Warfarin/cough lozenges interaction
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Subtherapeutic INR in an elderly patient: case report A 67-year-old man, whose INR was stable on warfarin, began to use cough lozenges; his INR fell below therapeutic levels. The man began receiving warfarin after discovery of atrial fibrillation and a right occipital infarction on 19 May 2011. At discharge his INR was stable at 2.7. At a visit 5 days later, in June 2011, he continued on warfarin at 4 mg/day [route not stated]. His INR remained within the target range of 2–3 over two further weekly visits. On the next visit, 14 days after his previous visit, his INR was 1.7. In the week before this visit, he had begun taking cough lozenges containing eucalyptus oil, menthol and liquorice [Fisherman’s Friend] for a dry cough, at a rate of about 4–6 per day. He did not mention the lozenges at this stage, and continued taking them. He was instructed to take an extra 7mg warfarin dose, then continue on 4 mg/day. Two weeks later, his INR was still low; his dosage was increased to 4mg Mon–Fri and 5mg on weekend days. After a further 2 weeks his INR was 1.6, and his dosage was raised to 4mg and 5mg on alternate days. At about this time, he stopped taking the lozenges, which he had been using for about 1 month. One week later, on August 8th, his INR was 2.3; it remained stable. Author comment: "[T]he sequence of events reported here would be classified as being a ’probable’ drug interaction on the Naranjo scoring system for likelihood of drug interactions." Boyle S, et al. Clinically significant interaction between warfarin and popular cough lozenges ’Fisherman’s Friend’. BMJ Case Reports : 28 Oct 2011. Available from: URL: http://dx.doi.org/10.1136/bcr.09.2011.4791 - United 803065799 Kingdom
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Reactions 21 Jan 2012 No. 1385
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