Metformin: Uses and Side Effects

Medically Reviewed by Jabeen Begum, MD on August 18, 2023
6 min read

Metformin is often the first medicine you’ll take if you have type 2 diabetes—for many reasons.

It helps bring down your blood sugar level in three ways:

  • It tells your liver to make less glucose.
  • It lowers your insulin resistance, which means it makes your muscles use insulin better, allowing glucose to get into them instead of staying in your blood.
  • It helps your intestines absorb less glucose from your food.

It can lower your A1c, which reflects the “average” of your blood sugar control over a few months. It can also delay prediabetes from becoming diabetes.

Some brand names are:

  • Fortamet
  • Glucophage
  • Glumetza
  • Riomet

All of those are pills except Riomet, which is a liquid.

Some “combination” pills have metformin along with other medicines, including:

  • Glipizide and metformin (Metaglip)
  • Glyburide and metformin (Glucovance)
  • Pioglitazone and metformin (Actoplus Met)
  • Repaglinide and metformin (Prandimet)
  • Saxagliptin and metformin (Kombiglyze)
  • Sitagliptin and metformin (Janumet)

As with any medicine, metformin can have side effects. Most are mild, but a few can be serious. Talk to your doctor about what you can expect.

What is the lowest dose of metformin you can take?

Everyone is different, and your doctor will prescribe you the appropriate dose. For oral dosage form (extended-release tablets), your doctor will likely prescribe you either 500 or 1,000 milligrams to be taken once a day with your evening meal. Your doctor may increase your dose but it likely won't go over 2,000-2,500 milligrams. Your dosage will also depend on what other medications you're taking.

Stomach trouble is the most common metformin side effect. About 25% of people have problems such as:

  • Bloating
  • Gas
  • Diarrhea
  • Belly pain
  • Constipation
  • Little appetite

Taking it with food can help. If you increase your dose, these side effects may return.

While doctors used to avoid prescribing this drug to people with a history of kidney trouble, it may be OK for someone with mild or moderate kidney disease.

You might see the shell part of an extended-release pill in your poop. If you do, don't worry. The medication has gone into your body, and you shouldn't take any extra pills.

Metformin's side effects can include an increased risk of urinary tract infections. If your pee is cloudy or bloody, or if it hurts when you pee, check with your doctor right away.

One large study has linked long-term use of metformin to higher chances of getting Alzheimer's disease and Parkinson's disease. But we need more research to understand the connection better and what it means.

Some people (in one study, it was less than 5%) reported heartburn, headaches, upper respiratory infection, or a bad taste in the mouth when they took extended-release metformin. Up to 12% of people on the regular formula had those side effects. They also reported flu-like symptoms, sweating, flushing, heart palpitations, rashes, and nail problems.

Lactic acidosis

This is a dangerous condition caused by the buildup of lactic acid, a chemical that your muscles and red blood cells make naturally. When it happens while taking metformin, it’s called metformin-associated lactic acidosis (MALA).

The problem is very rare, happening in a tiny fraction of people who take the drug.

It’s more likely to happen if you:

  • Have kidney or liver disease
  • Drink a lot of alcohol
  • Have severe congestive heart failure
  • Are sick with fever, diarrhea, or throwing up
  • Are dehydrated

Many of the warning signs are similar to some metformin side effects, such as stomach pain, dizziness, and weakness. Others are numbness or a cold feeling in your limbs, or changes in your heart rate. Call your doctor right away if you notice any of these problems.

Vitamin B12 deficiency

A lack of this B vitamin can happen to anyone, but the risk is higher if you are on metformin, especially over time. When you don’t get enough, it can cause peripheral neuropathy, the numbness or tingling in your feet and legs that’s already a risk with diabetes. It can also cause anemia, which refers tolow levels of red blood cells.

Ask your doctor to check your B12 level regularly. Don’t wait until you have symptoms. It’s also a good idea to add foods naturally high in B12 to your diet. Beef liver and clams have the most. Chicken, beef, eggs, dairy products, and fortified cereals are good sources of B12. Supplements can also bring your levels back to normal, especially if you’re a vegetarian. Just talk to your doctor before you start taking one.


Your blood sugar may fall too low if you take this medication while fasting or doing very heavy physical activity.

If you're taking a combination pill, or metformin with other diabetes medications or insulin, check with your doctor about how likely you are to have low blood sugar. If you take metformin by itself, you probably won't have low blood sugar.

Because of the risk of serious problems, your doctor will probably recommend a different medication if you:

  • Have had an allergic reaction to metformin or other medicines
  • Have diabetes that isn’t under control
  • Have liver or kidney problems
  • Have a severe infection
  • Recently had a heart attack or heart failure
  • Have breathing or blood flow problems
  • Drink a lot of alcohol

Some side effects go away on their own over time. There are a few ways you can ease or avoid problems:

  • Ask to start at a low dose. This makes it easier for your body to adjust to the medicine.
  • Take metformin with food. It’s OK to take the medicine on an empty stomach, but having it with a meal makes it easier to handle.
  • Ask about the extended-release form of metformin. You’ll take it once a day rather than twice. Because it doesn’t release the drug in one burst, side effects are often milder. In one study, just 10% of people who took the extended-release form had diarrhea, compared with 53% of those who took the standard formula. Just 7% reported nausea with the extended-release form, in contrast to 26% with the standard formula. And fewer than 1% of those on extended-release metformin had to stop taking it because of side effects.

Metformin can cause problems with other drugs you take, including:

  • Birth control pills and other estrogen drugs
  • Calcium channel blockers
  • Corticosteroids
  • Diuretics
  • Glaucoma medications
  • Thyroid drugs

Also, if you take metformin along with medicines for acid reflux, you could be more likely to have a vitamin B12 deficiency. Be sure to go over everything you take with your doctor.

Before you have an imaging test that uses contrast dye, such as a CT scan or MRI, you’ll need to stop taking metformin. The combination of the dye and the drug can cause a reaction that leads to lactic acidosis. Let your health care team know that you take it before you have an imaging test.

Metformin is often the first line of defense when it comes to fighting type 2 diabetes. It helps bring down your blood sugar levels. It's important to start metformin at a low dose to give your body time to adjust to the medication. Also, take it with food to make it easier on your stomach. Ask your doctor about drug interactions, as metformin may affect other drugs you're taking.

  • What is the most common side effect of metformin?

The most common side effect of metformin is stomach trouble such as gas and bloating.

  • What are the toxic side effects of metformin?

Lactic acidosis, or a buildup of lactic acids, is one of the most serious possible side effects. But it's very rare. You could also get a B12 deficiency or hypoglycemia, which is when your blood sugar falls too low.

  • What happens to your body when you start taking metformin?

Metformin tells your liver to make less glucose. It then makes your muscles use insulin better so glucose can get into them instead of staying in your blood. Finally, it helps your intestines absorb less glucose from your food.

  • Does metformin cause weight loss?

Taking metformin may lead to a small amount of weight loss.