Interaction of allopurinol with phenprocoumon in man.

Conditions in two patients on long-term phenprocoumon (Marcumar®) treatment are reported who had signs of phenprocoumon overdosage when given simultaneously allopurinol. The determination of phenprocoumon plasma concentrations in one patient showed that phenprocoumon accumulates for several weeks during treatment with allopurinol. Signs of phenprocoumon overdosage thus can appear long time after starting allopurinol treatment.