A man on long-term warfarin with a stable INR is prescribed clarithromycin for a chest infection. What is the expected effect?
Answer and explanation
Correct answer: D. A rise in INR with a higher bleeding risk
- Step 1: Many antibiotics, azoles and alcohol potentiate warfarin. Step 2. Clarithromycin inhibits warfarin metabolism and may disturb gut flora producing vitamin K, so the INR rises. Answer: A rise in INR with a higher bleeding risk. Why not the others: A fall in INR is expected with enzyme inducers such as rifampicin. A change is expected rather than none. Absorption of clarithromycin is not the concern.
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