Metformin can help lower blood sugar and HbA1c levels (a measure of glucose control over the previous 2 to 3 months) and fight insulin resistance (a condition in which the body does not use insulin efficiently). 

Although it is not approved by the U.S. Food and Drug Administration (FDA) as a weight-loss treatment, it can also help people lose weight in some cases.

This article will outline everything you need to know about this popular diabetes drug.

Box of metformin tablets

Key facts

  • Metformin functions by reducing glucose production in the liver, increasing insulin sensitivity, and decreasing sugar absorption from food. 
  • It’s typically taken daily or two or three times daily with meals and is available in immediate-release (IR) and extended-release forms (XR or ER).
  • Metformin is considered safe, with a low risk of severe low blood sugar. Common side effects include gastrointestinal symptoms such as stomachache and nausea.
  • Beyond type 2 diabetes, metformin is being explored for treating conditions such as gestational diabetes and polycystic ovary syndrome (PCOS). It’s also used off-label for type 1 diabetes and weight management.
  • A variety of medications may interact negatively with metformin, such as certain diuretics and antibiotics. Excessive alcohol consumption is also advised against while taking metformin.
  • The medicine is highly accessible and affordable and covered by most health insurance plans, including Medicare and Medicaid.

Table of Contents

What is metformin?

Approved in the United States in 1994 and studied in humans since the 1950s, metformin is a prescription drug taken to help manage diabetes. It is the only available medicine in a class of drugs called biguanides, and it comes in pill form and liquid form. 

Metformin pills are available in both immediate-release and extended-release formulations. Depending on your management needs and the type of medicine you are taking, you may be instructed to take your medication either daily or two or three times daily with meals.

Metformin brand names

Some brand names include:

  • Fortamet
  • Glucophage
  • Glucophage XR
  • Glumetza
  • Riomet
  • Riomet ER

Metformin is also an ingredient in a variety of combination medications. 

Read more: Metformin Combination Drugs for Type 2 Diabetes.

How does metformin work? 

Metformin lowers blood sugar levels by three methods:

  • Reducing the production of glucose from the liver
  • Reducing insulin resistance, improving the ability of the body’s cells to consume and use sugar for energy
  • Decreasing sugar absorption in the intestines

In combination with adopting a healthy diet and engaging in regular physical activity, this can lead to lower blood sugar levels, reduced HbA1c levels, and, sometimes, weight loss.

“We had been steadily working on improving my glucose control and A1c, but I was still running high quite often and my A1c had plateaued in the 7s, so [my healthcare provider] recommended starting metformin to address that without increasing my risk for lows.” 

Kat Schroeder, metformin user (read about Kat’s experience with metformin)

Common side effects of metformin

Most people tolerate metformin well. It is considered extremely safe and does not typically cause dangerous hypoglycemia (low blood sugar) unless you take significantly more than prescribed or are using it in combination with other medicines that can cause low blood sugar, such as insulin or a sulfonylurea.

It is an inexpensive and effective medication that is widely accessible. However, some people may experience mild to moderate side effects, including:

  • Stomachache
  • Stomach cramps
  • Nausea 
  • Vomiting
  • Diarrhea
  • Loss of appetite
  • Weight loss
  • Metallic taste in the mouth
  • Weight gain (rare)

Less common but more serious potential side effects of metformin include:

  • Hypoglycemia (if taken in combination with insulin or sulfonylureas)
  • Lactic acidosis (a potentially fatal condition in which lactic acid builds up in the body)
  • Vitamin B12 deficiency

You can learn more about potential side effects in our article: Metformin Side Effects: What You Need to Know.

Hypoglycemia

Metformin rarely causes hypoglycemia in people with diabetes who are not also on insulin (or another drug that increases insulin production) because it doesn’t increase insulin production like many 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

Lactic acidosis

Metformin comes with a “black box” warning about the risk of lactic acidosis, the most severe warning that the FDA issues.

Symptoms of this condition can include:

  • Nausea
  • Vomiting
  • Weakness
  • Fatigue
  • Fast, deep breathing
  • Muscle cramps and body aches

However, in the study “Quality and Characteristics of 4241 Case Reports of Lactic Acidosis in Metformin Users Reported to a Large Pharmacovigilance Database,” researchers examined over 4,000 cases where metformin was suspected in lactic acidosis incidents, as reported to a major database. 

