Avoid excessive alcohol intake while taking metformin. Together, alcohol and metformin may increase the risk of lactic acidosis and hypoglycemia. Follow diet, medication, and exercise routines closely. Changing any of these things can affect blood sugar levels. Tell your doctor or other health care provider that you are taking this medication if you need to have surgery or x-ray procedures that require injection of contrast agents. Treatment with metformin may need to be stopped for a short period of time. Do not take any over-the-counter cough, cold, allergy, pain, or weight-loss medications without first talking to your doctor. Some conditions may increase the risk of developing lactic acidosis while taking metformin. Tell your doctor that you are taking metformin if you become ill; if you have a heart attack; have a stroke; develop congestive heart failure; experience diarrhea, vomiting, fever, or dehydration from any cause; decrease in the amount of food or liquid in your normal diet, or develop other health conditions. You may need to stop treatment with metformin for a short amount of time until you are feeling better. Cimetidine (Tagamet), by decreasing the elimination of metformin from the body, can increase the amount of metformin in the blood by 40%. This may increase the frequency of side effects from metformin. There are no adequate studies in pregnant women. Most experts agree that insulin is the best treatment for pregnant women with diabetes. Metformin is excreted into breast milk and can, therefore, be transferred to the nursing infant. Nursing mothers should not use metformin.

The most common side effects with metformin are nausea, vomiting, gas, bloating, diarrhea and loss of appetite. These symptoms occur in one out of every three patients. These side effects may be severe enough to cause the therapy to be discontinued in one out of every 20 patients. These side effects are related to the dose of the medication and may decrease if the dose is reduced. A serious-though rare-side effect of metformin is lactic acidosis. Lactic acidosis occurs in one out of every 30,000 patients and is fatal in 50% of cases. The symptoms of lactic acidosis are weakness, trouble breathing, abnormal heartbeats, unusual muscle pain, stomach discomfort, light-headedness and feeling cold. Patients at risk for lactic acidosis include those with the reduced function of the kidneys or liver, congestive heart failure, severe acute illnesses, and dehydration.

Stop taking metformin and seek emergency medical attention if you experience an allergic reaction (difficulty in breathing; closing of the throat; swelling of the lips, tongue, or face; or hives). A small number of people who have taken metformin have developed a serious condition, called lactic acidosis that has been fatal in up to 50% of cases. Lactic acidosis has occurred most often in people whose kidneys were not working properly. Liver problems may also increase the risk of developing lactic acidosis. Stop taking metformin and call your doctor immediately if you experience a feeling of general discomfort or sickness; weakness; sore or aching muscles; trouble breathing, unusual drowsiness, dizziness or lightheadedness; unusual or unexplained stomach upset (after the initial stomach upset that may occur at the start of therapy with metformin); or the sudden development of a slow or irregular heartbeat. These may be signs of lactic acidosis. Metformin does not usually cause hypoglycemia (low blood sugar). Nevertheless, hypoglycemia may occur in the treatment of diabetes, as a result of skipped meals, excessive exercise, or alcohol consumption. Know the signs and symptoms of low blood sugar, which include hunger, headache, drowsiness, weakness, dizziness, a fast heartbeat, sweating, tremor, and nausea. Carry a non-dietetic candy or glucose tablets to treat episodes of low blood sugar. Other, less serious side effects may be more likely to occur. Continue to take metformin and talk to your doctor if you experience nausea, vomiting, abdominal pain, or diarrhea at the start of therapy; abdominal bloating or increased gas production; or decreased appetite or changes in taste (metallic taste in your mouth). Side effects other than those listed here may also occur. Talk to your doctor about any side effect that seems unusual or that is especially bothersome.