Phenformin is a biguanide hypoglycemic agent with actions and uses similar to those of metformin. It activates AMP-activated protein kinase (AMPK) and inhibits mTORC1 signaling. Phenformin used for the treatment of diabetes. Phenformin was removed from the U.S. market 20 years ago because of a high incidence of lactic acidosis. Risk factors for the development of lactic acidosis include renal deficiency, hepatic disease, cardiac disease, and drug interaction such as cimetidine. Phenformin exerts potential anti-neoplastic action.