Biguanides (Metformin)

Reviewed by: HU Medical Review Board | Last reviewed: May 2022 | Last updated: October 2022

Biguanides are a class of drugs used to treat people with type 2 diabetes (T2D). They work by reducing the amount of glucose produced by your liver. This helps lower your blood sugar.

Only 1 biguanide is currently used to treat T2D: metformin. Metformin is often the first treatment for T2D because of its effectiveness, safety, and low cost. Before taking metformin, tell your doctor about your full medical history. Ask them about what side effects to expect.

How do biguanides work?

In people with T2D, 2 problems affect how the body uses sugar (glucose). First, the pancreas does not produce enough insulin (a hormone that regulates sugar movement). Second, your 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 suggest that biguanides lower the risk of certain heart problems and cancers.2-4

Examples of biguanides

The only biguanide prescribed to treat T2D is currently metformin. It is sold under several brand names, including:5

  • Fortamet®
  • Glumetza®
  • Glucophage®
  • Glucophage® XR
  • Riomet®
  • Riomet ER™
  • D-Care DM2®

What are some possible side effects?

Side effects of metformin can vary, but are usually mild and temporary. The risk of side effects is lower when metformin is taken with food. The most common side effects of metformin are:2,5,6

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

Metformin also reduces how well your body absorbs vitamin B12. So some people experience vitamin B12 deficiency and may need to take supplements. In very rare cases, metformin can cause lactic acidosis (too much lactic acid in the body).1,2

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

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 therapy recommended to people with T2D. 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

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. This includes:2,3,6

  • Any other medical conditions, especially problems with your:
    • Kidneys
    • Heart
    • Liver
  • Your level of 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 the drug

Even if you have a positive response to metformin at first, high blood sugar may return later. If this happens, your doctor may suggest adding another drug.

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