Interaction between warfarin and sulphamethoxazole.

A patient is described in whom there was strong circumstantial evidence of an interaction between warfarin and co-trimaxazole. To test the validity of this hypothesis a series of in vitro and in vivo human studies were untaken. The results indicate that interaction may occur in which the sulphamethoxazole moiety of co-trimoxazole displaces warfarin from its binding sites on plasma albumin. Nevertheless, this interaction occurs only at high plasma levels of warfarin and is most likely to be clinically significant only in patients receiving high doses of warfarin who have low plasma albumin concentrations. These studies indicate that the use of co-trimoxazole is not contra-indicated in patients receiving warfarin therapy, but care should be exercised, especially in patients with high plasma warfarin:albumin ratios.