Metformin (Riomet, Glumetza, Fortamet) is a generic prescription medication used to help manage blood sugar levels. It’s approved for use in adults and children ages 10 years and older with type 2 diabetes. For this purpose, metformin is used in combination with diet and exercise. You’ll typically take metformin long term.
Metformin comes as an immediate-release (IR) oral tablet, extended-release (ER) oral tablet, and oral solution. IR tablets release the medication all at once into your body. ER tablets release the medication slowly into your body over several hours. This article mainly focuses on the IR and ER oral tablets of metformin.
As with other drugs, metformin can cause side effects (adverse effects). Read on to learn about potential common, mild, and serious side effects of metformin. For a general overview of metformin, including details about its use, refer to this article. Your doctor can also tell you more about metformin.
Metformin can cause certain side effects, some of which are more common than others. These side effects may be temporary, lasting a few days to weeks. However, if the side effects last longer than that, bother you, or become severe, be sure to talk with your doctor or pharmacist.
These are just a few of the more common side effects reported by people who took metformin in clinical trials:
- flatulence (gas)
- indigestion
- weakness
- diarrhea*
- nausea and vomiting†
* To learn more about this side effect, see the “Metformin and diarrhea” section below.
† For more information about this side effect, see “Side effect specifics” below.
Mild side effects can occur with metformin use. This list doesn’t include all possible mild side effects of the drug. For more information, you can refer to metformin’s prescribing information.
Mild side effects that have been reported with metformin include:
- headache
- increased sweating
- flatulence (gas)
- indigestion
- weakness
- dizziness
- changes in taste
- diarrhea*
- constipation†
- nausea and vomiting†
- mild allergic reaction†
These side effects may be temporary, lasting a few days to weeks. However, if the side effects last longer than that, bother you, or become severe, be sure to talk with your doctor or pharmacist.
Note: After the Food and Drug Administration (FDA) approves a drug, it tracks side effects of the medication. If you develop a side effect while taking metformin and want to tell the FDA about it, visit MedWatch.
* To learn more about this side effect, see the “Metformin and diarrhea” section below.
† For more information about this side effect, see “Side effect specifics” below.
Metformin may cause serious side effects. The list below may not include all possible serious side effects of the drug. For more information, you can refer to metformin’s prescribing information.
If you develop serious side effects while taking metformin, call your doctor right away. If the side effects seem life threatening or you think you’re having a medical emergency, immediately call 911 or your local emergency number.
Serious side effects that have been reported and their symptoms include:
- Low vitamin B12 levels. Symptoms can include:
- problems with thinking
- constipation*
- Low blood sugar level.*
- Lactic acidosis.†
- Severe allergic reaction.‡
* For more information about this side effect, see “Side effect specifics” below.
† Metformin has a
‡ An allergic reaction is possible after using metformin. However, it’s not clear whether this side effect occurred in clinical trials. For details, see the “Side effect specifics” section below.
Metformin can cause the same side effects in both females* and males* who take the medication. Examples of these side effects are described throughout this article.
However, females may be more likely than males to have side effects when starting treatment with metformin. In a
When you first start metformin treatment, your doctor will typically prescribe a low dose. And then they’ll increase it gradually over time. This can help prevent side effects, especially digestive side effects such as nausea.
Note: If you’re female, haven’t gone through menopause, and have ovulation problems, taking metformin may cause ovulation. (With menopause, you no longer have periods. With ovulation, an egg is released from your ovary.) This could increase your risk of becoming pregnant. Talk with your doctor about whether you should use birth control while taking metformin.
* Sex and gender exist on spectrums. Use of the terms “female” and “male” in this article refers to sex assigned at birth.
Some people may have diarrhea while taking metformin. Diarrhea was the most common side effect reported in clinical trials of metformin.
Diarrhea is more common when starting metformin treatment. It tends to ease as you continue treatment.
