Metformin

Key facts

• Metformin (sold as Glucophage and in many generics) is a biguanide. It lowers blood sugar in type 2 diabetes by cutting the liver's glucose output and helping muscles use insulin better, without pushing the pancreas to release more insulin.
• You take it with food, usually once or twice a day, or once daily for the extended-release form. Blood sugar improves gradually over one to two weeks, not overnight.
• Stop metformin before and after any scan that uses iodinated contrast dye, and pause it if you become seriously ill, dehydrated, or unable to eat or drink normally. Continuing it through these situations can trigger lactic acidosis, a rare but life-threatening buildup of lactic acid in the blood.
• Seek urgent care for unusual muscle pain, unexplained breathlessness, or extreme tiredness together with a fast or irregular heartbeat.

What metformin treats

Metformin is the usual first choice for type 2 diabetes, where the body no longer responds properly to its own insulin. It's sometimes prescribed for prediabetes to slow progression to full diabetes, and off-label for polycystic ovary syndrome (PCOS) to improve insulin resistance. It does not treat type 1 diabetes, where the pancreas makes no insulin at all.

How metformin works

Metformin reduces the amount of glucose the liver releases into the blood and helps muscle cells take up glucose more readily. It also slows glucose absorption in the gut. Because it does not force the pancreas to make more insulin, it carries a low risk of hypoglycemia when used on its own.

Before you take it

• Do not take metformin if you have severe kidney impairment. Your kidneys clear the drug, and reduced clearance lets it build up and raises the risk of lactic acidosis.
• Tell your prescriber about liver disease, unstable heart failure, or heavy alcohol use. Each of these adds to the same risk.
• Your kidney function is normally checked before you start and at least once a year afterward, more often if you're older or your kidneys are borderline.
• Tell any doctor, dentist, or radiology department that you take metformin before surgery or a scan involving contrast dye.

Side effects

Common early effects settle within a few weeks: nausea, diarrhea, a metallic taste, bloating, and reduced appetite. Extended-release tablets tend to cause fewer stomach effects and should never be crushed or chewed.

Stop and seek urgent medical care for any of these:

• Unusual muscle pain, cramping, or weakness.
• Trouble breathing or unusually fast breathing.
• Extreme tiredness, stomach pain, or a slow or irregular heartbeat.
• Severe or persistent vomiting, or signs of dehydration.

Safety essentials

• Pause metformin before scheduled contrast-dye imaging and during any illness that brings vomiting, diarrhea, poor fluid intake, or fever. Restart only once you're eating and drinking normally and your kidney function is confirmed stable. This is the single most important rule with metformin.
• Long-term use can lower vitamin B12; your doctor may check levels periodically, especially if you notice tingling or numbness in your hands or feet.
• Limit alcohol, since heavy drinking compounds the lactic acidosis risk.

This page is educational and does not replace advice from a doctor or pharmacist who knows your health history.