BMA Concise Guide to Medicine & Drugs

Phenobarbital (Phenobarbitone)

Brand name None

Used in the following combined preparations None


Drug group Barbiturate anticonvulsant drug

Overdose danger rating High

Dependence rating High

Prescription needed Yes

Available as generic Yes


Phenobarbital belongs to the group of drugs known as barbiturates. The drug is used mainly in the treatment of epilepsy, although this use is declining. It was also used as a sleeping drug and sedative before the development of safer drugs. In the treatment of epilepsy, the drug is usually given together with another anticonvulsant drug such as phenytoin. The main disadvantage of the drug is that it often causes unwanted sedation. However, tolerance develops within a week or two, and most patients have no problem in long-term use. In children and the elderly, it may occasionally cause excessive excitement. Because of their sedative effects, phenobarbital and other barbiturates are sometimes abused.


Your drug prescription is tailored for you. Do not alter dosage without checking with your doctor.

How taken/used Tablets, liquid, injection.

Frequency and timing of doses Once daily, usually at night.

Dosage range Adults 60–180mg daily.

Onset of effect 30–60 minutes (by mouth).

Duration of action 24–48 hours (some effect may persist for up to 6 days).

Diet advice People taking the drug long-term should eat plenty of fresh green vegetables to prevent possible deficiency of vitamins A, D, K, and folic acid.

Storage Keep in original container at room temperature out of the reach of children.

Missed dose Take as soon as you remember. If the next dose is due within 10 hours, take a single dose now and skip the next.

Stopping the drug Do not stop taking the drug without consulting your doctor, who may supervise a gradual reduction in dosage. Abrupt cessation may cause seizures or lead to restlessness, trembling, and insomnia.


Seek immediate medical advice in all cases. Take emergency action if unsteadiness, severe weakness, confusion, or loss of consciousness occur.


Most of the adverse effects of phenobarbital are the result of its sedative effect. They can sometimes be minimized by a medically supervised reduction in dosage. Common side effects include drowsiness, clumsiness, unsteadiness, dizziness, and fainting. Discuss with your doctor if these are severe. More rarely, the drug may cause confusion, mood changes, or memory problems. Consult your doctor if any of these occur. If you develop a rash, localized swellings, or mouth ulcers, you should stop taking the drug and contact your doctor immediately.


General note Phenobarbital interacts with a wide range of other drugs. Consult your doctor or pharmacist before taking any new drugs, including herbal remedies.

Sedatives All such drugs are likely to increase the sedative properties of phenobarbital.

Anticoagulants, corticosteroids, oral contraceptives, and protease inhibitors Their effect may be decreased by phenobarbital.

Antipsychotics, antidepressants, St John’s wort may reduce the anticonvulsant effect of phenobarbital.


Be sure to tell your doctor if:

· You have long-term liver or kidney problems.

· You have heart problems.

· You have poor circulation.

· You have porphyria.

· You have breathing problems.

· You have depression.

· You are taking other medicines.

Pregnancy The drug may affect the fetus and increase the tendency of bleeding in the newborn. Discuss with your doctor.

Breast-feeding The drug passes into the breast milk and could cause drowsiness in the baby. Discuss with your doctor.

Infants and children Reduced dose necessary.

Over 60 Increased likelihood of confusion. Reduced dose may therefore be necessary.

Driving and hazardous work Your underlying condition, in addition to the possibility of reduced alertness while taking phenobarbital, may make such activities inadvisable. Discuss with your doctor.

Alcohol Never drink alcohol while under treatment with phenobarbital. Alcohol may interact dangerously with this drug.


With prolonged use, tolerance to the drug’s sedative effects may develop. Dependence may also result, and withdrawal symptoms may occur if the drug is stopped suddenly. Long-term use may also lead to deficiency of vitamins A, D, K, and folic acid.

Monitoring Blood samples may be taken periodically to test blood levels of the drug.