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Vitamin K Antagonist (Anticoagulant) Pregnancy: Contraindicated in 1st and 3rd trimesters; LMWH preferred throughout

Warfarin

Brand names: Coumadin, Marevan

Adult dose

Dose: Loading: 5–10 mg; Maintenance: 1–9 mg
Route: Oral
Frequency: Once daily (same time each day)
Max: Titrated to INR target
INR targets: AF / DVT / PE = 2.0–3.0; mechanical heart valves = 2.5–3.5. Loading: 10 mg for 2 days (if no bleeding risk), then dose per INR. Always prescribe by brand (Marevan) per MHRA.

Paediatric dose

Dose: 0.1–0.2 mg/kg
Route: Oral
Frequency: Once daily
Max: 10 mg/day
Specialist initiation only. INR monitoring essential. Labile INR common in children.

Dose adjustments

Renal

Use with caution in renal impairment — increased bleeding risk. Increase INR monitoring frequency.

Hepatic

Significant hepatic impairment increases sensitivity to warfarin — reduce dose and monitor closely.

Paediatric weight-based calculator

Specialist initiation only. INR monitoring essential. Labile INR common in children.

Clinical pearls

  • Antidote: Vitamin K (phytomenadione) — oral for INR >5 without bleeding; IV for serious bleeding. Prothrombin complex concentrate (PCC, e.g. Beriplex) for life-threatening bleeding.
  • MHRA: prescribe by brand name (Marevan) to avoid formulation switching
  • CYP2C9 and VKORC1 gene variants affect warfarin dose requirements — pharmacogenomics testing available
  • Bridging anticoagulation needed perioperatively in high-risk patients (mechanical valves)

Contraindications

  • Active significant bleeding
  • Severe hepatic impairment
  • Pregnancy (1st and 3rd trimesters — warfarin embryopathy)
  • Within 72 hours of surgery with significant bleeding risk

Side effects

  • Bleeding (major risk)
  • Skin necrosis (rare — protein C deficiency)
  • Purple toe syndrome
  • Alopecia
  • Teratogenicity

Interactions

  • Many interactions via CYP2C9 — antibiotics, NSAIDs, amiodarone, fluconazole, SSRIs all increase bleeding risk
  • Rifampicin markedly reduces anticoagulant effect
  • Cranberry juice increases INR
  • Vitamin K-rich foods reduce INR

Monitoring

  • INR: daily initially, then weekly until stable, then monthly
  • Signs of bleeding

Reference: BNFc; BNF 90; NICE NG196 (Atrial Fibrillation); MHRA Warfarin Safety Update; 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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