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Metformin: Important Things Every Diabetic Should Know

This year marks 65 years since the first clinical use of metformin to treat diabetes — one of the OG (original) diabetes drugs. 

Today, it’s a first-line treatment for people with prediabetes and type 2 diabetes and has promise in treating and preventing a host of other medical conditions. And for good reason: The oral drug lowers blood sugar (AKA blood glucose) in those who have these conditions.

What is metformin?

Metformin belongs to a class of drugs called biguanides. Some brand names include Fortamet, Glucophage, Glumetza and Riomet. It comes in an immediate-release tablet, oral solution and extended-release tablet and helps people ages 10 years old and older.

If your provider has recently prescribed you this oral medication, we share everything you need to know and break down misconceptions.

How does metformin work?

We mentioned that metformin lowers blood sugar, but how does it exactly do this?

Metformin doesn’t increase insulin levels in the body but instead lowers the amount of sugar the body produces and absorbs.

“It works through the liver to decrease liver glucose production,” said Rong Guo, MD, Ph.D., an endocrinologist at Banner Health in Arizona. “As it lowers glucose production, it also works through the gut to decrease the amount of glucose the body absorbs from the foods we eat and works through muscle and fat tissue to lower blood sugar by increasing the body’s insulin sensitivity.”

What is metformin used for?

Metformin, in combination with diet and exercise, can lead to lower blood glucose levels, improved hemoglobin A1C levels and weight loss, as well as cardiovascular benefits.

Besides treating diabetes, metformin is used off-label – meaning it isn’t approved by the U.S. Food and Drug Administration (FDA), but is still a recognized use of the drug – to treat several health conditions, like prediabetes, gestational diabetes and polycystic ovary syndrome (PCOS).

“Metformin usually does not cause low blood sugars in diabetic patients or those without the disease,” Dr. Guo said. “It has been prescribed for years to help with menstrual regulation, fertility and high blood sugar levels.”

In addition, researchers are looking into the potential use of metformin to lower the risk of cancer such as colon and breast cancer, dementia and stroke and to prevent other age-related diseases.

Are there any side effects with metformin?

Metformin is considered a safe, cheap and effective medication worldwide, but it can cause some minor side effects in some people. Many of these mild symptoms are associated with taking the medicine for the first time.

Common side effects of metformin include diarrhea, nausea, upset stomach, stomach pain, dizziness, muscle pain, headache and taste disturbance. However, these side effects generally go away after you take the medication for a while.

“Taking metformin with meals can help reduce the side effects,” Dr. Guo said. “We also gradually increase your dose of metformin by adding 500 mg weekly to avoid the side effects. Changing metformin to metformin extended release can help reduce the side effects as well.”

In rare cases, it can cause metformin associated lactic acidosis, a condition caused by a buildup of lactic acid in the blood, and can increase the risk of hypoglycemia, or low blood sugar. Lactic acid is a very rare side effect of metformin. However, your risk is higher if you have advanced renal failure or very severe heart failure. In these two conditions, metformin should not be used.

Importantly, taking metformin on its own doesn’t significantly raise your risk of low blood sugar.

“Drinking alcohol can severely increase the risk for lactic acidosis in patients on metformin,” Dr. Guo said. “I can’t highlight enough that if you’re taking metformin, you should avoid excessive alcohol intake.”

What should I know if I’m taking other medications?

Adding certain medications to the mix can raise the risk of medication interactions. Talk to your doctor or pharmacist about what medications you’re taking before starting on metformin.

[Also read “Medication Safety Tips for Older Adults with Diabetes.”]

“When metformin is taken in combination with other anti-diabetic medications including insulin or sulfonylureas, such as glipizide, glimepiride or glyburide, it can sometimes cause low blood sugar,” Dr. Guo said. “In this case, the other anti-diabetic medication dosages need to be adjusted.”

Some medications including ranolazine (Ranexa), vandetanib, dolutegravir, and cimetidine (Tagamet HB) could keep metformin around in your body longer, which can raise your risk of lactic acid buildup.

“Also talk to your doctor if you’re taking acetazolamide (Diamox), dichlorphenamide (Keveyis), methazolamide, topiramate (Topamax) or zonisamide (Zonegran),” Dr. Guo said. “When metformin is used with these medications, it might increase the risk for lactic acidosis. This may require additional monitoring.”

Who shouldn’t take metformin?

If you have any of the following, metformin should be avoided.

“If you can’t tolerate or are unable to take metformin, other anti-diabetic medications including glucagon-like peptide 1 receptor agonist (GLP-1R agonist) and sodium-glucose cotransporter-2 inhibitors (SGLT2i) are good choices,” Dr. Guo said. “These medications can lower blood sugars, decrease body weight and protect heart and kidneys. They have been widely used in treating type 2 diabetes.”

Can metformin damage the kidneys?

One misconception is that metformin can cause kidney damage, but this isn’t true.

“The kidneys process and clear the drug out of your system via urine,” Dr. Guo said. “However, in those with severe renal impairment, this medication isn’t recommended.”

Talk to your provider if you have questions regarding your kidney function.

Does metformin cause cancer?

This is another misconception. Metformin itself isn’t known to cause cancer. In fact, as mentioned earlier, it might actually help prevent it.

“Long-term studies in animals showed no evidence of carcinogens in their bodies,” Dr. Guo said. “On the contrary, some human studies showed metformin has beneficial effects in various cancers including gastric cancer, pancreatic cancer, liver cancer, uterine cancer, prostate cancer, colon cancer and thyroid cancer by reducing the growth of cancer cells.”

This misconception about metformin may have sprouted from some extended-release (ER) metformin products that were recalled in 2020. A potentially harmful chemical called N-nitroso dimethylamine (NDMA) was found in some metformin ER products.

While NDMA is commonly found in foods and drinking water at low levels, NDMA may raise your risk of cancer if you take a large amount for a long time. This is true for any product with high levels of NDMA.

“It’s important to note that NDMA hasn’t been found in any immediate-release metformin tablets,” Dr. Guo said.

You can check if your metformin has been recalled here.

Tips for taking metformin

  • Take metformin and metformin extended-release (ER) with meals. Check out these tips on planning your meals.
  • Start at a low dose once daily with the evening meal before increasing to twice a day. Dosages may vary from person to person.
  • Do not drink large amounts of alcohol while taking metformin because it can increase your risk of lactic acidosis.
  • Talk to your provider if you need to have radiological testing or an upcoming surgery or procedure. 
  • Monitor blood sugars and other regulatory lab tests including kidney function as needed.
  • Let your provider know before taking any other medications to avoid potential drug interactions.

Bottom line

Metformin is a safe, effective medication to treat type 2 diabetes, but it can have other health benefits besides helping with blood sugar control. It can be used to treat prediabetes and PCOS and may aid in weight loss and help prevent some types of cancer.

If you experience side effects or have questions or concerns about metformin, call your doctor. To find a health care specialist near you, visit

For more information and resources about living with diabetes, visit our education and support page.

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