[The effect of sulfinpyrazone on the coagulation-inhibiting action of acenocoumarol]

To investigate the possible clinical interaction between the platelet function regulator sulphinpyrazone (SP) and the oral anticoagulant acenocoumarol (AC), 22 in-patients of either sex were included in a single blind within-patient trial vs. placebo. After one week of stabilizing treatment with AC alone the patients were randomly allocated to two sequences (11 patients each), either SP + AC for weeks 2--3 followed by placebo + AC for weeks 4--5, or the reverse sequence, i.e. placebo + AC followed by SP + AC for the same periods. 17 patients completed the full five weeks. Four dropped out during SP treatment, all but one following some form of bleeding episode. One patient dropped out for the same reason during placebo. Apart from the bleeding episodes, no other undesirable effects were recorded. The daily dose of SP was always 800 mg. A statistically highly significant interaction (p less than 0.01) between SP and AC was found. The addition of SP to AC led to a drop in mean prothrombin time and rendered necessary a consequent reduction (of about 20%) in mean AC dosage. It is concluded that when initiating and withdrawing treatment with SP in a patient receiving AC, the prothrombin time should be checked daily for a few days to adapt (reduce) the dosage of AC to the change in prothrombin time induced by SP.