming from impaired judgment. In addition, even occasional use of hallucinogens may precipitate a psychiatric illness in predisposed subjects that is exacerbated by repeated use. A poorly understood aspect is the “flashback,” in which previous users re-experience aspects of intoxication while drug-free. Flashbacks also may occur in those who have used marijuana or PCP. They may be no more than déjà vu experiences, or they may be episodes that reflect an emerging psychopathological condition. LSD does not produce withdrawal symptoms or the intense craving caused by other drugs of abuse. However, strong tolerance occurs, with cross-tolerance to other hallucinogens, and increasingly higher doses may be used, leading to unpredictable consequences.

In addition to hallucinogens, other psychoactive drugs can also dramatically alter consciousness and perception. Some are used as adulterants or substitutes in “street drugs” and are frequently misrepresented as LSD or other drugs. The most notable class is antimuscarinics, which may be diverted from medical sources. Atropine and scopolamine are present in many plants and mushrooms, and certain groups of Native Americans and Central Americans practice their ritual use to produce profound alterations in consciousness. Antimuscarinic intoxication can be accompanied by signs of poisoning, and in severe cases, appropriate treatment with the cholinesterase inhibitor physostigmine should be instituted (see Chapter 10).

Inhalants

Inhalants are abused by young children who obtain these compounds readily at low to no cost. Abuse of the inhalants peaks at eighth grade and represents a cheap entry into drug abuse.

Volatile solvents are toluene-containing materials including paint thinners and sprays, correction fluids, and plastic adhesives; solvents and cleaners contain alkylbenzenes and chlorinated hydrocarbon cleaners and degreasers. Aerosol propellants include ethyl chloride and chlorofluorocarbon-containing compounds.

The most abused gas is N2O. Gases are also found in household products such as whipped cream dispensers and butane and propane sources. Motor performance deficits similar to those produced by alcohol and depressant drugs occur in those who abuse solvents. Prolonged use can lead to arrhythmias, heart failure, and death, especially with butane, propane, and aerosols. Death may also occur because of suffocation, asphyxiation, choking, and accidents while intoxicated. Inhalants are highly toxic, and chronic use can cause damage to the central and peripheral nervous systems, including well-defined axonopathies. In addition, chlorofluorocarbons are cardiotoxic. Treatment of acute toxicity is supportive to stabilize vital signs. There is no antidote or treatment for chronic exposure.

Amyl nitrite ampules are diverted from medical supplies. Other organic nitrites are available in specialty stores as room “odorizers.” Nitrites are vasodilators, and dizziness and euphoria result from the hypotension and cerebral hypoxia caused by peripheral venous pooling. Nitrites are often abused in conjunction with sexual activity and are popular among homosexual men for their ability to enhance orgasms, probably as a result of penile vasodilation. Nitrite use can result in accidents related to syncope.

Anabolic Steroids

Steroids are abused largely by body-builders and athletes to improve performance. However, they are also used by others to improve muscle size and reduce body fat and by adolescents who partake in high-risk behaviors. Athletes and body-builders often “stack” steroids, taking different combinations of two or more compounds in cycles of weeks or months in the belief that this improves effects. Because amounts taken are far in excess of natural hormone levels, they result in negative feedback and prevent production of natural hormones. This often results in testicular atrophy, reduced sperm production, and gynecomastia. Steroids in high doses also increase irritability and aggression and produce depression on withdrawal. Many users report that steroids make them feel good and are addictive. A complete description of the side effects of these agents is presented in Chapter 41.

Dietary supplements including dehydroepiandrostenedione and androstenedione (see Chapter 7) are also abused and are converted to anabolic steroids in the body. The use of steroids and steroid supplements is banned by the governing bodies of most sports.

Pharmacotherapies for Substance Abuse

In considering treatment, it is important to remember that substance abuse is a chronic, relapsing disease. Treatment can be very effective, especially if one accepts reduction in drug use and its resultant harm as an important goal. Complete cessation of drug use can also be achieved but may require multiple attempts. Thus repeated treatment of drug abuse should be considered in a similar light as treatment of other chronic diseases. It is important to distinguish physically dependent from nondependent abusers, because treatment strategies differ considerably. Unfortunately, most treatment programs are for “hard-core” dependent abusers. Strategies for preventing escalation from occasional use would be desirable but are rare.

Although numerous medications have been investigated for their ability to treat stimulant dependence, to date, no benefits have been reported for either antidepressants or DA agonists. Antipsychotics can be useful for the treatment of overdoses and toxic psychoses.

