Metformin (Fortamet, Glumetza) - Uses, Side Effects, and More

Medically Reviewed by Michelle Vermeulen, PharmD on May 07, 2024 | Written by Beth Johnston, PharmD, BCPS

Uses

What is metformin used for?

Metformin is used to help lower blood sugar levels in people with type 2 diabetes. 

Metformin may also be used for other conditions as determined by your healthcare provider.

How does metformin work (mechanism of action)?

Metformin works in a few different ways to help keep your blood glucose (sugar) from getting too high.

• Metformin decreases the amount of glucose your body absorbs from things you eat and drink. 
• Metformin reduces the amount of glucose that your liver makes. 
• Metformin also helps your body’s own insulin to work better. (Insulin is a hormone that helps your body use glucose as a source of energy.) 

How is metformin supplied (dosage forms)?

Metformin is available as Fortamet, Glumetza, and generic metformin in the following dosage forms that are taken by mouth.

• 500 mg, 625 mg, 750 mg, 850 mg, 1000 mg oral tablets
• 500 mg, 750 mg, 1000 mg extended-release oral tablets
• 500 mg/5 mL oral solution

How should I store metformin?

Metformin should be stored at room temperature, between 68 F to 77 F (20 C to 25 C). It can be exposed to temperatures between 59 F to 86 F (15 C to 30 C), for shorter periods of time, such as when transporting it. Store in a cool, dry place.

Side Effects

What are the most common side effects of metformin?

The most common side effects of metformin are listed below. Tell your healthcare provider if you have any of these side effects that bother you.

• Diarrhea
• Nausea or vomiting
• Gas or feeling bloated
• Feeling unusually weak or tired
• Stomach pain, upset stomach, heartburn
• Headache
• Constipation
• Change in how things taste or a metallic taste
• Infection in the nose or throat, sore throat, or common cold symptoms

There may be other side effects of metformin that are not listed here. Contact your healthcare provider if you think you are having a side effect of a medicine. In the U.S., you can report side effects to the FDA at www.fda.gov/medwatch or by calling 800-FDA-1088. In Canada, you can report side effects to Health Canada at www.health.gc.ca/medeffect or by calling 866-234-2345.

What are the serious side effects of metformin?

While less common, the most serious side effects of metformin are described below, along with what to do if they happen.

Severe Allergic Reactions. Metformin may cause allergic reactions, which can be serious. Stop using metformin and get help right away if you have any of the following symptoms of a serious allergic reaction.

• Breathing problems or wheezing
• Racing heart
• Fever or general ill feeling
• Swollen lymph nodes
• Swelling of the face, lips, mouth, tongue, or throat
• Trouble swallowing or throat tightness
• Itching, skin rash, or pale red bumps on the skin called hives
• Nausea or vomiting
• Dizziness, feeling lightheaded, or fainting
• Stomach cramps
• Joint pain

Lactic Acidosis. Metformin may cause lactic acidosis (a buildup of lactic acid in your blood), which can be serious. Stop using metformin and get help right away if you have any of the following symptoms of lactic acidosis.

• General ill feeling
• Muscle pain
• Difficulty breathing
• Drowsiness 
• Stomach pain 

Vitamin B12 Deficiency. Metformin may cause you to have low levels of vitamin B12 in your blood (possibly by decreasing the amount of vitamin B12 your body absorbs from things you eat or drink), which may lead to anemia (low levels of red blood cells). Your healthcare provider will likely use lab tests to check your vitamin B12 levels and for anemia. Tell your healthcare provider right away if you have any of the following symptoms of anemia.

• Unusual weakness or tiredness
• Cold hands and feet
• Fast or abnormal heartbeat
• Pale or yellowish skin
• Dizziness, lightheadedness, or feeling like you are about to pass out
• Shortness of breath

Low Blood Sugar (Hypoglycemia). When metformin is used with insulin or other medicines that lower your blood sugar by increasing your body’s own insulin levels, it can cause hypoglycemia (low blood sugar). The doses of the other medicines may need to be changed when taken with metformin. Call your healthcare provider right away if you have any of the following symptoms of low blood sugar.

• Headache
• Crankiness or anxiety
• Hunger
• Dizziness or confusion
• Blurry vision
• Slurred speech
• Sweating
• Feeling jittery or shakiness
• Fast heartbeat
• Seizures

Warnings & Precautions

Who should not use metformin?

Allergies to Ingredients. People who are allergic to any of the following should not take metformin.

• Metformin
• Fortamet
• Glumetza
• Any of the ingredients in the specific product dispensed

Your pharmacist can tell you all of the ingredients in the specific metformin products they stock.

