Warfarin and a New Antibiotic in an Elderly Patient
A 78-year-old woman (weight 52 kg, eGFR 48 mL/min/1.73 m2) has been stable on warfarin 4 mg daily for two years, with INRs between 2.2 and 2.9. She presents to her GP with a productive cough and fever. The GP plans to prescribe a five-day course of an antibiotic and asks you, the intern pharmacist, whether anything needs adjusting. Her current medicines are warfarin, amlodipine 5 mg, atorvastatin 20 mg and omeprazole 20 mg. She reports no bleeding, no recent falls, and no changes to diet. Her last INR was 2.6 three weeks ago. The GP is considering either amoxicillin or trimethoprim-sulfamethoxazole and wants your recommendation today.
1. Compare the interaction risk of the two antibiotic options and recommend one, justifying your choice.
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Trimethoprim-sulfamethoxazole is the higher-risk choice. Both trimethoprim and sulfamethoxazole inhibit CYP2C9, the main enzyme metabolising S-warfarin, and trimethoprim additionally displaces warfarin from albumin and can impair vitamin K synthesis by altering gut flora. The combined effect is a well-documented rise in INR, often within 3-5 days, with bleeding reports in elderly patients. Amoxicillin has a much weaker interaction: it may modestly disturb gut flora and has isolated case reports of raised INR, but it does not inhibit CYP2C9 meaningfully. Given her age, low body weight and reduced renal function, the safer choice is amoxicillin at a standard dose, provided she has no penicillin allergy. If trimethoprim-sulfamethoxazole were clinically essential (for example, culture-proven resistance), you would need to reduce the warfarin dose pre-emptively and check INR within 3 days, but that is not justified here.
2. Outline the monitoring plan and the advice you would give the patient, including red-flag symptoms.
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Check INR within 3-5 days of starting amoxicillin, and again at the end of the course, because the effect on gut flora can be delayed. Do not change the warfarin dose pre-emptively; adjust only if the INR drifts outside her target range of 2.0-3.0. Advise her to watch for bruising, nosebleeds, gum bleeding, dark or bloody stools, blood in urine, unusual headache, and any fall or head injury, and to seek urgent review if these occur. Reinforce that she should not start any over-the-counter NSAID or aspirin-containing cold remedy, since these add bleeding risk and interact with warfarin. Ask her to keep her diet consistent, especially green vegetables, and to report any new medicine, including herbal products, before taking it. Document the interaction in her record and flag it for the next prescriber.
3. The GP asks whether her omeprazole should be reviewed at the same time. What is your response?
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Omeprazole is a moderate CYP2C19 inhibitor and has a modest, well-recognised potential to raise INR with warfarin, though her INR has been stable on the combination for two years, so the risk is currently controlled. There is no need to stop it urgently. However, if she has no ongoing indication for gastroprotection, it would be reasonable to review at a later, non-urgent visit, since long-term PPI use in older adults is associated with fracture, hypomagnesaemia and C. difficile risk. Do not make two changes at once: changing omeprazole at the same time as starting an antibiotic would make any INR change impossible to attribute. Keep omeprazole unchanged now, monitor the INR, and revisit the PPI at a separate review.