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Oral anticoagulant (Vitamin K antagonist) Pregnancy: Contraindicated in first trimester (warfarin embryopathy) and near term (fetal/neonatal bleeding). LMWH preferred.

Warfarin

Brand names: Coumadin, Marevan

Adult dose

Dose: Loading: 5–10 mg day 1; then adjusted per INR
Route: Oral
Frequency: Once daily (same time each day)
Max: Adjusted by INR — no fixed maximum
Start 5 mg (or 3 mg in elderly/hepatic/cardiac failure). Check INR on day 3–4 and adjust. Target INR: AF/DVT/PE 2–3; mechanical heart valves 2.5–3.5. Induction protocols available (e.g., Fennerty chart).

Paediatric dose

Dose: 0.1 mg/kg
Route: Oral
Frequency: Once daily
Max: 10 mg/day initially
Initial: 0.1–0.2 mg/kg (max 10 mg) day 1. Day 2 onwards: adjust per INR. Infants (especially with formula feeding) may need higher doses. Paediatric haematology specialist input essential.

Dose adjustments

Renal

No dose adjustment but INR monitoring more variable; increased bleeding risk.

Hepatic

Use with extreme caution — reduced clotting factor synthesis increases sensitivity.

Paediatric weight-based calculator

Initial: 0.1–0.2 mg/kg (max 10 mg) day 1. Day 2 onwards: adjust per INR. Infants (especially with formula feeding) may need higher doses. Paediatric haematology specialist input essential.

Clinical pearls

  • Self-monitoring INR devices (CoaguChek) available for stable patients
  • Warfarin reversal: Vitamin K 5–10 mg oral/IV + PCC 4-factor (Beriplex) for urgent reversal
  • Hold 5 days before elective surgery (check INR 24–48h pre-op)
  • DOACs now preferred over warfarin for most AF indications — simpler dosing
  • Diet education: consistent green vegetable intake; avoid major fluctuations in Vitamin K

Contraindications

  • Active significant bleeding
  • Pregnancy (first and third trimester — teratogenic and bleeding risk)
  • Haemorrhagic stroke (recent)
  • Severe uncontrolled hypertension
  • Bacterial endocarditis (relative)

Side effects

  • Bleeding (major or minor)
  • Skin necrosis (especially with protein C/S deficiency)
  • Purple toe syndrome
  • Calciphylaxis (rare)
  • Hypersensitivity reactions

Interactions

  • Numerous — major interaction potential
  • Antibiotics (especially metronidazole, ciprofloxacin, co-trimoxazole) — increase INR
  • NSAIDs — increased bleeding risk and may alter INR
  • Amiodarone — significantly increases INR; reduce warfarin dose by 30–50%
  • CYP2C9 inducers (rifampicin, carbamazepine) — reduce warfarin levels
  • Simvastatin, fluconazole — increase warfarin effect

Monitoring

  • INR (frequency varies by stability)
  • Signs of bleeding
  • BP

Reference: BNFc; BNF; NICE NG196 Anticoagulation; BNFc. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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