Major
paracetamol × warfarin
Anilides. Analgesics and antipyretics×Vitamin K antagonist anticoagulants
Mechanism
Paracetamol above 2 g/day for more than 7 days raises INR by 1–3 units. The mechanism is not fully understood – the NAPQI metabolite probably inhibits vitamin K-dependent γ-carboxylation. A single 0.5–1 g dose is clinically insignificant. Occasional use up to 2 g/day for less than 7 days is safe.
Management
In warfarin-treated patients, paracetamol remains the preferred analgesic over NSAIDs. For courses longer than 7 days or doses above 2 g/day, check INR at 7 days. After the course ends, recheck INR in 7–10 days.
Sources
- Pharmaceutical Press: Stockley's Drug Interactions, 12th edition (2024)— Preston CL (ed.). Stockley's Drug Interactions. 12th ed. London: Pharmaceutical Press; 2024
- AGS: American Geriatrics Society 2023 Updated AGS Beers Criteria for Potentially Inappropriate Medication Use in Older Adults (2023)— J Am Geriatr Soc 2023;71(7):2052–2081