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Warfarin dosing and reversal

Ward jobs

Know the indication and target INR before changing a dose (usually 2.5; 3.5 for some mechanical valves).

Major bleeding: stop warfarin, give prothrombin complex concentrate and IV vitamin K — don't wait for the INR.

Act now

  1. 1

    Major bleeding: stop warfarin; vitamin K 5 mg IV; prothrombin complex concentrate (FFP only if PCC is unavailable).

  2. 2

    INR over 8 with minor bleeding: stop warfarin; vitamin K 1–3 mg IV; restart when INR under 5.

  3. 3

    INR over 8, no bleeding: stop warfarin; vitamin K 1–5 mg orally (IV preparation by mouth); repeat if INR still too high after 24 hours; restart when INR under 5.

  4. 4

    INR 5–8 with minor bleeding: stop warfarin; vitamin K 1–3 mg IV; restart when INR under 5.

  5. 5

    INR 5–8, no bleeding: withhold 1 or 2 doses and reduce the maintenance dose.

Assess

  • Indication, target INR, INR trend, who manages it (anticoagulation clinic).
  • Bleeding: site and severity. Head injury on warfarin needs a CT head.
  • Interactions: clarithromycin, erythromycin, metronidazole, co-trimoxazole, fluconazole, amiodarone; also poor intake, alcohol, liver disease.

Investigations

  • INR, FBC, U&E, LFT; repeat INR the next day after any change or reversal.

Manage

  • Loading and dosing: follow your local warfarin chart (age-adjusted regimens are common).
  • Subtherapeutic INR with a high-risk indication (mechanical valve, recent VTE): discuss LMWH bridging.
  • Before elective procedures: usually stop 5 days before; discuss bridging for high-risk patients.

Escalate when

  • Major or intracranial bleeding: senior and haematology. Mechanical valve with a high INR: discuss before giving vitamin K.

Doses and prescribing

Phytomenadione (vitamin K)

5 mg IV for major bleeding; 1–3 mg IV for minor bleeding with INR over 5.

Prothrombin complex concentrate

25–50 units/kg IV, product-specific, for major bleeding.

Check the BNF and your local formulary before prescribing.

Written to current guidance (NICE NG232) · sources checked 14 Nov 2026. We watch NICE for updates weekly.

Sources (2)

Reviewed by Aboodee Alhasso, MPharm MRCPharm IP on 14 Nov 2026

Calculators for this

For healthcare professionals, as an aid to clinical judgement — not a substitute for it. Follow your local policy and ask a senior when in doubt.