They found that actual metformin-associated lactic acidosis was confirmed in only 9 percent of these cases. 

The study highlighted that high levels of lactate in the blood, rather than metformin levels, were linked to higher risks of death, especially when coupled with kidney issues. 

This research is the most extensive to date in exploring the true impact of metformin on severe lactic acidosis, suggesting that the drug is largely safe (but warrants further investigation in specific groups of people).

The majority of cases of lactic acidosis from metformin use are results of overdoses — not normal use of the drug.

Read more: Can Metformin Cause Lactic Acidosis?

Metformin and vitamin B12 deficiency

In studies, researchers have found that long-term use of metformin can cause vitamin B12 deficiency. Left untreated, this can lead to significant nerve damage. 

Everyone taking metformin should have their B12 levels tested annually. In consultation with their healthcare provider, anyone with inadequate levels should start taking a B12 supplement immediately. (Vitamin B12 injections every few months may be recommended in some cases.)

Because vitamin B12 isn’t produced by plants, supplementing with the vitamin if you are vegan or eat a mostly plant-based diet may be especially important. 

Metformin and “false lows”

Some people experience “false lows” (symptoms of hypoglycemia even when blood sugar levels are within normal range) when starting metformin. 

This can happen if you’re used to having near-constant high blood sugar levels. Because your body is used to these high levels, a normal blood sugar level feels low. 

The false lows should go away as your body adjusts to your new blood sugar level. (Always check your blood sugar if you are feeling low, as it may not be a false low.)

How to take metformin

Metformin should be taken whole (not crushed, broken, or chewed) with a drink of water with a meal to help avoid an upset stomach. 

Try to take it with a higher-protein meal or snack, and never take metformin on an empty stomach. The medicine is usually better tolerated in the evening, so taking it with dinner is best. 

If you are experiencing severe gastrointestinal issues while taking metformin, speak with your doctor. They may reduce your dose or switch you to an extended-release version of the medicine. 

Because the extended-release medicine is released over a few hours instead of immediately, you may experience fewer digestive side effects with this approach.

(Note that when using the extended-release form of the tablet, it’s common and not a cause for concern to find part of the tablet in your stool. This occurs after the medication has been absorbed by your body.)

Dosages of metformin

The most common doses for adults with diabetes are:

Immediate-release tablets:

  • 500 milligrams (mg)
  • 850 mg
  • 1,000 mg

Extended-release tablets and liquid form:

  • 500 mg
  • 850 mg
  • 1,000 mg

It’s recommended to start with a low dose and increase it if you tolerate the drug well and need a higher dose to regulate your blood sugar levels. Always check with your doctor first before increasing your dose. 

Read more: Metformin Dosage Guide (Min and Max Doses).

What should you do if you miss a dose?

Take your metformin as soon as you can, unless it’s almost time for your next dose. In that case, skip the missed dose and resume your regular schedule unless your provider advises otherwise. Do not take two doses at one time.

Can you overdose on metformin?

Yes, this is possible, but it’s very rare. A large overdose can cause severe and potentially dangerous hypoglycemia or lactic acidosis. 

Seek emergency medical attention if you think you have overdosed.

When will I see results after starting metformin? 

The Mayo Clinic notes that “You may notice an improvement in your blood glucose control in 1 to 2 weeks, but the full effect of blood glucose control may take up to 2 to 3 months.” 

Be patient. Keep regularly checking your blood sugar levels and taking your medication, and you should eventually see significant improvements from this medication. 

Work with your healthcare provider to determine how your current metformin dose is working for you, and what (if any) adjustments need to be made to improve results or lessen any negative side effects you may be experiencing. 

How do I know if the medication is working?

Metformin’s effectiveness can be gauged through several indicators. A reduction in blood sugar levels and HbA1c, noticeable after a few weeks to months, generally indicates that it is working. Some people may also experience modest weight loss. 

However, persistent high blood sugar levels, no improvement in HbA1c, or unexplained weight gain despite a healthy lifestyle might suggest that it is not working as intended. 

For a more detailed understanding, read: Signs Metformin Is Working (Or Isn’t Working).

