Actually, metformin is usually not the original cause of kidney problems. However, metformin is eliminated by the kidneys and when a patient has poor kidney function, the metformin can build up in the blood and cause a rare but serious condition called lactic acidosis. Lactic acidosis affects the chemistry balance of your blood and can lead to kidney failure and other organ failure. The risk of lactic acidosis is very low and most often occurs in patients with poor kidney function - so for most patients, the benefits of metformin outweigh the risks of treatment. Most doctors will regularly perform kidney function tests to make sure the kidney is working well in patients who are taking metformin. With that said, if you are taking metformin, contact your doctor immediately if you experience unexplained weakness, muscle pain, difficulty breathing, or increased drowsiness - these can be early signs of lactic acidosis. Also, if you are taking metformin and going to receive a radiocontrast dye study or have surgery, tell your doctors that you are taking metformin - in most cases, your doctor will instruct you to temporarily stop taking metformin during these procedures to help decrease the risk of lactic acidosis.