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Vitamin K Antagonist Anticoagulant Pregnancy: Contraindicated in pregnancy (teratogenic, fetotoxic)

Warfarin

Brand names: Coumadin, Marevan

Adult dose

Dose: Individualised; typical maintenance 3-9 mg/day
Route: Oral
Frequency: Once daily (same time each day)
Max: Dose adjusted to achieve target INR
Target INR: 2.0-3.0 (AF, DVT/PE, mechanical heart valves require higher target)

Paediatric dose

Dose: Seek specialist opinion N/A/kg
Route: Oral
Frequency: Seek specialist opinion
Max: Seek specialist opinion
Seek specialist opinion

Dose adjustments

Renal

Dose adjustment not required for renal function alone, but bleeding risk increases in renal impairment

Hepatic

Avoid in severe hepatic impairment — impaired clotting factor synthesis makes INR unreliable

Paediatric weight-based calculator

Seek specialist opinion

Clinical pearls

  • In elderly with AF, DOACs (apixaban, rivaroxaban, edoxaban) are now preferred over warfarin per NICE NG196 and ESC 2020 — lower intracranial haemorrhage risk and no INR monitoring required
  • Warfarin still preferred in: mechanical heart valves, antiphospholipid syndrome with arterial events, or cost/access constraints
  • HAS-BLED score: use to identify modifiable bleeding risk factors (hypertension, labile INR, alcohol, NSAIDs) rather than to withhold anticoagulation
  • Antidote: Vitamin K (slow reversal); Beriplex/Octaplex (PCC — rapid reversal for major bleeding or emergency surgery)
  • MHRA 2014: Warfarin-cranberry juice interaction confirmed — advise patients to avoid

Contraindications

  • Active bleeding
  • Severe hepatic impairment
  • Pregnancy (teratogenic — especially weeks 6-12)
  • Thrombocytopenia

Side effects

  • Bleeding (the primary risk)
  • Skin necrosis (rare — protein C deficiency)
  • Purple toe syndrome
  • Alopecia
  • Osteoporosis (long-term)

Interactions

  • Extensive interactions — antibiotics (altered gut flora), NSAIDs, antifungals, amiodarone, SSRIs, statins, herbal medicines (St John's Wort reduces effect)
  • Always check interactions before prescribing any new drug in warfarin patients

Monitoring

  • INR (weekly initially; then every 4-12 weeks when stable)
  • Signs of bleeding
  • Blood pressure

Reference: BNFc; BNF 90; NICE NG196 (AF); ESC AF Guidelines 2020; BNF Interactions Appendix. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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