Biguanides (Metformin) for Prediabetes and Type 2 Diabetes

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Biguanides (Metformin) for Prediabetes and Type 2 Diabetes


Generic NameBrand Name
long-acting metforminGlucophage XR, Glumetza, Fortamet
liquid metforminRiomet

Metformin is also available as a combination pill, combined with the sulfonylurea medicine glyburide (Glucovance), glipizide (Metaglip), or glimepiride (Duetact); the thiazolidinedione medicine pioglitazone (Actoplus Met) or rosiglitazone (Avandamet), the DPP-4 inhibitor medicine sitagliptin (Janumet), and the meglitinide medicine repaglinide (PrandiMet).

How It Works

Biguanides lower blood sugar by:

  • Decreasing the amount of sugar produced by the liver.
  • Increasing the amount of sugar absorbed by muscle cells.
  • Decreasing the body's need for insulin.

Metformin does not cause the pancreas to produce more insulin. It should not cause low blood sugar (hypoglycemia) or weight gain, unless it is taken in combination with medicines that do. Some people may lose weight when starting this medicine.

Why It Is Used

These medicines are used to treat insulin resistance common to people with prediabetes, type 2 diabetes, and polycystic ovarian syndrome. It has also been studied for use in cancer prevention and treatment, but more research is needed.

How Well It Works

Type 2 diabetes is a disease that can get worse over time, so medicines may need to change.

Diabetes medicines work best for people who are being active and eating healthy foods. Studies have suggested that metformin lowers hemoglobin A1c by 1% to 2%.1

Side Effects

All medicines have side effects. But many people don't feel the side effects, or they are able to deal with them. Ask your pharmacist about the side effects of each medicine you take. Side effects are also listed in the information that comes with your medicine.

Here are some important things to think about:

  • Usually the benefits of the medicine are more important than any minor side effects.
  • Side effects may go away after you take the medicine for a while.
  • If side effects still bother you and you wonder if you should keep taking the medicine, call your doctor. He or she may be able to lower your dose or change your medicine. Do not suddenly quit taking your medicine unless your doctor tells you to.

Call 911 or other emergency services right away if you have:

  • Trouble breathing.
  • Swelling of your face, lips, tongue, or throat.

Call your doctor right away if you have:

  • Hives.
  • Symptoms of lactic acidosis, such as rapid breathing, excessive sweating, cool and clammy skin, sweet-smelling breath, belly pain, nausea or vomiting, and/or confusion.

Common side effects of this medicine include:

  • Temporary nausea and/or diarrhea.
  • Loss of appetite.
  • Increased abdominal gas.
  • A metallic taste.

See Drug Reference for a full list of side effects. (Drug Reference is not available in all systems.)

What To Think About

When a person begins taking metformin, the dosage usually is increased gradually to prevent side effects. You may also reduce nausea by taking the medicine with food.

Over time, blood levels of vitamin B12 can decrease in some people who take metformin. If you have been taking metformin for more than a few years, check with your doctor about getting a vitamin B12 test.

Lactic acidosis may occur in people who have kidney or liver failure, have low levels of oxygen in their blood (hypoxia), abuse alcohol, have a severe infection, or are dehydrated. It can also result if metformin is taken when a person has surgery or X-ray studies that use a dye. Be sure all your doctors know that you are taking this medicine if you need a test that involves the use of a dye or if you are having surgery. You may have to stop taking metformin temporarily.

Women who have stopped menstruating before they start taking metformin may begin menstruating again and may become pregnant.

Taking medicine

Medicine is one of the many tools your doctor has to treat a health problem. Taking medicine as your doctor suggests will improve your health and may prevent future problems. If you don't take your medicines properly, you may be putting your health (and perhaps your life) at risk.

There are many reasons why people have trouble taking their medicine. But in most cases, there is something you can do. For suggestions on how to work around common problems, see the topic Taking Medicines as Prescribed.

Advice for women

If you are pregnant, breast-feeding, or planning to get pregnant, do not use any medicines unless your doctor tells you to. Some medicines can harm your baby. This includes prescription and over-the-counter medicines, vitamins, herbs, and supplements. And make sure that all your doctors know that you are pregnant, breast-feeding, or planning to get pregnant.


Follow-up care is a key part of your treatment and safety. Be sure to make and go to all appointments, and call your doctor if you are having problems. It's also a good idea to know your test results and keep a list of the medicines you take.

Complete the new medication information form (PDF)(What is a PDF document?) to help you understand this medication.



  1. American Diabetes Association (2009). Medical management of hyperglycemia in type 2 diabetes: A consensus algorithm for the initiation and adjustment of therapy. Diabetes Care, 32: 193–203.


ByHealthwise Staff
Primary Medical ReviewerE. Gregory Thompson, MD - Internal Medicine
Specialist Medical ReviewerJennifer Hone, MD - Endocrinology, Diabetes and Metabolism
Last RevisedMay 2, 2012

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