Metformin helps your body better use its own insulin. It lowers the amount of sugar the 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 cause 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 when you’re taking the drug.
In general, up to 25% of people who take metformin will have side effects involving their digestive system. The most common ones are bloating, gas, diarrhea, belly pain, and constipation. But most people find that these problems go away over time, often on their own.
There are a few ways you can ease these problems or avoid them:
Ask to start at a low dose. This makes it easier for your body to adjust to the medicine.
Take metformin with food. While it’s OK to take the medicine on an empty stomach, 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, studies show that side effects are often milder.
In one study, just 10% of people who took the extended-release form had diarrhea compared to 53% of those who took the standard formula, and just 7% had nausea compared to 26%. Also, fewer than 1% of those on extended-release metformin had to stop taking it because of side effects.
Other Minor Side Effects
Some people (less than 5% in one study) 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, palpitations, rashes, and nail 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. 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 also good sources. Supplements can also bring your levels back to normal, especially if you are a strict vegetarian. Just talk to your doctor before you start taking one.
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 MALA (metformin-associated lactic acidosis). The problem is extremely rare, happening in a tiny fraction of people who take the drug. It’s more likely to happen in people whose kidneys don’t work properly and can’t clear metformin from the body. That’s why metformin isn’t recommended if you have kidney problems or kidney failure, liver disease, or congestive heart failure. Many of the condition’s 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 possibly changes in your heart rate. Call your doctor right away if you notice any of these signs.
What Else You Should Know
Because alcohol can prompt problems with lactic acid when you’re on metformin, limit how much you drink. Talk to your doctor about what’s safe for you.
Metformin can cause problems with other drugs that you may 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 start taking the drug.
If you need to have an imaging test that uses contrast dye, such as a CT scan or MRI, you’ll need to stop taking metformin because the combination of the dye and the drug can cause a reaction that leads to lactic acidosis. Let your health care team know you take metformin before you have an imaging test.