BMA Concise Guide to Medicine & Drugs

Warfarin

Brand name Marevan

Used in the following combined preparations None

QUICK REFERENCE

Drug group Anticoagulant drug

Overdose danger rating High

Dependence rating Low

Prescription needed Yes

Available as generic Yes

GENERAL INFORMATION

Warfarin is an anticoagulant widely used to prevent blood clots, mainly where blood flow is slowest, particularly the leg and pelvic veins (deep-vein thromboses). Such clots can break off and travel to the lungs, where they cause pulmonary embolism. The drug is also used to reduce the risk of clots in the heart in people with atrial fibrillation (irregular heart rhythm) or artificial heart valves. These clots may travel to the brain and cause a stroke. Regular monitoring is needed to ensure warfarin’s proper maintenance, dosage and safety, using the INR (International Normalised Ratio) blood test. As warfarin’s full beneficial effects are not felt for two to three days, a faster-acting drug such as heparin is often used initially in people with, or at high risk of developing, a clot.

The most serious adverse effects is the risk of excessive bleeding, because of excessive dosage or interaction with other drugs.

INFORMATION FOR USERS

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

How taken/used Tablets.

Frequency and timing of doses Once daily, taken at the same time each day.

Dosage range Large variation in starting and maintenance dose, according to patient factors, but usually 10mg for 2 days (starting dose); 3–9mg daily at same time, determined by blood tests (maintenance dose).

Onset of effect Within 24–48 hours; full effect after several days.

Duration of action 2–3 days.

Diet advice Avoid cranberry juice and major diet changes (especially of salads and vegetables).

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

Missed dose Take as soon as you remember. Take the following dose on your original schedule.

Stopping the drug Do not stop taking the drug without consulting your doctor; stopping the drug may lead to worsening of the underlying condition.

OVERDOSE ACTION

Seek immediate medical advice in all cases. Take emergency action if severe bleeding or loss of consciousness occur.

POSSIBLE ADVERSE EFFECTS

Bleeding is the most common adverse effect of warfarin. If you notice excessive bruising, very prolonged bleeding from a minor wound, or blood in your urine or faeces, contact your doctor immediately and stop taking the drug. Rarely, warfarin may cause abdominal pain, diarrhoea, rash, and hair loss. Consult your doctor if any of these occur. If you develop fever or jaundice, you should stop taking the drug and contact your doctor urgently.

INTERACTIONS

General note A wide range of drugs, (such as aspirin and other non-steroidal anti-inflammatory drugs (NSAIDs), diuretics, chemotherapy, oral contraceptives, lipid-lowering drugs, amiodarone, barbiturates, cimetidine, steroids, and certain laxatives, antidepressants, antibiotics and herbal medicines) interact with warfarin to affect the risk of bleeding. Consult your pharmacist before using over-the-counter medicines, and inform your warfarin clinic of any changes to your medicines.

SPECIAL PRECAUTIONS

Be sure to tell your doctor if:

· You have long-term liver or kidney problems.

· You have high blood pressure.

· You have a history of peptic ulcers.

· You have a bleeding disorder.

· You are taking other medicines.

Pregnancy Not prescribed. Given in early pregnancy, the drug can cause malformations in the unborn child. Taken near the time of delivery, it may cause the mother to bleed excessively. Discuss with your doctor, who will prescribe alternative treatment.

Breast-feeding The drug passes into the breast milk, but at normal doses adverse effects on the baby are unlikely. Discuss with your doctor.

Infants and children Reduced dose necessary.

Over 60 No special problems.

Driving and hazardous work Use caution. Even minor bumps can cause bad bruises and excessive bleeding.

Alcohol Avoid major changes in alcohol consumption.

Surgery and general anaesthetics Warfarin may need to be stopped before surgery. Discuss with your doctor or dentist.

PROLONGED USE

No special problems.

Monitoring Regular INR blood tests are carried out. Dose is adjusted accordingly and recorded in a treatment book, which should be carried with you at all times. More frequent testing may be needed if there is a significant change in your health.