What to Know About Biguanides (Metformin)

An Oral Medication Used to Treat Diabetes

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Biguanides are a class of medications used to treat type 2 diabetes and other conditions. They work by reducing the production of glucose that occurs during digestion.

Metformin is the only biguanide currently available in most countries for treating diabetes. Glucophage (metformin) and Glucophage XR (metformin extended-release) are well-known brand names for these drugs. Others include Fortamet, Glumetza, and Riomet. Metformin is also available in combination with several other types of diabetes medications, such as sulfonylureas.

Metformin pouring out of a bottle
Francis Dean / Getty Images


Metformin is often prescribed for type 2 diabetes once the disease cannot be managed by lifestyle changes alone. If you've been diagnosed with type 2 diabetes and require medication, metformin is likely to be the first drug you'll take. As diabetes progresses, insulin injections may be needed to control blood sugar, but metformin may be continued to enhance your body's ability to use insulin.

The American Diabetes Association (ADA) guidance on treatment for type 2 diabetes recommends people with certain high-risk factors, including cardiovascular and kidney issues, start taking metformin plus an additional therapy (with proven cardiovascular disease benefit) to help improve outcomes. Your healthcare provider will review your medical history to determine if you fall into this category.

Metformin works by controlling the amount of sugar in your blood. It does not affect how much insulin your body produces, but it increase sensitivity to insulin. This helps your cells take in glucose to use as energy, decreases the production of glucose in your liver, and reduces the concentration of glucose in your bloodstream.

Other Types of Biguanides

Biguanides were first derived from the French lilac, also called goat's rue (Galega officinalis). Some herbal remedies may include this plant. If you are using diabetes medications, tell your healthcare provider about any herbal supplements you're taking to avoid interactions.

Phenformin was introduced in 1957 at the same time as metformin but then withdrawn in the late 1970s because it was associated with a fatal risk of lactic acidosis. Buformin was developed in Germany in 1957 but was never sold in the United States. It also was found to cause an increased risk of lactic acidosis. These forms of biguanides may still be available in some countries.

Other types of biguanides, called proguanil and chlorproguanil, are used as antimalarial drugs.

Off-Label Uses

Metformin may occasionally be prescribed off-label for type 1 diabetes, obesity, and polycystic ovarian syndrome (PCOS). It is also being investigated for potential cancer-fighting and cardioprotective benefits.

Before Taking

In addition to standard testing for diabetes, your healthcare provider will test your kidney function to estimate glomerular filtration rate, or eGFR, before prescribing metformin for the first time.

Precautions and Contraindications

According to ADA treatment guidelines, people with advanced kidney disease should not take metformin. However, for those with mild kidney impairment, and for some people with moderate kidney function, substantial evidence now suggests metformin combined with careful monitoring is beneficial.

If you drink alcohol frequently or tend to drink a lot at one time, share this information with your healthcare provider before starting metformin. Since alcohol significantly lowers blood sugar, it may increase the risk of lactic acidosis as well as risky glucose imbalances.

If you easily become dehydrated during exercise or for other reasons, this is also important for your healthcare provider to know, since dehydration increases the risk of lactic acidosis.

Warning signs of lactic acidosis include stomach pains, nausea, and/or vomiting; unusual muscle pain; sleepiness or fatigue; a slow or irregular heartbeat; and trouble breathing. If you experience any of these symptoms, seek immediate medical attention.

Women who are pregnant are usually advised to stop taking metformin once pregnancy is confirmed due to possible adverse effects of the drug on fetal development.

In late September 2020, eight drug companies voluntarily complied with a request made by the U.S. Food and Drug Administration (FDA) on May 28, 2020 to recall certain metformin products from the market. The FDA previously identified unacceptable levels of N-Nitrosodimethylamine (NDMA) in certain lots.

People taking metformin should continue taking their medication as prescribed until their health professional prescribes an alternative treatment, if applicable. Stopping metformin without a replacement can pose serious health risks to those with type 2 diabetes.


Metformin is taken at doses of 500 milligrams (mg) to 2550 mg a day. Your healthcare provider will start your prescription at a low dose and gradually increase it according to your body's needs. Depending on the form, it is taken once to three times daily.

How to Take and Store

Metformin is an oral medication that can be taken as a tablet or liquid. Instructions must be followed for the safe use of each product. It can safely be stored at room temperature.

Metformin should be taken with meals to prevent side effects. If you forget a dose, take your next dose at the usual time rather than double up. If you accidentally take too much, seek medical attention: It's important to stabilize your blood sugar right away to avoid serious health issues.

Side Effects

Metformin does not cause excessive hypoglycemia, which is an advantage over some other diabetes medications. It also doesn't cause weight gain, and it has benefits for some cardiovascular risk factors. It may even help with weight loss and lowering cholesterol.


Metformin can cause nausea, upset stomach, and diarrhea, particularly when you first start taking it. It should always be taken with food to reduce the risk of gastrointestinal issues.

Over time, metformin may blocdk vitamin B12 absorption in the body. Ask your healthcare provider whether B12 vitamin supplements are right for you.


People with severe kidney impairment or heart failure should not take metformin since in rare cases it can cause lactic acidosis. The risk is very low—around one in 30,000 people taking metformin—but the condition can be fatal.

Warnings and Interactions

While metformin is generally well tolerated and has a good safety profile, if you combine this medication with others such as insulin or sulfonylureas, you'll need to work with your healthcare provider to be especially careful about side effects such as low blood sugar. Report any changes or unusual symptoms to your healthcare provider right away when you're combining metformin with other medications.

4 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. American Diabetes Association Professional Practice Committee. 9. Pharmacologic approaches to glycemic treatment: Standards of medical care in diabetes—2022Diabetes Care. 2022;45(Supplement_1):S125-S143. doi:10.2337/dc22-S009

  2. MedlinePlus. Metformin.

  3. U.S. Food and Drug Administration. FDA alerts patients and health care professionals to nitrosamine impurity findings in certain metformin extended-release products.

  4. Wang YW, He SJ, Feng X, et al. Metformin: a review of its potential indicationsDrug Des Devel Ther. 2017;11:2421-2429.

Additional Reading

By Debra Manzella, RN
Debra Manzella, MS, RN, is a corporate clinical educator at Catholic Health System in New York with extensive experience in diabetes care.