Metformin: what it is used for and what to know

Metformin is an oral medicine in the biguanide class. It requires a prescription and is not a controlled substance: its FDA label lists no DEA schedule. It comes as regular (immediate-release) tablets, extended-release tablets and an oral liquid.

How it works. According to MedlinePlus, metformin lowers the amount of glucose you absorb from food and the amount your liver makes, and it helps your body respond better to its own insulin. It controls blood sugar but does not cure diabetes, which is why you keep taking it even when you feel well.

Yes, they are the same medicine. Glucophage was metformin's original brand name; according to MedlinePlus, that brand is no longer on the market; today it is sold as generic metformin. Metformin also comes combined with other diabetes medicines in a single tablet.

According to the FDA label, metformin is approved as an adjunct to diet and exercise to improve blood sugar control in type 2 diabetes: the regular tablets in adults and in children 10 and older, and the extended-release tablets in adults. It is not used for type 1 diabetes. Doctors also sometimes use it off-label (for uses not on the label):

Regular tablets. Per the FDA label, adults usually start at 500 mg twice a day or 850 mg once a day, with meals, and the dose is raised gradually up to a maximum of 2,550 mg a day in divided doses. In children 10 and older, the maximum is 2,000 mg a day.

Extended-release tablets. These usually start at 500 mg once a day with the evening meal, up to a maximum of 2,000 mg a day. Swallow them whole: do not split, crush or chew them.

The doctor decides the dose, based on your blood sugar, your kidneys and how well you tolerate the medicine. Do not change or stop it on your own.

Stomach upset is metformin's most common side effect, and it can usually be managed. Per the FDA label and MedlinePlus:

According to the FDA label and MedlinePlus, the most common metformin side effects are:

The metformin label carries a boxed warning about lactic acidosis: a buildup of acid in the blood that is rare but serious. Per the label, the risk is higher with:

• Kidney problems.
• Age 65 or older.
• Imaging tests with iodinated contrast (injected dye).
• Surgery and other procedures.
• Conditions with low oxygen, such as a heart attack, a serious infection or decompensated heart failure.
• Heavy alcohol use.
• Liver problems.
• Some medicines, such as carbonic anhydrase inhibitors (for example, topiramate).

Warning signs: extreme tiredness, weakness, nausea, vomiting, stomach pain, fast or deep breathing, dizziness, an irregular heartbeat or feeling very cold. MedlinePlus says to stop taking it and call your doctor right away if you have them. If they are severe, call 911 or go to the emergency room.

Your kidneys clear metformin, so the FDA label sets clear rules based on your kidney filtration rate (eGFR):

• eGFR below 30: it must not be used (contraindicated).
• eGFR 30 to 45: starting it is not recommended.
• If you already take it and your eGFR falls below 45: the doctor weighs whether the benefits still outweigh the risks.
• Before starting and at least once a year: your kidney function is checked.

Contrast dye and surgery. Per the label, in some people (for example, with an eGFR of 30 to 60 or with liver or heart problems) metformin is stopped at the time of, or before, a contrast imaging test, and the eGFR is rechecked 48 hours later so it can be restarted if the kidneys are stable. It is also paused during surgery when you cannot eat or drink. Tell the doctor you take metformin before any imaging test or surgery. If your lab showed high creatinine, the doctor reviews your case.

Metformin can lower vitamin B12 when taken for a long time. The FDA label recommends checking blood counts every year and B12 every 2 to 3 years, and the ADA advises periodic B12 checks, especially with anemia or tingling in the feet. Signs of B12 deficiency include tiredness, pale skin and tingling in the hands and feet. Do not stop metformin on your own: the doctor can order a B12 test.

Before starting, tell the doctor and the pharmacist everything you take, including supplements. Per the FDA label:

Pregnancy. Per the FDA label, data in pregnant women are limited and not enough to tell whether metformin causes birth defects or miscarriage; the label also explains that poorly controlled diabetes in pregnancy carries risks for mother and baby. If you are pregnant or planning a pregnancy, your treatment is decided with the doctor. Do not stop or change your medicines on your own.

Breastfeeding. The label says metformin passes into breast milk in small amounts, with limited data. If you breastfeed, tell the doctor.

Ovulation. The label warns that in some women who do not ovulate, metformin can restart ovulation, so an unplanned pregnancy is possible. Talk with the doctor about birth control if you are not trying to conceive.

The FDA has not approved metformin for weight loss: its label approves it only for blood sugar control. Some people lose a little weight on it, but it is not a weight-loss drug. Read more about metformin for weight loss and the approved options.