THE administration of the pyrazolone derivatives, phenylbutazone and oxyphenbutazone, to a patient already being treated with coumarin anticoagulant drugs can lead to serious hemorrhagic complications.1 2 3 4 5 6 The means whereby these drugs potentiate the prothrombinopenic effect of the coumarins is not clear. It is not caused by impaired liver function or defective vitamin K metabolism since these drugs do not alter blood coagulation when given alone.7 , 8 In 1956 Sigg et al.1 reported a decrease in the disappearance rate of ethylbiscoumacetate in 1 of their patients when phenylbutazone was given. They attributed this to delayed renal excretion of the coumarin drug. Weiner and . . .
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Aggeler et al. (1967) studied this question.
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