Similarly, there are no medications to treat dependence on CNS depressants, marijuana, hallucinogens, dissociative compounds, inhalants, or anabolic steroids. However, progress in the pharmacotherapy for opioid and nicotine addiction is increasing. The treatment of alcohol dependence is discussed in Chapter 32.

Opioids

Detoxification of patients receiving opioids for pain relief is accomplished by tapering the dose of a prescribed opioid or by substituting a longer-acting drug. Only rarely does such iatrogenic dependence lead to illicit opioid use, and development of dependence should not be a consideration in providing adequate relief of terminal pain.

Although medications play an important role in treatment of opioid abuse, they are adjuncts to psychosocial and educational interventions. Simple detoxification alone is not usually sufficient to prevent relapse. Most abusers undergo detoxification numerous times, either medically or as a result of interrupted drug supply or incarceration. Non-drug detoxification can be used, or alternatively, abusers can be stabilized on a long-acting oral medication such as methadone. The daily dose is gradually decreased over 30 days (inpatient) or 180 days (outpatient). Withdrawal signs are mild, although the patient will be uncomfortable for most of the withdrawal period. Another approach is to terminate opioids abruptly with an antagonist and treat the symptoms of withdrawal, many of which reflect stress and sympathetic nervous system activation. Medications such as the α2-receptor antagonist clonidine have been used successfully for this purpose (see Chapter 11), particularly in mildly dependent subjects. Antianxiety agents may also be useful.

Maintenance therapies are based on cross-dependence. Methadone is used because it has good oral bioavailability and a long duration of action. Methadone maintenance patients receive single, daily oral doses that are chosen to prevent withdrawal but not large enough to produce significant intoxication. Urinalysis to detect continued illicit drug use is important. Methadone maintenance is effective and has many proven benefits. It breaks the destructive pattern of continued drug abuse and lessens criminal behavior. It is also attractive to abusers who would not otherwise seek treatment and provides an opportunity for other interventions to be implemented. Methadone has dependence-producing properties that help ensure continued patient participation. Take-home medications and other clinic privileges can be used to reinforce positive changes in behavior. The long-acting methadone analog l-methadyl acetate may provide some advantages, because it has active metabolites with a longer duration than methadone and is administered three times per week, thereby reducing the need for daily clinic visits.

The partial agonist buprenorphine is also available for the treatment of dependence. Buprenorphine is a Schedule III drug and is administered IV or intramuscularly. When buprenorphine is used in dependent heroin abusers, it can prevent withdrawal. In addition, eventual discontinuation may be accompanied by decreased withdrawal symptoms, perhaps reflecting its slow rate of dissociation from its receptor. Because buprenorphine has agonist activity, to overcome the possibility of dependent individuals using buprenorphine for its opioid properties, it is available in combination with the antagonist naloxone in a sublingual preparation.

Nicotine

Drug therapy has been increasingly relied upon to assist in smoking cessation. Approaches currently used include nicotine replacement therapy (NRT), antidepressants, and the newly developed partial agonist, varenicline.

NRT products include both nonprescription nicotine gum, patches, and lozenges, and prescription nicotine nasal sprays and inhalers. The rationale for the use of NRT is to prevent the user from smoking and inhaling tobacco and to decrease the rituals associated with smoking while initially maintaining nicotine intake. Eventually the dose of nicotine used is decreased. These products have aided cessation and achieved success in some, but not all individuals.

The use of antidepressants for nicotine dependence is based on evidence that nicotine exhibits antidepressant effects that maintain the smoking habit, and nicotine withdrawal may produce depressive symptoms or precipitate a major depressive episode. The antidepressant bupropion has been shown to aid in long-term smoking cessation. Although the specific mechanism mediating this effect is not fully understood, bupropion inhibits NE, DA, and 5-HT reuptake and is a noncompetitive nicotinic receptor antagonist. Again, bupropion has aided cessation and achieved success in some, but not all individuals.

