Metformin and Type 2 Diabetes

Medically Reviewed by Michael Dansinger, MD on August 25, 2022

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 so glucose can get into them instead of staying in your blood.
  • It helps your intestines absorb less glucose from your food.

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

Some brand names are:

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

Some "combination" pills have metformin with another medicine, including:

Like any medicine, metformin can have side effects. Most are mild, but a few can be serious. Keep these in mind and talk to your doctor about what you can expect.

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

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

While doctors used to avoid prescribing this drug to people who've had 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.

One large study has linked long-term metformin use to higher chances of getting Alzheimer's disease and Parkinson's disease. But more research is needed 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, and a bad taste in their 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:

Many of the warning signs are similar to some metformin side effects, like 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 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, low 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. 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 metformin 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 sugars. If you're taking metformin by itself, you probably won't have low blood sugars.

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% had nausea, compared with 26%. 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 diuretics, glaucoma medications, corticosteroids, thyroid drugs, birth control pills and other estrogen drugs, and calcium channel blockers. 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 metformin before you have an imaging test

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