Warfarin, Ciprofloxacin and a Well-Meant Antacid
Mrs X, 68, takes warfarin 5 mg daily for atrial fibrillation (target INR 2.0-3.0) and attends the pharmacy with a prescription for ciprofloxacin 500 mg twice daily for 5 days for dysuria. Her INR today is 3.9; she has no bleeding, no bruising and no fall. She also buys aluminium hydroxide antacid tablets to take at bedtime for 'gas', and asks for ibuprofen for low back pain. She lives alone, cooks for herself, and her daughter usually reminds her about tablets by phone. She has never had a bleed but was hospitalised once for a high INR after a course of antibiotics. She asks, 'Can I just take everything together at night so I don't forget?'
1. Rank the three interaction concerns and justify which one you would act on first.
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Most urgent: ciprofloxacin plus warfarin. Fluoroquinolones inhibit warfarin metabolism and can raise INR within days; her INR is already 3.9, and she has had a high-INR admission after antibiotics, so she is a proven responder. Action: arrange an INR check in 2-3 days, consider holding 1-2 doses or a small dose reduction with the prescriber, and warn her about bleeding.
Second: ibuprofen plus warfarin. This is a pharmacodynamic interaction - platelet inhibition plus gastric mucosal injury - in a 68-year-old living alone. Avoid it; paracetamol is the analgesic of choice at the lowest effective dose.
Third: antacid plus ciprofloxacin. Aluminium and magnesium ions chelate fluoroquinolones and cut absorption, risking treatment failure for the UTI - serious but not immediately dangerous.
2. INR 3.9 with no bleeding - do you recommend vitamin K? What do you advise and what follow-up?
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No vitamin K. An INR of 3.9 without bleeding is above target but below the range (roughly 4.5-10) at which oral vitamin K is usually considered, and over-correction in atrial fibrillation risks a stroke.
Withhold or reduce the next one or two warfarin doses as agreed with the prescriber, look for bleeding (bruising, gums, stool colour, urine), check for new drugs, alcohol or diet changes, and repeat the INR in 2-3 days. Give clear return advice: black stools, blood in urine, severe headache, prolonged nosebleed or a fall mean immediate hospital attendance.
The antibiotic course should be completed, and the UTI reviewed if symptoms persist.
3. Give the counselling plan, including exact timing of the antacid and what to do if she cannot manage complex timing.
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Practical plan: ciprofloxacin at, say, 8 am and 8 pm; the antacid at least 2 hours after the morning dose and never at the same time, because the aluminium binds the antibiotic and both are wasted. Ibuprofen is out; offer paracetamol for back pain.
Use a weekly pill organiser and ask the daughter to supervise by phone, or ask the prescriber whether the antacid can be replaced with a preparation that does not chelate ciprofloxacin. Warn that ciprofloxacin can cause tendon pain and that any new bleeding needs a same-day INR. Emphasise no alcohol binges, consistent green vegetables, and no new over-the-counter medicines without asking.
If she cannot manage the timing, ask the prescriber about an alternative antibiotic.