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Anticoagulant Pregnancy: X — teratogenic in first trimester (warfarin embryopathy). Use LMWH throughout.

Warfarin

Brand names: Coumadin, Marevan

Adult dose

Dose: Individualised — typically 1–10mg OD (INR-guided)
Route: Oral
Frequency: Once daily (at same time each day)
Loading: 10mg OD day 1 (if not elderly/frail/malnourished), then 10mg day 2, then dose-adjust by INR. Alternative: Fennerty loading protocol (10mg, 10mg, 5mg, 5mg then guide by INR). Typical maintenance 3–5mg OD but highly variable.

Dose adjustments

Renal

Use with caution in severe CKD — increased bleeding risk. Frequent INR monitoring required.

Hepatic

Use with extreme caution in hepatic impairment — reduced clotting factor synthesis, unpredictable INR response.

Clinical pearls

  • INR targets: AF/VTE prevention: 2–3. Mechanical prosthetic heart valves: 2.5–3.5 (mitral) or 2–3 (aortic).
  • Reversal for bleeding: Vitamin K 1–5mg IV or oral; if life-threatening — 4-factor PCC (Beriplex, Octaplex) 25–50 units/kg IV + vitamin K 5–10mg IV. Do NOT wait for FFP effect in emergency.
  • Sick day rules for patients: check INR if unwell, vomiting, starting antibiotics, or new medications.
  • Consider switching to DOAC (apixaban, rivaroxaban) for non-valvular AF — equivalent efficacy, safer profile, no monitoring. DOACs contraindicated in mechanical valves and antiphospholipid syndrome.

Contraindications

  • Active major bleeding
  • First trimester of pregnancy (Warfarin embryopathy risk) and near term
  • Peptic ulcer disease with high bleeding risk (relative)
  • Poorly controlled hypertension (SBP >180 mmHg — bleeding risk)

Side effects

  • Bleeding (major — intracranial, GI, retroperitoneal)
  • Skin necrosis (rare — first days of treatment, protein C deficiency risk)
  • Purple toe syndrome (rare — cholesterol microemboli)
  • Alopecia (reversible)
  • Teratogenicity (first trimester and near term)

Interactions

  • Many interactions — check every new drug for effect on INR
  • Amiodarone: markedly increases INR — reduce warfarin dose 30–50%
  • Antibiotics (metronidazole, ciprofloxacin): increase INR via gut flora reduction / CYP inhibition
  • NSAIDs: additive bleeding risk (GI and COX inhibition on platelet function)
  • St John's Wort: reduces INR — enzyme induction
  • Cranberry juice: increases INR

Monitoring

  • INR — daily until stable
  • then weekly
  • then monthly (when stable). Monitor more frequently with illness
  • new drugs.

Reference: NICE NG196 AF; NICE BNF 84; BNF Warfarin monograph. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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