What are the side effects of metformin?

Donna Hill Howes, RN
Family Practitioner

Side effects may include vomiting, diarrhea, gas, indigestion, and abdominal bloating or cramping. Other side effects may include headaches, a low vitamin B12 level, and low blood sugar. Lactic acidosis, a serious condition involving excess lactic acid in the blood, may occur. This is more common in people with kidney disease. 

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Metformin is prescribed for people with type 2 diabetes. Like any medication, it carries the risk of side effects. The most common side effects from metformin include nausea, diarrhea, gas and upset stomach. These are most likely when you first start taking it and usually go away on their own. Until they do, you should try to take your metformin with a meal. You can also try reducing the amount you take for a few days and gradually increasing it until you’ve reached the amount your doctor has prescribed for you.

Lactic acidosis is a very rare—but very serious—side effect. It happens most often in people with liver, kidney or respiratory diseases. Call your doctor right away if you have any of these symptoms of lactic acidosis: weakness, stomach pain or discomfort, fast and shallow breathing, sleepiness and muscle cramping.

Although metformin doesn’t cause hypoglycemia by itself, if combined with other medications, vigorous exercise or too little food, it can make your blood glucose drop too low.

Since low blood glucose can be dangerous, make sure that you and your family know the symptoms. These include feeling shaky, sweaty, hungry, and irritable. If you have these symptoms, take some quick-acting sugar. Good sources are three or four glucose tablets, a half-cup of fruit juice or regular soda, or a tablespoon of honey or sugar.

You should call your doctor if you have side effects that don’t go away or if you have any symptoms of lactic acidosis.

Diarrhea, abdominal cramps, and nausea and vomiting are common side effects of metformin. When starting metformin, using a lower starting dose and gradually increasing the dosage may help to minimize these gastrointestinal symptoms. Usually these side effects will go away as your body becomes used to metformin. Taking metformin with food may also help with these gastrointestinal symptoms.

Dr. Audrey K. Chun, MD
Geriatric Medicine Specialist

Metformin (prescribed to treat type 2 diabetes) can cause excessive gas and bloating, heartburn, headaches, a cough, muscle pain and a metallic taste in the mouth, but these side effects typically ease after a few weeks. Very rarely, metformin may cause a serious condition called lactic acidosis. Key signs include weakness, trouble breathing, abnormal heartbeat, unusual muscle pain, stomach discomfort, lightheadedness and feeling cold. You're more at risk if you have reduced kidney function, worsening congestive heart failure or are dehydrated.

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