Are brand‐name and generic warfarin interchangeable?

Although Health Canada approved generic warfarins in Year 2000, there is still controversy over the interchangeability of these products with brand‐name Coumadin. Since a large population of patients require warfarin and it has a narrow margin of efficacy and safety, it is vital to ensure that generic brands can be safely interchanged with Coumadin. Multiple N‐of‐1 randomized double blind crossover trials switched outpatients between generic warfarin and Coumadin over a period of thirty weeks. Study patients were on each drug for 5 × 3‐week periods with International Normalized Ratio (INR) measurements twice per period. Outcomes included mean INR results and proportion of dosage adjustments that were required with each brand. Inter‐patient and intra‐patient differences between generic warfarin and Coumadin were compared. Study patient results were also compared with those of a retrospective control group, maintained on Coumadin only. The 6 patients (96% power to detect a difference of greater than 0.5 INR units between warfarin brands) showed no significant difference between warfarin brands in terms of mean INR results (p = 0.485) and proportion of dosage adjustments required with each brand (p = 0.264). Study patients experienced less inter‐ and intra‐subject INR variation than the historical control group, though the difference was not statistically significant. Patients can safely and effectively switch between generic warfarin and Coumadin. Clinical Pharmacology & Therapeutics (2004) 75, P27–P27; doi: 10.1016/j.clpt.2003.11.103