Metformin helps your body use its own insulin better. It lowers the amount of sugar your liver makes, which lowers blood sugar levels. And there’s less risk of your blood sugar level falling too low, the way it can with some other diabetes drugs. Also, unlike many of the other drugs, it doesn’t cause weight gain. You may even lose some weight while you take it.
But 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.
Common Side Effects of Metformin
Stomach trouble is the most common metformin side effect. About 25% of people have problems like:
- Belly pain
Rare Side Effects of Metformin
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.
Serious Side Effects of Metformin
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.
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.
Who’s at High Risk of Serious Side Effects?
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
Managing Metformin Side Effects
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 Drug Interactions
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