To help prevent diarrhea, your doctor will typically prescribe a low dose of metformin when you first start treatment. This allows your body to get used to the medication. If needed, your doctor will increase your dosage gradually over time. Taking your doses with food may also help prevent diarrhea.
If you have diarrhea with metformin, it’s important to drink plenty of fluids to avoid getting dehydrated. Dehydration can increase your risk of a serious side effect called lactic acidosis.* (With lactic acidosis, you have a buildup of lactic acid in your blood. This can be life threatening.)
If you have diarrhea that’s severe, bothersome, or doesn’t ease, talk with your doctor. They may recommend taking medication to treat the diarrhea. They may also reduce your metformin dosage or suggest switching to a different form of the drug. Diarrhea is less common with extended-release (ER) metformin tablets than with immediate-release (IR) tablets.
* Metformin has a
Metformin may cause several side effects. Here are some frequently asked questions about the drug’s side effects and their answers.
Do side effects of metformin vary between 500-mg and 1,000-mg strengths?
Possibly. Immediate-release (IR) and extended-release (ER) forms of metformin both come in 500-milligram (mg) and 1,000-mg strengths. Both strengths can cause the same side effects. However, as with many drugs, side effects tend to be more common with higher dosages of metformin than with lower dosages.
If you’re concerned about the risk of side effects with the dosage of metformin you’ve been prescribed, talk with your doctor.
What are the long-term side effects of metformin treatment?
Long-term side effects of metformin treatment may include anemia (low red blood cell count) and nerve problems.
Metformin can affect the way your body absorbs vitamin B12. As a result, long-term use of metformin can sometimes cause low vitamin B12 levels. Your body needs vitamin B12 to make red blood cells and nerve cells. So, a low level of vitamin B12 could lead to anemia and nerve problems.
Most people take metformin as a long-term treatment. If you take metformin for longer than 4 months, your doctor will usually check your vitamin B12 levels every 2 to 3 years. And they’ll order yearly blood tests to check for anemia.
If you develop a low vitamin B12 level, your doctor may recommend taking a vitamin B12 supplement to help correct this. However, in some cases, your doctor may recommend stopping metformin treatment.
Does metformin cause hair loss?
Hair loss wasn’t reported in clinical trials of metformin. However, long-term use of metformin can cause low vitamin B12 levels. And this can sometimes lead to hair loss.
Hair loss may also be related to high blood sugar levels, which metformin is used to treat. Also, hair loss may be associated with polycystic ovary syndrome (PCOS). PCOS is a hormonal disorder that causes sacs of fluid to develop in the ovaries. Metformin is commonly used off-label to treat PCOS. With off-label use, a drug is prescribed for a use other than what it’s been approved for by the Food and Drug Administration (FDA).
If you have bothersome hair loss while taking metformin, talk with your doctor. They can investigate the possible cause and suggest ways to manage it.
Does metformin cause weight loss or weight gain?
Weight gain wasn’t reported in clinical trials of metformin. However, metformin was associated with some weight loss. Metformin lowers your blood sugar levels. This could lead to weight loss in some people.
Also, changes in taste and digestive problems have been reported with metformin use. If you have these side effects, they may reduce your appetite, which could lead to weight loss.
You’re unlikely to gain weight as a result of taking metformin.
If you’re concerned about how metformin may affect your weight, talk with your doctor. They can recommend ways to maintain a weight that’s suitable for you.
Learn more about some of the side effects that metformin may cause. To find out how often side effects occurred in clinical trials, see the prescribing information for metformin.
Lactic acidosis
Metformin has a
Symptoms of lactic acidosis can include muscle or stomach pain, trouble breathing, extreme sleepiness, or slow heartbeat. It can also include feeling weak, dizzy, unwell, or cold, especially in your arms or legs.
