Phenformin interaction

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

Antidiabetic drugs No expected interaction No expected interaction with chlorpropamide, insulin, phenformin, tolazamide, or tolbutamide No expected interaction with glibenclamide (in vitro study) No expected interaction... 

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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