Major
Atorvastatin × Warfarin
HMG-CoA reductase inhibitors (statins)×Vitamin K antagonist anticoagulants
Mechanism
Atorvastatin weakly inhibits CYP3A4 and CYP2C9 – the warfarin clearance route. Some patients show a 20–30% INR rise within 2–4 weeks; a small share experience clinically significant effects.
Symptoms
Gum bleeding, epistaxis, bruising without trauma, blood in urine or stool, menorrhagia. Severe cases include gastrointestinal or intracranial haemorrhage. Risk rises in patients over 65 and with prior peptic ulcer disease.
Management
The combination is acceptable. After starting atorvastatin, check INR at 2 and 4 weeks, then resume routine monitoring. If INR rises above target, reduce warfarin by 10–20%.
Sources
- Lexicomp: Lexicomp Drug Interactions (2024)– Wolters Kluwer Clinical Drug Information, Inc. Lexi-Interact Online, 2024
- Pharmaceutical Press: Stockley's Drug Interactions, 12th edition (2024)– Preston CL (ed.). Stockley's Drug Interactions. 12th ed. London: Pharmaceutical Press; 2024