Phenformin Warfarin

Genetically determined defects in the phase I oxidative metabolism of debrisoquin, phenacetin, guanoxan, sparteine, phenformin, warfarin, and others have been reported (Table 4-4). The defects are apparently transmitted as autosomal recessive traits and may be expressed at any one of the multiple metabolic transformations that a chemical might undergo. 

Potentiation of the anticoagulant action of warfarin by phenformin has been reported (151). 

Hamblin TJ. Interaction between warfarin and phenformin. Lancet 1971 2(7737) 1323. 

One patient taking warfarin developed haematuria with a therapeutic prothrombin time three months after starting phenformin. Another patient developed metformin-induced lactic acidosis after warfarin-induced bleeding caused renal obstruction. However, these interactions are isolated and seem unlikely to be of general importance. 

The information about a biguanide interaction with warfarin appears to be limited to these isolated reports, neither of which definitively suggest that the biguanide altered anticoagulant effects. In general no interaction would be expected between metformin or phenformin and warfarin. 

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