Warfarin

Class: anticoagulant (coumarin)

Indications: thrombotic disorders prophylaxis

Contraindications/cautions: potential haemorrhagic conditions; caution in hepatic and renal impairment

Adverse reactions: common: bleeding; less common: hair loss; rare – purple toe syndrome

Metabolism/clearance: metabolised by the metabolising enzymes CYP 1A2, 2C19 and 2C9 mainly in the liver

Interactions:

• increased clinical effect/toxicity of warfarin (due to increased blood concentrations) may occur with some CYP metabolising enzyme inhibitors (see above) e.g. ciprofloxacin, fluconazole, fluoxetine, ketoconazole, pantoprazole
• decreased clinical effect/toxicity of warfarin (due to decreased blood concentrations) may occur with some CYP metabolism enzyme inducers (see above) e.g. broccoli like vegetables, carbamazepine, phenobarbitone, phenytoin, rifampicin, smoking
• increased risk of bleeding with aspirin, SSRIs (e.g. fluoxetine), NSAIDs (e.g. diclofenac)
• increased clinical effect of warfarin may occur with paracetamol
• decreased clinical effect of warfarin may occur with phytomenadione (vitamin K) and foods rich in vitamin K

NB Any changes in drug therapy should be accompanied by an INR check

Syringe driver: not available

Mechanism of action: interferes with vitamin K synthesis

Notes:

• a low molecular weight heparin e.g. enoxaparin may be better tolerated
• sifferent brands are not proven to be equivalent

*recurrence despite prothrombin ratio between 2 and 3

Table from Management Guidelines for Common: Medical Conditions, 15th Edition 2013, Canterbury District Health Board, New Zealand