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Metformin is a well-studied, generic drug, used by millions of people around the world.

How does metformin work?

Metformin lowers the amount of glucose the body (mainly the liver) produces and absorbs to help people manage blood sugar levels. The drug lowers glucose production in a person’s liver and thereby lowers the amount of sugar in the blood by increasing the body’s sensitivity to insulin (this allows cells to take up glucose from the blood). It also decreases the amount of glucose that our bodies absorb from the foods we eat – meaning that less glucose makes it into our blood stream to begin with.

Who uses metformin?

Metformin is the most common glucose-lowering drug used by people with type 2 diabetes. For most people, metformin works to bring down blood sugar gradually when combined with weight loss, nutritious food and physical activity.

Healthcare professionals may prescribe metformin off-label for people with prediabetes, especially people at high risk of developing diabetes, based on data from the Diabetes Prevention Program (DPP). The American Diabetes Association recommends metformin for people under the age of 60, with a BMI greater than 35, or with a history of gestational diabetes – and especially for people whose rising glucose levels do not respond to major lifestyle changes.

Metformin vs. extended-release (ER) metformin:

Both forms of metformin are the same medication, but they are taken in different ways. Standard metformin is taken two or three times per day. It is recommended that people take it with meals to reduce the stomach and bowel side effects that can occur – most people take metformin with breakfast and dinner. Extended-release metformin is taken once a day and should be taken at night, with dinner. This can help to treat high glucose levels overnight.

Commonly used metformin drugs:

  • Glumetza
  • Fortamet
  • Riomet
  • Glucophage (normal or extended release)

What are the benefits?

  • Metformin is inexpensive and available as a generic drug.
  • It is an oral medication (pill), so no injections are needed.
  • The drug may decrease insulin resistance.
  • It does not cause weight gain and may cause weight loss.
  • There’s a low risk of hypoglycemia unless used together with sulfonylureas or insulin.
  • It’s been widely used for over 50 years.
  • Some evidence suggests that metformin may reduce the risk of heart disease.

What are the side effects?

Like most prescription drugs, metformin can cause side effects in some people, but many of these are mild. These side effects are also associated with taking the medication for the first time.

The most common side effects among people starting on metformin include:

  • Stomach pain
  • Gas
  • Bloating
  • Diarrhea

These problems usually go away after some time or can be prevented by starting at a lower dosage of metformin and then gradually increasing the dosage as the drug is tolerated. Tracking your diet and taking metformin with a meal can help reduce these side effects.

Rare side effects of metformin:

  • Lactic acidosis – a severe condition caused by buildup of lactic acid in the blood
  • Increased risk of hypoglycemia (low blood sugar) – particularly for those who take insulin and other drugs that increase insulin secretion (such as sulfonylureas), but also when combined with excessive alcohol intake.
  • Vitamin B12 deficiency can lead to anemia – some people taking metformin need to take a vitamin B12 supplement

Who should not use metformin?

  • People with more severe kidney damage as metformin accumulates in the blood and may then cause lactic acidosis (see rare side effects of metformin)
  • For the same reason mentioned above, metformin use should also be stopped for certain major surgeries or acute serious illness. 
  • Consult with your healthcare provider to see if metformin is right for you.

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Last updated: August 2, 2021