Metformin is primarily used to treat type 2 diabetes by bringing blood sugar to a healthy range, although it doesn’t cure the disease. Most side effects are mild, though some can require medical attention.

Recall of metformin extended release

In May 2020, the Food and Drug Administration (FDA) recommended that some makers of metformin extended release remove some of their tablets from the U.S. market. This is because an unacceptable level of a probable carcinogen (cancer-causing agent) was found in some extended-release metformin tablets. If you currently take this drug, call a healthcare professional. They will advise whether you should continue to take your medication or if you need a new prescription.

Metformin is a prescription drug used to treat type 2 diabetes. It belongs to a class of medications called biguanides.

People with type 2 diabetes have blood sugar (glucose) levels that rise higher than normal. Metformin doesn’t cure diabetes. Instead, it helps lower your blood sugar levels to a safe range.

Metformin is also often prescribed to treat polycystic ovary syndrome (PCOS), but the FDA hasn’t approved it.

Metformin can cause mild and serious side effects. Most side effects are mild and primarily affect your digestive system. Severe side effects such as lactic acidosis are less common but require prompt medical attention.

Here’s what you need to know about metformin side effects and when you should call your doctor.

Metformin causes some common side effects. These can occur when you first start taking metformin but usually go away over time. Tell your doctor if these symptoms are severe or cause a problem.

The most common side effects of metformin include:

Nausea, vomiting, and diarrhea are some of the most common side effects people have when they first start taking metformin. These problems usually go away over time.

You can reduce these effects by taking metformin with a meal. Also, to help lessen your risk of severe diarrhea, your doctor will likely start you on a low dosage of metformin and then increase it slowly.

Metformin is sometimes used to improve insulin sensitivity, reduce insulin resistance, and help regulate ovarian function in people with PCOS. Reducing insulin levels can, in turn, help improve PCOS symptoms such as irregular cycles and acne. It’s used off-label for this purpose. The side effects of this use are the same as those for other uses.

Off-label drug use

Off-label drug use means a drug approved by the FDA for one purpose is used for a different purpose that hasn’t yet been approved.

However, a doctor can still use the drug for that purpose. This is because the FDA regulates the testing and approval of drugs but not how doctors use drugs to treat their patients. So your doctor can prescribe a drug however they think is best for your care.

Metformin can cause some serious side effects.

Lactic acidosis

Although very rare, metformin’s most serious side effect is lactic acidosis. Metformin has a “boxed” — also referred to as a “black box” — warning about this risk. A boxed warning is the most severe warning that the Food and Drug Administration (FDA) issues.

Lactic acidosis is a rare but serious problem that can occur due to a buildup of metformin in your body. This buildup causes a pH imbalance in your body. It’s a medical emergency that must be treated immediately in the hospital.

The death rate of metformin-associated lactic acidosis is between 30% and 50%, but it’s only reported ten times per every 100,000 patient years. For example, if 200,000 people took metformin for one year each, there would be about 20 cases expected.

See the precautions section for more information about factors that raise your risk of lactic acidosis.

Call your doctor right away if you have any of the following symptoms of lactic acidosis. If you have trouble breathing, call 911 or emergency services or go to the nearest emergency room.


Metformin can decrease the levels of vitamin B12 in your body. In rare cases, this can cause anemia or low levels of red blood cells. If you don’t get much vitamin B12 or calcium through your diet, you may be at higher risk of very low vitamin B12 levels.

Your vitamin B12 levels can improve if you stop taking metformin or take vitamin B12 supplements. Make sure to talk with your doctor before you stop taking metformin.

The more common symptoms of anemia include:

  • tiredness
  • dizziness
  • lightheadedness

If you think you may have anemia, make an appointment with your doctor to have your red blood cell levels checked.


Metformin doesn’t usually cause hypoglycemia or low blood sugar. However, in rare cases, you may develop hypoglycemia if you combine metformin with:

  • a poor diet
  • strenuous exercise
  • excessive alcohol intake
  • other diabetes medications

Call your doctor if you have any symptoms of hypoglycemia, which can include:

  • weakness
  • tiredness
  • nausea
  • vomiting
  • stomach pain
  • dizziness
  • lightheadedness
  • abnormally fast or slow heartbeat

How to prevent hypoglycemia

  • Take your medications on schedule.
  • Follow a well-balanced diet.
  • Exercise as directed by your doctor.
  • Tell your doctor about all other medications you take.
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According to the National Health Service, metformin is usually safe to take by itself or with insulin while pregnant or nursing.