Can I stop taking metformin?

Stopping metformin should be a carefully considered decision, prompted by lack of effectiveness, severe side effects, or changes in your healthcare needs and lifestyle. 

It’s crucial to not stop the medicine abruptly, since this can lead to negative health effects such as high blood sugar and potential diabetes complications. Always speak with your doctor for guidance on how to wean off the drug. This may involve adjusting the dosage over time and exploring alternative medicines or lifestyle changes. 

For further insights, read: Stopping Metformin: When and How You Can Stop Taking Metformin.

Are there alternative medications?

While metformin is a widely used and effective treatment for type 2 diabetes, it may not suit everyone due to side effects or lack of efficacy. Fortunately, a variety of alternative medication options exist, each with their own benefits and potential side effects. 

Lifestyle changes like diet and exercise can also be effective, either for use alongside a different medicine or as standalone treatments, in some cases. 

It’s important to speak with your healthcare provider to find the most suitable alternative based on your individual needs and circumstances. 

For an in-depth look at these options, see: The Best Alternatives to Metformin for Type 2 Diabetes Management.

Who should not take metformin?

People with certain conditions may be advised against taking metformin. Speak with your doctor prior to taking this medicine if you:

What drugs to avoid while taking metformin

The Mayo Clinic lists the drugs that may interact negatively with metformin. 

Using metformin with the following medicines is not recommended. If you are on any of these medications, work with your doctor to determine the best approach: 

  • Acetrizoic Acid
  • Diatrizoate
  • Ethiodized Oil
  • Iobenzamic Acid
  • Iobitridol
  • Iocarmic Acid
  • Iocetamic Acid
  • Iodamide
  • Iodipamide
  • Iodixanol
  • Iodohippuric Acid
  • Iodopyracet
  • Iodoxamic Acid
  • Ioglicic Acid
  • Ioglycamic Acid
  • Iohexol
  • Iomeprol
  • Iopamidol
  • Iopanoic Acid
  • Iopentol
  • Iophendylate
  • Iopronic Acid
  • Ioseric Acid
  • Iosimide
  • Iotasul
  • Iothalamate
  • Iotrolan
  • Iotroxic Acid
  • Ioxaglate
  • Ioxitalamic Acid
  • Ipodate
  • Metrizamide
  • Metrizoic Acid
  • Tyropanoate Sodium

Using metformin with any of the following medicines is typically not recommended, but may be required. In this case, your doctor may adjust the dose or how often one or both of the medicines is used. Speak with your doctor to decide what will work best for you: 

  • Abiraterone Acetate
  • Acetazolamide
  • Alogliptin
  • Aspirin
  • Bexagliflozin
  • Bictegravir
  • Brinzolamide
  • Bupropion
  • Capmatinib
  • Chloroquine
  • Chlorpropamide
  • Cimetidine
  • Ciprofloxacin
  • Dasabuvir
  • Delafloxacin
  • Dichlorphenamide
  • Dofetilide
  • Dolutegravir
  • Dorzolamide
  • Enoxacin
  • Fedratinib
  • Fexinidazole
  • Gatifloxacin
  • Gemifloxacin
  • Glimepiride
  • Glipizide
  • Glyburide
  • Grepafloxacin
  • Hydroxychloroquine
  • Insulin
  • Insulin Aspart, Recombinant
  • Insulin Bovine
  • Insulin Degludec
  • Insulin Detemir
  • Insulin Glargine, Recombinant
  • Insulin Glulisine
  • Insulin Lispro, Recombinant
  • Iopromide
  • Ioversol
  • Isavuconazole
  • Lanreotide
  • Levofloxacin
  • Levoketoconazole
  • Liraglutide
  • Lomefloxacin
  • Methazolamide
  • Moxifloxacin
  • Nateglinide
  • Norfloxacin
  • Octreotide
  • Ofloxacin
  • Ombitasvir
  • Paritaprevir
  • Pasireotide
  • Pioglitazone
  • Pramlintide
  • Ranolazine
  • Repaglinide
  • Semaglutide
  • Sitagliptin
  • Somatrogon-ghla
  • Sotagliflozin
  • Sparfloxacin
  • Tafenoquine
  • Thioctic Acid
  • Tolazamide
  • Tolbutamide
  • Topiramate
  • Trovafloxacin
  • Vandetanib
  • Zonisamide

Avoid excessive alcohol use when taking metformin. Speak with your doctor about how much, if any, alcohol use is safe for you.

