Metformin very rarely causes a dangerous lactic acidosis (roughly 1 case in every 10,000 patient years, mortality rate about 40%). Most cases affect individuals with risk factors, especially impaired renal functions. Other biguanides cause lactic acidoses (e.g buformin) more often. The risk of a lactic acidosis under metformin is no greater than the risk of a severe hypoglycemia under sulfonylureas.

Metformin quite frequently (5 to 20%) causes gastrointestinal problems (nausea, vomiting, abdominal pain, diarrhea) and malabsorption of vitamin B₁₂ and folic acid. Skin rashes are rare. The platelet inhibition hardly has any clinical disadvantages.

Metformin: Interactions

Alcohol can reinforce the tendency of hyperlactatemia.

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