Metformin can cause short-term side effects like stomach discomfort. The most serious of these is lactic acidosis, a condition caused by buildup of lactic acid in the blood. This can occur if too much metformin accumulates in the blood due to chronic or acute (e.g. dehydration) kidney problems. Generally, though, metformin is considered to be a safe and effective treatment for type 2 diabetes and prediabetes.