Do not use metformin if you have kidney disease, or if you are in a state of diabetic ketoacidosis (call your doctor for treatment with insulin).
Before taking metformin, tell your doctor if you have liver disease or a history of heart disease.
Some people have developed a life-threatening condition called lactic acidosis while taking metformin. Get emergency medical help if you have any of these symptoms of lactic acidosis: weakness, increasing sleepiness, slow heart rate, cold feeling, muscle pain, shortness of breath, stomach pain, feeling light-headed, and fainting.
If you need to have any type of x-ray or CT scan using a dye that is injected into a vein, you may need to temporarily stop taking metformin. Be sure the doctor knows ahead of time that you are using this medication.
Know the signs of low blood sugar (hypoglycemia) and how to recognize them, including hunger, headache, confusion, irritability, drowsiness, weakness, dizziness, tremors, sweating, fast heartbeat, seizure (convulsions), fainting, or coma (severe hypoglycemia can be fatal). Always keep a source of sugar available in case you have symptoms of low blood sugar.