Which outcome is most likely when a patient on warfarin starts Metronidazole?

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Study for the Foundation Year Pharmacy – Clinical Practice Test. Prepare with detailed questions, step-by-step explanations, and test format insights. Enhance your readiness and confidence!

Multiple Choice

Which outcome is most likely when a patient on warfarin starts Metronidazole?

Explanation:
When warfarin is taken with metronidazole, the antibiotic can slow the liver’s breakdown of warfarin by inhibiting the cytochrome P450 enzyme system, especially CYP2C9 which metabolizes the more potent S-warfarin. With reduced clearance, warfarin plasma levels rise, the INR increases, and the anticoagulant effect becomes stronger. This makes bleeding risk higher, so the most likely outcome is an increased anticoagulant effect. Clinically, monitor the INR more closely after starting metronidazole, and be prepared to adjust the warfarin dose accordingly. Advise the patient on signs of bleeding and to report them promptly.

When warfarin is taken with metronidazole, the antibiotic can slow the liver’s breakdown of warfarin by inhibiting the cytochrome P450 enzyme system, especially CYP2C9 which metabolizes the more potent S-warfarin. With reduced clearance, warfarin plasma levels rise, the INR increases, and the anticoagulant effect becomes stronger. This makes bleeding risk higher, so the most likely outcome is an increased anticoagulant effect. Clinically, monitor the INR more closely after starting metronidazole, and be prepared to adjust the warfarin dose accordingly. Advise the patient on signs of bleeding and to report them promptly.

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