Moran et al. [1] recently surveyed 181 patients who were receiving warfarin therapy with regard to their knowledge of the potential drug interactions, adverse effects and immediate actions in the event of haemorrhage during warfarin treatment. We were particularly interested to note that almost 47.9 % of patients surveyed were unaware of potential interactions between warfarin and other medications and that only 31.8 % of patients were aware that antibiotics could interact with warfarin. These results highlight our recent experience with four patients who were taking concomitant topical miconazole and warfarin, all of whom developed dramatically elevated INR levels. Miconazole is an imidazole anti-fungal medication with actions against a broad variety of fungal species and is widely available in pharmacies as an oral gel without prescription [2]. Although clinically significant interactions between topical miconazole and warfarin have previously been reported, we believe that this potential interaction is not widely recognised by clinicians, pharmacists and patients [3]. Warfarin is mainly metabolised via cytochrome P-450 hepatic microsomal enzyme CYP2C9, and thus drugs that inhibit the actions of this enzyme may lead to supratherapeutic INR levels in patients taking warfarin. Importantly, miconazole, even in its topical form, is known to be a highly potent inhibitor of this enzyme [4–6]. The four cases described below were all attending the oral anticoagulation clinic (OAC) at Beaumont Hospital which has approximately 2,400 patients under review at the present time. Importantly, all the four patients had stable INRs prior to taking topical miconazole (patient details are summarised in Table 1). Amongst the four cases, the indications for warfarin were atrial fibrillation (AF, n = 2), venous thromboembolism (VTE, n = 1) and arterial thromboembolism (ATE, n = 1) with a target INR of 2.5 in all cases. The duration of combined warfarin and topical miconazole ranged from 3 to 7 days and in only one case was accompanied by other new medications (Chlarithromycin, and mycostatin, case four, Table 1). Despite INR values that were broadly within range before commencing miconazole, the INR levels increased rapidly in all the four cases, reaching levels of [10 in 3/4 cases. Minor bleeding (haematuria, case four) complicated one case, but resolved with intravenous vitamin K. Two of the other patients received oral vitamin K and there were no other bleeding complications. Our case series is characterised by the dramatic and rapid increase in INR values following the commencement of miconazole, a feature which has been highlighted in a number of publications to date. In 2002, Devaraj reported the case of an 80-year-old man whose INR rose to 21.4 following concomitant warfarin and topical miconazole treatment [7]. More recently, Kovac et al. [8] reported on 32 warfarinised patients who also received miconazole oral gel. Bleeding occurred in 15/32 patients (7 minor, 8 major), and in keeping with our experience the mean INR increased from 2.44 to 8.8 with concomitant warfarin and miconazole. We believe this interaction to be particularly important because miconazole gel can be purchased in pharmacies without prescription, highlighting the importance of educating patients, their carers and all health care C. C. Ufondu (&) P. Ferrins J. Quinn Departments of Haematology, Beaumont Hospital, Dublin 9, Beaumont, Ireland e-mail: ufondunonso@yahoo.com