Kidney Problems. Metformin should not be used if your kidneys are not working as well as they should be. If there is a concern about the health of your kidneys, your healthcare provider may do tests to determine if they are working well enough to take this medicine.

Metabolic Acidosis, Including Diabetic Ketoacidosis. Metformin should not be used in people with acute or chronic metabolic acidosis (when the chemical balance of acids and bases in your blood gets thrown off). Tell your healthcare provider if you have a history of metabolic acidosis or diabetic ketoacidosis.

What should I know about metformin before using it?

Do not take metformin unless it has been prescribed to you by a healthcare provider. Take it as prescribed.

Do not share metformin with other people, even if they have the same condition as you. It may harm them.

Keep metformin out of the reach of children.

Take metformin with meals to help decrease the chance you will have an upset stomach. Certain forms of metformin should be taken with the evening meal. Ask your pharmacist or healthcare provider about the best way to take your medicine. 

If you take the liquid form of metformin, use an accurate measuring device to measure your dose. A household spoon is not an accurate measuring device and may cause you to take the wrong dose. Ask your pharmacist to recommend an appropriate measuring device.

If you take an oral tablet form of metformin, you may sometimes notice something in your stool (poop) that looks like the tablet. This is normal and should not cause concern.

Do not chew, cut, or crush extended-release metformin tablets, they should be swallowed whole.

Metformin can cause ovulation (release of an egg from your ovary) if you have not gone through menopause, even if you do not have regular periods. This can increase your chance of getting pregnant.

Certain conditions that cause increased stress on your body, like fever, trauma (such as a car accident), infection, or surgery, can change the amount of diabetes medicines you need to take to control your diabetes. Talk to your healthcare provider if you have any of these conditions to decide if your medicine needs to be changed.

Talk to your healthcare provider about how to prevent, recognize, and manage low blood sugar (hypoglycemia), high blood sugar (hyperglycemia), and diabetes-related problems.

People who are 65 years or older can be at greater risk for some side effects from metformin. Talk to your healthcare provider about your risks if you are in this age group.

What should I tell my healthcare provider before using metformin?

Tell your healthcare provider about all of your health conditions and any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using. This will help them determine if metformin is right for you. 

In particular, make sure that you discuss any of the following.

Liver Problems. Tell your healthcare provider if you have liver problems. Your risk of lactic acidosis is increased if your liver is not working as well as it should be.

Current and Past Medical Conditions. Tell your healthcare provider if you have any of the following. 

• Kidney problems
• Heart problems, including congestive heart failure or a heart attack
• Drink alcohol often or in large amounts
• Have an upcoming medical test that uses contrast (a special dye), surgery, or other procedure
• Dehydration 
• Severe infection
• Stroke

Other Medicines and Supplements. Metformin may interact with other medicines and supplements. Before taking metformin, tell your healthcare provider about any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using. See the Interactions section for more details.

Pregnancy. It is not known if or how metformin could affect pregnancy or harm an unborn baby. Tell your healthcare provider if you are or plan to become pregnant. Your healthcare provider will advise you if you should take metformin while you are pregnant or trying to get pregnant. 

Breastfeeding. Metformin passes into breast milk, but the effects on the baby or on milk production are not clear. Tell your healthcare provider if you are breastfeeding or plan to breastfeed. Your healthcare provider will advise you if you should take metformin while breastfeeding. 

Interactions

Does metformin interact with foods or drinks?

There are no known interactions between metformin and foods or drinks.

Alcohol can increase the risk of lactic acidosis (a serious side effect of metformin discussed above). It is unknown if drinking alcohol will affect metformin in other ways, but alcohol may affect blood sugar levels in people with diabetes. This may interfere with the effect of metformin. It is best to limit the amount of alcohol you drink.

Does metformin interact with other medicines (drug interactions)?

Always tell your healthcare provider about any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using.

In particular, make sure that you discuss if you are using any of the following before using metformin.

• A carbonic anhydrase inhibitor, such as zonisamide (Zonegran), acetazolamide (Diamox), or methimazole
• Medicines to treat diabetes, especially insulin or medicines that increase your insulin levels

Many other medicines can increase your blood sugar and can interfere with the effect of metformin. Talk to your pharmacist or healthcare provider before starting or stopping other medicines while taking metformin. You may need to monitor your blood sugar levels more closely during these times.

Some other medicines may alter the blood levels of metformin. Tell your healthcare provider about all medicines that you take or have recently taken.

Overdose/Missed Dose

What should I do if I accidentally use too much metformin?

If you or someone else has taken too much metformin, get medical help right away, call 911, or contact a Poison Control center at 800-222-1222.

What should I do if I miss a dose of metformin?

If you miss a dose, take your next dose as you normally would unless your healthcare provider tells you something different. Do not take double or extra doses.

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