Metformin crosses the placenta but hasn’t been linked to increased rates of fetal development issues or complications.

A 2018 study found no significant difference between the rate of serious adverse events in pregnant women who either took a placebo or metformin. Mild side effects such as nausea, vomiting, and diarrhea were reported more often in women who took metformin.

A 2016 review of studies found evidence that metformin may reduce the rate of miscarriage in women with PCOS.

Most of the common side effects of metformin involve your digestive system. You can minimize your chances of developing side effects by:

  • Starting with a low dose: It’s best to start at a low dose and work up over time to reduce the chances of developing side effects. A typical starting dose is 500 milligrams (mg).
  • Taking metformin with a meal: Taking metformin with a meal can help reduce your chances of developing an upset stomach or gastric discomfort.
  • Taking extended-release metformin: You can talk with a doctor to see if extended-release metformin might be right for you. This type of metformin releases slowly over time and typically has milder side effects. Be sure to discuss the FDA recall of certain brands of metformin extended release to ensure you get a safe version.
  • Taking pills whole: You shouldn’t crush up pills. Doing so can speed up the rate that they’re absorbed.

If you develop uncomfortable side effects, it’s a good idea to contact your doctor. They may recommend changing the dosage of the type of metformin you take. You may need to change your dosage, particularly during stress.

It’s also a good idea to avoid drinking heavily when taking metformin because it can increase your chances of developing lactic acidosis.

Several factors raise your risk of lactic acidosis while taking metformin. If any of these factors affect you, discuss them with your doctor before taking this drug.

Kidney problems

Your kidneys remove metformin from your body. If your kidneys don’t work well, you’ll have higher levels of metformin in your system. This raises your risk of lactic acidosis.

If you have mild or moderate kidney problems, your doctor may start you at a lower dosage of metformin.

If you have severe kidney problems or are age 80 or older, metformin may not be right for you. Your doctor will likely test your kidney function before you take metformin and then again each year.

Heart problems

If you have diabetes, you are at an increased risk of heart disease. That’s because high blood sugar can eventually damage your blood vessels. Therefore managing your diabetes by taking medications like metformin may help lower your risk of heart problems.

Studies suggest that metformin may reduce the risk of heart-related death and events among people with type 2 diabetes. It may also lower the risk of death from and reoccurrence of heart failure for those who’ve already experienced it. These benefits are not seen in non-diabetics, however.

Liver problems

You should not take metformin if you have severe liver problems. Your liver clears lactic acid from your body.

Severe liver problems could lead to a buildup of lactic acid. Lactic acid buildup raises your risk of lactic acidosis. Metformin also raises your risk, so taking it if you have liver problems is dangerous.

Alcohol use

Drinking alcohol while taking metformin raises your risk of hypoglycemia. It also raises your risk of lactic acidosis because it increases lactic acid levels in your body.

You should not drink large amounts of alcohol while taking metformin. If you drink alcohol, talk with your doctor about how much alcohol is safe for you while you take metformin.

For more information, read about the dangers of drinking with metformin use and how alcohol affects diabetes.

Surgical or radiologic procedures

If you plan to have surgery or a radiology procedure that uses iodine contrast, you’ll need to stop taking metformin before the procedure. These procedures can slow the removal of metformin from your body, raising your risk of lactic acidosis. Talk with your doctor about the specific timing of when to stop taking metformin.

Your doctor can also let you know when it’s safe to resume taking metformin after the procedure. You’ll typically get the go-ahead when your kidney function tests have returned to normal.

If your doctor has prescribed metformin and is concerned about its side effects, talk with your doctor. You may want to review this article with them. Be sure to ask any questions you have, such as:

  • What side effects should I watch out for?
  • Am I at high risk of lactic acidosis?
  • Is there another medication I could take that might cause fewer side effects?

Your doctor can answer your questions and work with you to manage any side effects you may have.