While taking metformin, you may have an increased risk of lactic acidosis if you:
- have kidney or liver problems
- drink alcohol very often or drink large amounts
- have alcohol use disorder
- become dehydrated
- have low oxygen levels due to complications, such as sudden heart failure, heart attack, breathing problems, severe infection, or shock
- have a radiologic imaging procedure, such as an X-ray or magnetic resonance imaging (MRI) scan with injectable dyes or contrast agents
- have surgery or another medical procedure
- are age 80 years or older and haven’t had your kidney function tested
What you can do
If you develop symptoms of lactic acidosis while taking metformin, call your doctor right away. But if your symptoms feel life threatening, call 911 or your local emergency number. This condition is life threatening and needs to be treated in the hospital.
Be sure to talk with your doctor about your health history before you start taking metformin. This will help them determine if metformin is safe for you.
To help prevent lactic acidosis while taking metformin, you should:
- avoid drinking large amounts of alcohol
- drink plenty of nonalcoholic fluids to prevent dehydration if you have diarrhea or vomiting
- ask your doctor if you should stop taking metformin temporarily before planned imaging procedures or surgeries
Constipation
Constipation is possible with metformin treatment. In clinical trials of metformin, constipation was rarely reported.
It’s important to note that constipation could also be a symptom of low vitamin B12 levels. Low B12 levels may occur with long-term use of metformin.
What you can do
If you have constipation while taking metformin, this typically gets better over time. During your treatment, it may help to:
- drink plenty of water
- eat plenty of fiber-rich foods
- do gentle exercise, such as walking
If you have constipation that’s bothersome or doesn’t get better, talk with your doctor. They may recommend taking a laxative medication or other treatments to ease it. They may also order a blood test to check your vitamin B12 level.
Nausea and vomiting
Some people may have nausea and vomiting while taking metformin. In clinical trials, these side effects were common with metformin treatment.
Nausea and vomiting are more likely to occur when you first start taking metformin. But they usually ease as you keep taking the medication.
What you can do
To help prevent nausea and vomiting, and other digestive side effects, your doctor will likely prescribe a low dose of metformin when you start treatment. This allows your body to get used to the medication. If needed, your doctor may gradually increase your dosage over time. Taking your doses with food may also help prevent nausea and vomiting.
If you have vomiting with metformin, it’s important to drink plenty of fluids to avoid getting dehydrated. Dehydration can raise your risk of a serious side effect of metformin called lactic acidosis.*
If you have nausea or vomiting that’s severe, bothersome, or doesn’t ease, talk with your doctor. They may reduce your metformin dosage or suggest switching to a different form of the drug. Nausea and vomiting are less common with extended-release (ER) metformin tablets than with immediate-release (IR) tablets.
* Metformin has a
Low blood sugar levels
Some people may have low blood sugar levels while taking metformin. However, this side effect usually occurs if you take metformin with insulin or another diabetes drug called a sulfonylurea. Examples of sulfonylurea drugs include glipizide (Glucotrol) and glimepiride (Amaryl).
In clinical trials, low blood sugar was less common in people who took metformin alone.
Symptoms of low blood sugar can include:
- irritability
- hunger
- blurry vision
- dizziness
- shaking
- sweating
- nausea
- headache
- confusion
What you can do
If you have symptoms of low blood sugar, you should immediately eat or drink something containing sugar that that your body can quickly absorb. Examples include:
- jelly beans
- hard candies
- glucose tablets
- a tablespoon of honey or sugar
- sugary drinks (not diet or sugar-free drinks)
If you often have low blood sugar while taking metformin, talk with your doctor. They may lower your dosage of metformin. If you also take other medications for diabetes, your doctor may make changes to your treatment.
Allergic reaction
As with most drugs, metformin can cause an allergic reaction in some people. However, it’s not clear whether this side effect occurred in clinical trials.
Symptoms can be mild or serious and can include:
- skin rash
- itching
- flushing
- swelling under your skin, typically in your eyelids, lips, hands, or feet
- swelling of your mouth, tongue, or throat, which can make it hard to breathe
What you can do
For mild symptoms of an allergic reaction, call your doctor right away. They may recommend ways to ease your symptoms and determine whether you should keep taking metformin. However, if your symptoms are serious and you think you’re having a medical emergency, immediately call 911 or your local emergency number.
