Vitamin K antagonist (oral anticoagulant)
Warfarin
Oral vitamin K antagonist used for the prevention and treatment of thromboembolism. Narrow therapeutic index — INR monitoring is required.
Indications
- Treatment and secondary prevention of venous thromboembolism (DVT, PE)
- Stroke prevention in non-valvular and valvular atrial fibrillation
- Mechanical heart valve thromboprophylaxis
- Selected patients with antiphospholipid syndrome
Dosing
Initiation
Slow loading regimens (e.g. 5 mg daily) are commonly used in older adults; faster regimens may be appropriate in younger inpatients. Always confirm against the local hospital protocol and the BNF entry.
Maintenance
Adjusted to the patient's INR. Typical maintenance is 3–9 mg once daily but varies widely between individuals. Take at the same time each day, conventionally evening.
INR targets
Most indications target INR 2.5 (range 2.0–3.0). Mechanical mitral valves and recurrent VTE on warfarin target INR 3.5 (range 3.0–4.0). Always confirm against the indication-specific guidance.
Always confirm doses against the current BNF — this summary is for study, not prescribing.
Monitoring
- INR before initiation, then daily/alternate days during loading until stable
- Once stable, INR every 4–8 weeks (extend up to 12 weeks with sustained stability per local policy)
- Recheck INR after any dose change, intercurrent illness, antibiotic course, dietary change, or new medication
- FBC + LFTs as clinically indicated
Contraindications
- Pregnancy — teratogenic in the first trimester (fetal warfarin syndrome) and risk of fetal haemorrhage near term
- Active major bleeding
- Recent or planned major surgery or lumbar puncture
- Severe uncontrolled hypertension
- Severe hepatic impairment (especially with raised INR)
Important interactions
- Antibiotics — broad-spectrum (especially metronidazole, ciprofloxacin, clarithromycin, co-trimoxazole) potentiate warfarin via CYP2C9 inhibition and gut-flora disruption
- Amiodarone — markedly increases INR (CYP2C9 inhibition); reduce warfarin dose ~30–50% on initiation
- NSAIDs and aspirin — additive bleeding risk
- Cranberry juice and grapefruit can increase INR; St John's wort decreases INR (enzyme induction)
- Vitamin K-rich foods (leafy greens) reduce INR if intake changes substantially
- Many anticonvulsants (carbamazepine, phenytoin) reduce INR via induction
Counselling points
- Take at the same time every day — evenings are conventional
- Carry your yellow anticoagulation booklet at all times
- Tell every healthcare professional you are on warfarin before any procedure or new medication
- Avoid drastic changes in diet, especially leafy green vegetables and cranberry juice
- Limit alcohol — binge drinking destabilises INR
- Pregnancy is contraindicated; women of childbearing potential need reliable contraception
- Seek urgent help for unusual bleeding, severe headache, blood in urine/stool, or major trauma
Red flags
- Major bleeding, intracerebral haemorrhage — stop warfarin and reverse with IV vitamin K and/or four-factor PCC per local protocol
- INR > 8 without bleeding — withhold and consider oral vitamin K per BNF
- Pregnancy — switch to LMWH urgently
Practice for the CRA
Drug-specific questions, calculations, and full mock exams aligned to the GPhC Common Registration Assessment.