The Drug Interaction That Requires a Call
A 68-year-old patient with atrial fibrillation presents a new prescription for warfarin 5 mg daily, written by a cardiologist. The patient's medication profile shows the patient has been taking fluconazole 150 mg weekly for a fungal infection for the past 3 weeks, plus omeprazole and metoprolol. The patient's most recent INR was 2.4, drawn 2 weeks ago. The patient reports no bleeding but says she has been feeling more tired than usual. The pharmacist notices the warfarin prescription is new and the fluconazole was started 3 weeks ago.
1. What is the interaction concern, and what should the pharmacist do before dispensing?
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Fluconazole is a potent inhibitor of CYP2C9 and CYP3A4 and can significantly increase warfarin's anticoagulant effect, raising the risk of bleeding. The patient has been on fluconazole for 3 weeks, and the new warfarin prescription means the interaction has not yet been managed. The pharmacist should not dispense without addressing the interaction. The pharmacist should contact the prescriber to discuss the interaction, recommend an INR check and possible warfarin dose adjustment, and document the conversation. The pharmacist should also assess the patient's symptoms (fatigue) and ask about bleeding signs.
2. The prescriber says to dispense as written and will check an INR in a week. What should the pharmacist do?
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The pharmacist should document the prescriber's decision and the recommendation made. The pharmacist may dispense, but should counsel the patient on bleeding precautions, signs of bleeding (bruising, gum bleeding, dark stools, blood in urine, unusual fatigue), and the importance of the INR check. The pharmacist should also recommend an earlier INR if possible, given the interaction. The pharmacist should schedule a follow-up call and document the plan. The pharmacist should not refuse solely because the prescriber disagrees, but should ensure the patient is informed and monitored.
3. What counseling and monitoring should the pharmacist provide at dispensing?
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The pharmacist should counsel on: warfarin administration at the same time each day; the interaction with fluconazole and the need for INR monitoring; avoiding alcohol and interacting supplements (especially vitamin K–rich foods and NSAIDs); recognizing bleeding signs; and the importance of reporting fatigue, bruising, or dark stools. The pharmacist should verify the patient's understanding, provide written information, and document the counseling. The pharmacist should also check whether the patient is due for an INR and coordinate with the prescriber.