Letter to the editor: adverse interaction of warfarin and paracetamol: evidence from a post-mortem study

Abstract

Keywords Anticoagulants.Coumarins.Drugtoxicity.Hemorrhage.Safetymanagement.WarfarinDear Sir,We have read with great interest the article entitled“Adverse interaction of warfarin and paracetamol: evidencefrom a post-mortem study” by Launiainen et al. which hasbeen published online in European Journal of ClinicalPharmacology [1]. We would like to comment on thearticle by referring to our study on predisposing preventablefactors among patients admitted to the emergency depart-ment with bleeding due to warfarin usage [2]. Launiainen etal. studied the adverse interaction of warfarin with otherdrugs used concomitantly using data obtained from theFinnish post-mortem toxicology database. In contrast, theaim of our study was to determine the predisposingpreventable factors among patients admitted to the emer-gency department with bleeding while they were onwarfarin therapy. In Launiainen et al.’s study, at least oneinteracting drug was present in 33% of the 328 warfarin-positive cases, with paracetamol being the most prevalent(49%, n=53); the presence of a non-steroidal anti-inflammatory drug (NSAID) in combination with warfarinwas rare, with only six cases identified. In our study, themean number (± standard deviation) of drugs being usedother than warfarin was 4.8±2.5 (range 0–14, median 4drugs). In total, 112 patients (98.2%) were using at leastone drug in addition to warfarin at the time of theiradmission to the emergency department with bleeding. Themean number of drugs being used that are known to affectwarfarin metabolism was 1.7±1.3 (range 0–6, median 1.5drugs). Of the 112 patients, 95 (83.3%) were using at leastone drug known to affect warfarin metabolism at the timeof admission: 48 (42.1%) were using aspirin, 28 (24.6%)were using NSAIDs other than aspirin, and 18 (15.8%)were using paracetamol in combination with warfarin.Thirty-five of the patients (30.7%) were using theseanalgesics without a physician's prescription (in six cases,aspirin; in 12 cases, paracetamol; in 12 cases, NSAIDs). Inour study, another noteworthy finding was that only ninepatients of the 114 admitted to the emergency departmentwith bleeding due to warfarin use were aware of thepossibility that warfarin could interact with other drugs.Two possible explanations for the percentage differences inparacetamol or NSAID use with warfarin in these twostudies may be the use of different data sources (post-mortem toxicology database versus emergency departmentadmissions with bleeding due to warfarin usage) and socialfactors affecting the analgesic preferences in two countries.Both studies indicate that the use of analgesics (whichadversely affects warfarin metabolism) concomitantly withwarfarin is quite high. Most of the patients were able toeasily gain access to analgesics without a prescription. We