Learn more about this combination in: Metformin & Alcohol: Can You Drink While Taking Metformin?

Common questions about metformin

How should the pills be stored?

It’s recommended to store the pills at room temperature, between 68 to 77 degrees Fahrenheit (20 to 25 degrees Celsius) but they can be kept safely at 59 to 86 degrees Fahrenheit (15 to 30 degrees Celsius) for a while if you are traveling.

The pills should always be stored in a light-resistant container (like the container they originally came in) away from heat and excess moisture.

Can metformin cause hair loss?

No, metformin is not a known cause of hair loss. 

However, long-term use of metformin can cause vitamin B12 deficiency. This can in turn prevent hair follicles from receiving oxygenated blood, leading to hair loss. Ask your doctor for a vitamin B test if you notice hair loss or thinning hair.

Do I need to change my diet?

Adjusting your diet while taking metformin is critical for maximizing the medicine’s effectiveness and minimizing common side effects. Although no specific foods are strictly prohibited, it is advisable to limit alcohol, saturated and trans fats, refined carbohydrates, and high-sodium items. Focusing on a balanced diet rich in whole fruits, vegetables, lean proteins, and lower-fat dairy can help enhance metformin’s benefits in managing blood sugar levels.

Excessive consumption of certain items can lower the drug’s effectiveness and worsen side effects. 

For detailed guidance on optimizing your diet while on metformin, read: Foods to Avoid While Taking Metformin.

Can I use metformin if I am pregnant (or trying to become pregnant)?

It may be safe to consider taking metformin if you are pregnant or plan to become pregnant. However, always talk to your doctor about the medications you take when you’re pregnant or trying to become pregnant.

Can you become addicted to metformin?

No, there is no evidence that metformin is addictive.

Does metformin cause cancer?

In 2019, the FDA investigated whether some forms of metformin contained high levels of a chemical called N-nitrosodimethylamine (NDMA), which is potentially carcinogenic (cancer-causing).

In 2020 and 2021, several companies voluntarily recalled certain lots of extended-release metformin based on the FDA’s recommendations. You can check to see if your metformin was included in these recalls here. 

Even if you take a metformin product that has been recalled, it’s important that you continue to take your medication until you talk to your healthcare professional. 

There is no short-term health risk — no human cancer cases have been directly linked to NDMA at this time — and the negative effects of abruptly stopping metformin treatment far outweigh the benefits.

Testing by the FDA did not show NDMA in immediate-release metformin products.

Are there other benefits of metformin? 

Metformin is currently being used to treat gestational diabetes (a type of diabetes that occurs during pregnancy) and polycystic ovary syndrome (PCOS, a condition in which a woman’s hormones are out of balance, resulting in the development of small, cyst-like sacs on the ovaries). 

The American Diabetes Association has said that more doctors should be prescribing metformin to their patients to treat prediabetes (a condition in which blood glucose levels are high, but not in the range for a diabetes diagnosis) as well, especially for people under the age of 60. 

Researchers are also studying if metformin can help in the fight against conditions including cancer, heart disease, obesity, neurodegenerative conditions (like Huntington’s disease and Parkinson’s disease), age-related macular degeneration (AMD), and even serve as an anti-aging supplement. 

Metformin and type 1 diabetes

Metformin is not currently approved in the United States or Europe for the treatment of type 1 diabetes.

However, many people with type 1 diabetes take metformin “off-label” (in a way not approved by the FDA) to help manage blood sugar levels, lessen insulin resistance, and even lose weight. 

A 3-year study published in the journal The Lancet found that participants with type 1 diabetes taking metformin lost weight compared to placebo (inactive treatment).

Research has shown that metformin may help reduce blood glucose levels in people with type 1 diabetes. Metformin has been shown to reduce required insulin doses as well as HbA1c levels in people with type 1 diabetes.

Be aware that taking metformin while on insulin for type 1 diabetes can cause lower blood sugars and the need for less insulin. Therefore, it is important to be mindful of potential hypoglycemia, especially overnight.