A study of the interaction of omeprazole and warfarin in anticoagulated patients.

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Abstract
1. Thirty‐five patients on continuous therapy with warfarin were given omeprazole 20 mg once daily and placebo each for 3 weeks according to a two‐centre randomised double‐blind cross‐over design. 2. Blood samples were obtained once weekly during the run‐in and follow‐up periods as well as during the first 2 weeks of each treatment period, and twice during the last week of each treatment period. Plasma concentrations of R‐ and S‐warfarin were measured by h.p.l.c. and the anticoagulant effect was assessed using the Trombotest. 3. Twenty‐eight patients were evaluated. The mean plasma concentration of R‐warfarin was increased by 9.5% during omeprazole treatment compared with placebo, while that of S‐ warfarin, the more active isomer, was unaffected. The coagulation time was not significantly changed (106 s during omeprazole and 98 s during placebo). Corresponding TT‐values (Trombotest) were 8.8 and 9.9 (NS).