Varenicline is a newly developed partial nicotinic receptor agonist with selectivity at nicotinic α4β2 receptors. As a partial agonist, varenicline both stimulates and prevents nicotine from binding to these receptors. As a consequence of receptor activation, the activity of the mesolimbic DA pathway increases, but to a lesser extent than with nicotine. Absorption of varenicline is nearly complete after oral administration, with maximum plasma levels reached within 3 to 4 hours. Varenicline exhibits low (≤20%) plasma protein binding, has minimal metabolism, and has an elimination half-life of 24 hours, with >90% of an administered dose excreted unchanged in the urine. The most common adverse events of varenicline include nausea, sleep disturbances, constipation,

CLINICAL PROBLEMS

Societal

Loss of productivity; missed work

Constant drug seeking; criminal activity

Personal

Tolerance; need for increasing drug doses

Dependence; requirement for continuous exposure

Social; development of sociopathies and loss of personal relationships

Medical

Overdoses

Abscesses at injection sites, thrombophlebitis

Pregnancy complications and babies born dependent

Possibilities for subsequent infections:

HIV

Bacterial endocarditis

Hepatitis and hepatic dysfunction

Tuberculosis

Pneumonia

Septic pulmonary embolism

Tetanus

flatulence, and vomiting. Several post-marketing reports have indicated that varenicline may produce behavioral changes including suicidal ideation and suicide, and some have suggested that this may reflect exacerbation of an underlying psychiatric illness. Because this compound has not been on the market for a prolonged period of time (it was approved by the U.S. Food and Drug Administration in 2006), it is too early to judge either its rate of success relative to other treatment modalities or the incidence of suicidal ideation or suicide in the smoking population-at-large.

New Horizons

Increasing knowledge of the mechanisms of action of abused drugs could lead to medications that stabilize the user in the same way as methadone and buprenorphine do for opioid addicts, or even allow for abstinence. In a similar way, “substitute agonists” for the DA transporter, the site of action of cocaine and amphetamine, have been synthesized and shown to prevent self-administration of cocaine in non-human primates. Although indirectly acting DA agonists have yielded mixed results, clinical studies with disulfiram, which increases levels of DA in brain as a consequence of blocking the conversion of DA to NE, in cocaine addicts have been encouraging.

Most recently, vaccines for both nicotine and cocaine abuse have been developed and are being investigated for their efficacy. NicVax®, a nicotine conjugate vaccine, has been under study for several years. Phase II trials, which have been completed, indicate that at a 6-month endpoint, individuals with a high anti-nicotine antibody titer demonstrated complete abstinence. A vaccine for cocaine is currently in clinical trials.

TRADE NAMES

(In addition to generic and fixed-combination preparations, the following trade-named materials are some of the important compounds available in the United States.)

Opioid Dependence

Buprenorphine (Buprenex)

Buprenorphine + Naloxone (Suboxone)

Levomethadyl Acetate (LAAM, ORLAAM)

Nalmefene (Revex)

Naloxone (Narcan)

Naltrexone (ReVia)

Pentazocine + naloxone (Talwin Nx)

Nicotine Dependence

Bupropion (Zyban)

Nicotine gum (Nicorette)

Nicotine patch (Habitrol, NicoDerm, PROSTEP)

Nicotine inhaler/nasal spray (Nicotrol)

Varenicline (Chantix)

FURTHER READING

Benowitz NL. Neurobiology of nicotine addiction: Implications for smoking cessation treatment. Am J Med. 2008;121:S3-S10.

Howell LL, Kimmel HL. Monoamine transporters and psychostimulant addiction. Biochem Pharmacol. 2008;75:196-217.

Volkow et al. 2007 Volkow ND, Fowler JS, Wang GJ, Swanson JM, Telang F. Dopamine in drug abuse and addiction: Results of imaging studies and treatment implications. Arch Neurol. 2007;64:1575-1579.

For further information, see the National Institute for Drug Abuse at http://www.nida.nih.gov.

SELF-ASSESSMENT QUESTIONS

1. A person who has been taking one drug chronically and experiences a withdrawal syndrome upon discontinuing it finds relief from these symptoms by taking a second drug. This is an example of:

A. Craving.

B. Psychological dependence.

C. Cross-dependence.

D. Tolerance.

E. Drug addiction.

2. Which of the following drugs most likely results in a life-threatening withdrawal?

A. Cocaine

B. Pentobarbital

C. Heroin

D. LSD

E. Methamphetamine

3. Relative to barbiturates, the dose-effect curves for benzodiazepines are:

A. Steep.

B. Shallow.

C. Parallel.

D. Biphasic.

E. Inverted.

4. The problems of cocaine abuse are most similar to those of:

A. Heroin abuse.

B. Marijuana abuse.

C. Amphetamine abuse.

D. Alcoholism.

5. Barbiturate withdrawal symptoms are similar to the withdrawal symptoms from:

A. Heroin.

B. Alcohol.

C. Phenothiazines.

D. Morphine.

E. Nicotine.