The potential for interaction between warfarin and coprescribed medication: a retrospective study in primary care.

BACKGROUND AND OBJECTIVE

Warfarin is commonly involved in drug-related hospital admissions. The drug is most commonly prescribed to elderly patients in whom polypharmacy is common and, when administered in combination with other drugs such as NSAIDs, aspirin (acetylsalicylic acid) or macrolide antibacterials, is associated with increased bleeding risk. The aim of this study was to investigate the prevalence of prescriptions that might give rise to clinically relevant drug-drug interactions in a warfarinized population.

METHODS

Primary care prescribing of warfarin and potentially interacting medicines was assessed between 1 April 2005 and 31 March 2006 using computerized prescribing data retrieved from 321 primary care practices in Scotland.

RESULTS

A total of 17,861 registered patients were prescribed warfarin in the study year, of whom 68% (n=12,107) were concomitantly issued with a prescription for at least one potentially interacting medicine. For short-term use, 'one-off' prescriptions for antibacterials (sulfonamides, ciprofloxacin, and macrolides), nonselective NSAIDs and antithrombotics (fibrinolytics) were the most frequently prescribed drug groups with potential for interaction, being prescribed to 12.7%, 5.3%, and 1.4% of warfarinized patients, respectively. Macrolide antibacterials were prescribed in significantly fewer warfaranized patients than standardized population (3.84% vs 4.41%; p<0.001). For long-term use, nonselective NSAIDs and antithrombotics were the most frequently prescribed drug groups, being prescribed to 21.0% and 21.1% of warfarinized patients, respectively. When compared with a standardized population, NSAIDs and antithrombotics were prescribed to a significantly smaller proportion of the warfarin population, whereas selective cyclo-oxygenase 2 inhibitors were prescribed to a significantly greater proportion. Nevertheless for the whole warfarinized population 26.3% were prescribed nonselective NSAIDs and 22.5% antithrombotic agents.

CONCLUSIONS

The lower 'one off' use of macrolide antibacterials and long-term use of antithrombotics and NSAIDs in warfarinized patients observed in this study might suggest awareness among general practitioners of the increased risks for bleeding associated with concomitant use of these agents with warfarin therapy. However, despite this, the majority of warfarinized patients in this study were issued with a repeat prescription for at least one potentially interacting medicine.