A small number of people who have taken metformin have developed a serious condition called lactic acidosis. Lactic acidosis is caused by a buildup of lactic acid in the blood. This happens more often in people with kidney problems. Most people with kidney problems should not take metformin. The risk of lactic acidosis from metformin can be reduce by having regular evaluations by your doctor who will collect blood work to make sure your kidneys are working properly.