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Biguanides (Metformin)

Reviewed by: HU Medical Review Board | Last reviewed: August 2023

Biguanides are a class of drugs used to treat people with type 2 diabetes. These drugs work by reducing the amount of glucose produced by the liver. This helps lower blood sugar.1,2

Only 1 biguanide is currently used to treat type 2 diabetes: metformin. Metformin is often the first treatment for type 2 diabetes because of its effectiveness, safety, and low cost.1,2

How do biguanides work?

In people with diabetes, 2 problems affect how the body uses sugar (glucose). First, the pancreas does not produce enough insulin (a hormone that regulates sugar processing). Second, the cells do not respond well to insulin and take in less sugar. This often leads to high blood sugar (hyperglycemia).1

Biguanides work by reducing the amount of glucose produced by your liver. This improves how sensitive your body is to insulin. Your body can then use insulin better. Biguanides also seem to reduce food intake and body weight, which may help reduce blood sugar. Some data suggests that biguanides lower the risk of certain heart problems and cancers.2,3


The only biguanide currently prescribed to treat type 2 diabetes is metformin. It is sold under several brand names, including:4

  • Fortamet®
  • Glumetza®
  • Glucophage®
  • Glucophage XR®
  • Riomet®
  • Riomet ER™

What are the possible side effects?

Side effects of metformin can vary by person but are usually mild and temporary. The risk of side effects is lower when metformin is taken with food. The most common side effects are:2,4,5

  • Metallic taste in the mouth
  • Nausea
  • Stomach discomfort
  • Diarrhea
  • Bloating

Tell your doctor if you experience these side effects. Your doctor can pause treatment or reduce your dose to prevent side effects.

Metformin also reduces how well your body absorbs vitamin B12. Some people develop a vitamin B12 deficiency and may need to take supplements.1,2

Metformin carries a boxed warning, the strictest warning from the US Food and Drug Administration (FDA). It has this warning because it may cause a serious condition called lactic acidosis, a build-up of lactic acid in your tissues.3,6

The FDA warns that metformin may not be a good choice for:3

  • People with kidney disease
  • People age 65 or older who have had a heart attack, stroke, diabetic ketoacidosis, coma, heart disease, or liver disease

Tell your doctor if you are taking drugs from the following list, since these drugs may increase the risk of lactic acidosis when taken with metformin:3

  • Acetazolamide (Diamox®)
  • Dichlorphenamide (Keveyis®)
  • Methazolamide
  • Topiramate (Topamax®, in Qsymia®)
  • Zonisamide (Zonegran®)

These are not all the possible side effects of metformin. Talk to your doctor about what to expect when taking metformin. You also should call your doctor if you have any changes that concern you when taking metformin.

Other things to know

Metformin is usually the first drug recommended to people with type 2 diabetes. It is often combined with diet and exercise right after diagnosis. This is because, compared to many other options, metformin is:2,3

  • More effective
  • Safer
  • Cheaper

Even if your body responds well to metformin at first, high blood sugar may return later. If this happens, your doctor may suggest adding another drug.

Take metformin exactly as your doctor prescribes. It comes as either a liquid or tablet to take by mouth. Your doctor may start you with a low dose and slowly increase your dose every 1 to 2 weeks.3

Your doctor may perform certain kidney and liver tests before starting treatment. They may then monitor kidney and liver function every 6 to 12 months. Before you start metformin, tell your doctor your full medical history, including:2,3,5

  • Any other medical conditions, especially problems with your:
    • Kidneys
    • Heart
    • Liver
  • Alcohol use
  • Any recent infections or upcoming surgeries
  • Any other drugs you take, including:
    • Vitamins
    • Supplements
    • Over-the-counter drugs
    • Any allergies to ingredients in metformin

For more information, read more about metformin.3