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Summary and conclusionsTo assess the effects of oral contraceptives on anticoagulant treatment the prothrombin times of 12 patients were measured while they were taking both drugs simultaneously and while they were taking only anticoagulants. The mean prothrombin time ratio was significantly higher when patients were taking both drugs than when they were taking only anticoagulants and their doses of anticoagulant were significantly lower. During both periods most of the prothrombin values remained in the therapeutic range.These findings suggest that, contrary to the common belief that oral contraceptives diminish the effects of anticoagulants, contraceptives in fact potentiate the action of the anticoagulants. Patients and methodsAmong the 197 patients being followed in our anticoagulation control unit we found 12 women who were simultaneously taking an oral anticoagulant (nicoumalone) and an oestroprogestogenic oral contraceptive. In one case the contraceptive was a parenteral depot preparation, while the remaining 11 patients used oral preparations (see table). The patients were aged 27 to 44 years (mean 34-5), and none suffered from nephropathy, hepatopathy, or hypertensive disease.The indication for anticoagulation was the presence of a BjorkShiley valvular prosthesis in nine patients (five mitral, one aortic, three mitral and aortic), and the existence of embolic mitral valve disease in three patients.Anticoagulation control was carried out by measuring prothrombin activity according to Quick's method. Commercially obtained thromboplastin was used, but the reference values were from British comparative thromboplastin (BCT), and adequate correlations were established.2 The therapeutic range for commercial thromboplastin ranged generally from 1 3 to 1-8, corresponding to a prothrombin time ratio of 2 to 3 with BCT.The patients were followed for a total of 374 patient-months (mean

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