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Treatment with vitamin K antagonists (coumarins) is associated with an increased risk of bleeding. In order to elucidate the bleeding risk of users of antiplatelet drugs among users of cou-marins,we assessed the odds ratio of major bleeding associated with use of antiplatelet drugs in users of the coumarins aceno-coumarol and phenprocoumon.We used data from a Dutch rec-ord linkage system, including pharmacy and linked hospitaliz-ation records for approximately two million subjects,to conduct a nested case control study in a cohort of new users of couma-rins.Cases were patients who were hospitalized with a primary diagnosis of major bleeding while taking coumarin and were matched with up to four control subjects. Conditional logistic regression analysis was used to determine ORs and 95 % confi-Keywords Drug interaction, bleeding, coumarins, antiplatelet drug