Metformin is used to help treat type 2 diabetes. Mild side effects, such as nausea, vomiting, and diarrhea, are common and go away with time. But, some serious side effects may require medical attention.

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

Type 2 diabetes is the result of long-term insulin resistance. This means that your body produces insulin but does not use it efficiently. Insulin resistance can cause blood sugar (glucose) levels to rise to a potentially unsafe level. Over time, high blood sugar can cause complications such as kidney disease, nerve damage, and heart disease.

Metformin doesn’t cure diabetes. Instead, it helps lower your blood sugar levels into a safe range.

Doctors may also prescribe metformin off-label to treat polycystic ovary syndrome (PCOS).

Metformin can cause side effects ranging from mild to serious. 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 to get medical attention.

Recall of metformin extended release

In 2021, one brand recalled two lots of metformin extended-release tablets from the U.S. market, as reported by the Food and Drug Administration (FDA).

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, contact a healthcare professional. They will advise you on whether you should continue to take your medication or whether you need a new prescription.

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

Common side effects of metformin include:

Nausea, vomiting, and diarrhea are common when people start taking metformin, but they usually go away with time.

You can reduce the chances of side effects by taking metformin with a meal. To help reduce your risk of severe diarrhea, a doctor may start you on a low dosage of metformin and increase it slowly.

Doctors may also prescribe metformin to people with PCOS. The drug may help:

Lowering insulin levels can, in turn, improve PCOS symptoms such as irregular cycles and acne. Metformin is used off-label for this purpose. The side effects are the same as for other uses.

Off-label drug use

Off-label drug use means that a drug the FDA has approved for one purpose is being used for a different purpose that the FDA has not yet approved.

A doctor can still use the drug for another purpose because, while the FDA regulates the testing and approval of drugs, it does not regulate the ways doctors use drugs to treat their patients. So, your doctor can prescribe the drug they think is best for your care.

Metformin can potentially cause some serious side effects.

Lactic acidosis

Although it is rare, metformin’s most serious side effect is lactic acidosis. Metformin has a boxed warning — also called a black box warning — about this risk. A boxed warning is the most severe warning the FDA issues.

Lactic acidosis is a rare but serious problem resulting from a buildup of metformin in your body, which causes a pH imbalance. It’s a medical emergency that must be treated immediately in the hospital.

Symptoms can include:

Contact a doctor right away if you have any symptoms of lactic acidosis. If you have trouble breathing, call 911 or your local emergency number or go to the nearest emergency room.

Metformin-associated lactic acidosis has an estimated death rate of up to 50%.

Taking some other medications, including corticosteroids and blood pressure medications, with metformin may increase your risk of lactic acidosis. See the risk factors section for more information about factors that raise your risk of this complication.


Metformin can decrease the levels of vitamin B12 in your body. In rare cases, this can cause anemia (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 having very low vitamin B12 levels.

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.

Your vitamin B12 levels may improve if you stop taking metformin or take vitamin B12 supplements. Make sure to talk with the doctor who prescribed metformin before you stop taking it.

Stopping a medication

Always talk with a doctor before stopping any prescribed medication to make sure it is safe to do so. They may gradually lower your dose or prescribe a different medication.

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

Call your doctor if you have any symptoms of hypoglycemia, such as:

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

How to prevent hypoglycemia

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According to the United Kingdom’s National Health Service, metformin is usually safe to take while pregnant or nursing, either by itself or with insulin.

Metformin crosses the placenta but has not been linked to increased rates of fetal development issues or complications.

A 2022 study found no long-term negative effects of metformin use during pregnancy. The authors noted that metformin use may result in a fetus being small for its gestational age and recommended caution if there is a risk that a fetus will not get adequate nutrition.

The authors also noted that metformin use in females with PCOS is associated with a reduced risk of negative outcomes.

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

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 your chances of developing side effects. A typical starting dose is 500 milligrams.
  • Taking metformin with a meal: Taking metformin with a meal can help reduce your chances of developing an upset stomach or digestive discomfort.
  • Taking extended-release metformin: You can talk with a doctor to determine whether 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 extended-release metformin to ensure that you get a safe version.
  • Taking pills whole: You should not crush pills. Doing so can mean that your body will absorb them more quickly.

If you develop uncomfortable side effects, contact your prescribing doctor. They may recommend changing your dosage, particularly during times of stress.

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

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

Kidney problems

Your kidneys remove metformin from your body. If your kidneys are not working as they should, you’ll have higher metformin levels in your system. This raises your risk of lactic acidosis.

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

If you have severe kidney problems or are age 80 or older, metformin may not be right for you. A 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 such as 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 in people who have already experienced it. However, researchers found these benefits did not occur in people without diabetes.

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, which increases your risk of lactic acidosis. Metformin also raises your risk, so taking it is dangerous if you have liver problems.

Alcohol use

Drinking alcohol while taking metformin increases 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 a doctor about how much alcohol is safe for you to consume while you’re taking metformin.

For more information, read about the dangers of drinking with metformin 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, increasing your risk of lactic acidosis. Talk with your doctor about the specific time when you should stop taking metformin.

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

What exactly does metformin do to your body?

Metformin helps lower blood sugar levels by decreasing the amount of glucose produced by the liver and increasing the sensitivity of muscle cells to insulin, allowing them to take up more glucose from the blood.

What is the most serious side effect of metformin?

The most serious side effect of metformin is lactic acidosis, a rare but potentially life threatening condition characterized by the buildup of lactic acid in the bloodstream.

What should you avoid while taking metformin?

Metformin may interact with other medications, including those that help manage blood pressure, seizures, heartburn, and cholesterol.

Talk with your doctor about all the medications you’re taking before you begin taking metformin.

While taking metformin, you should avoid excessive alcohol consumption, as it can increase the risk of lactic acidosis.

Why do doctors no longer recommend metformin?

Doctors still recommend metformin as a first-line treatment for type 2 diabetes due to its effectiveness, safety profile, and low cost.

However, in some cases, doctors may consider other medications if metformin is not well-tolerated or if there are specific contraindications, such as kidney impairment.

If you’ve received a prescription for metformin and you’re 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 may have, such as:

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

A doctor can answer your questions and work with you to manage any side effects you experience.