Before taking metformin, there are certain precautions to consider. This includes a boxed warning from the Food and Drug Administration (FDA).
Boxed warning: Lactic acidosis
This drug has a
Other precautions
Be sure to talk with your doctor about your health history before you take metformin. This drug may not be the right treatment for you if you have certain medical conditions or other factors that affect your health. These are considered drug-condition or drug-factor interactions. The conditions and factors to consider include:
Allergic reaction. If you’ve had an allergic reaction to metformin or any of its ingredients, your doctor will likely not prescribe metformin. Taking metformin may cause you to have another allergic reaction. Ask your doctor what other medications may be better options for you.
Liver or kidney problems. If you have liver or kidney problems, you may have an increased risk of lactic acidosis with metformin treatment. Before you start taking metformin, your doctor will likely order tests to check your liver and kidney function. Based on the test results, your doctor will advise if metformin is safe for you to take. If you take metformin, your doctor will typically check your kidney function at least once per year to make sure the drug is still safe for you.
Diabetic ketoacidosis. Diabetic ketoacidosis is a complication of diabetes that’s a medical emergency. So, if you’ve ever had diabetic ketoacidosis or a risk of this condition, your doctor will likely not prescribe metformin. They’ll recommend what other medications may be better options for you.
Heart problems. If you have heart problems, including congestive heart failure, metformin may not be the best treatment for you. Talk with your doctor about what other medications may be better treatments for you.
Over 80 years old. If you are over 80 years old, your doctor will likely check your kidney function before taking metformin. They won’t recommend metformin unless your kidneys are functioning as usual. If you have questions about this, talk with your doctor.
Alcohol with metformin
You should limit the amount of alcohol you drink while you’re taking metformin. Drinking large amounts of alcohol can increase your risk of lactic acidosis.
Drinking large amounts means drinking a lot in one sitting (binge drinking). It can also refer to regularly drinking on most days. If you drink alcohol, talk with your doctor about how much, if any, is safe to drink while you’re taking metformin.
Pregnancy and breastfeeding while taking metformin
It’s unknown if metformin is safe to take during pregnancy. Studies haven’t found an increased risk of pregnancy loss, congenital abnormalities (also known as birth defects), or other risks when metformin is taken during pregnancy. However, further research may be needed to know for sure if the drug is safe to take during pregnancy.
If you’re pregnant or planning to become pregnant, talk with your doctor about the possible risks and benefits of taking metformin. They can recommend the best way to manage your blood sugar levels during pregnancy. You can also view the “Metformin: Side effects in women vs. men” section above for details about the risk of becoming pregnant while taking metformin.
It’s not known if it’s safe to breastfeed while taking metformin. Metformin may pass into breast milk in small amounts. However, it’s unknown if the drug can cause side effects in a child who’s breastfed. Talk with your doctor about the best way to feed your child while you’re taking metformin.
Metformin has common side effects, but these are usually mild and ease as your body gets used to the medication. As with many medications, serious side effects can also occur.
If you’d like to learn more about metformin, talk with your doctor or pharmacist. They can help answer any questions about the drug’s side effects. Referring to the following articles about metformin can provide you with additional information:
- More information about metformin. For details about other aspects of metformin, refer to this article.
- Cost. If you’d like to learn about metformin and its cost, see this article.
- A look at type 2 diabetes. For details about type 2 diabetes, see our diabetes hub.
Disclaimer: Medical News Today has made every effort to make certain that all information is factually correct, comprehensive, and up to date. However, this article should not be used as a substitute for the knowledge and expertise of a licensed healthcare professional. You should always consult your doctor or another healthcare professional before taking any medication. The drug information contained herein is subject to change and is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. The absence of warnings or other information for a given drug does not indicate that the drug or drug combination is safe, effective, or appropriate for all patients or all specific uses.