engleski

The incidence of potential clinically significant drug interactions of warfarin in elderly patients

Background and objective: Warfarin is known for its interactions with many drugs. Elderly patients are particularly sensitive to warfarin interactions. To evaluate the incidence of potential drug interactions when prescribing new drugs to elderly patients on warfarin, a prospective observational study was conducted. Setting and method: Patients on warfarin older than 65 years were included and monitored for 6 months in 4 community pharmacies in Croatia. Data regarding new prescribed drugs was obtained from pharmacy records at the moment of dispensing or by patient selfreporting. The potential interacting drugs were identified using the Lexicomp Lexi- Interact Online software. Only the clinically significant (levels C, D, X of clinical significance as classified by Lexicomp Lexi- Interact Online) interactions were included in this analysis. Main outcome measures: Number of new proscribed drugs, level of interaction with warfarin, mechanism of interactions. Results: We included 157 elderly patients with an average age of73 years. In the follow-up period, new drugs were prescribed to 54 patients (34.4%). There were 79 prescriptions of new drugs and 57 (72.2%) of those were drugs with a clinically significant interaction with warfarin. There were 39 prescriptions of drugs with level C of interaction (68.4%), and 18 (31.6%) with level D. There were no drug interactions of level X. In the group with level C the most prescribed drugs were antibiotics with 26 prescriptions: amoxicillin/clavulanate 28%, clindamycin 8%, ciprofloxacin 8%, norfloxacin 8%, azithromycin 5%, cefuroxime 5%, clarithromycin 3%, doxycycline 3%. The remaining 13 prescriptions included tramadol with paracetamol 18%, rosuvastatin 5%, simvastatin 3%, fluvastatin 3%, levothyroxine 3% and torasemide 3%.The dominant mechanism of the potential interactions was pharmacokinetic. In the group with level D the most prescribed drugs were nonsteroidal anti- inflammatory drugs with 12 prescriptions— diclofenac 35%, ibuprofen 23%, indomethacin 12%. Among other drugs, 6 prescriptions were antibiotic sulfamethoxazole with trimethoprim 12%, fenofibrate 6%, miconazole 6%, and fluconazole 6%.The dominant mechanism of the potential interactions waspharmacodynamic. Conclusion: Pharmacists should actively monitor prescribing of new drugs to elderly patients on warfarin in order to reduce the risk of clinically significant drug interactions.

drug interactions, warfarin

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