In short
- Metformin is a prescription medicine used to treat type 2 diabetes. It is not used for type 1 diabetes.
- Public health sources describe it as lowering blood glucose in three ways: the liver releases less glucose, the gut absorbs less from food, and the body responds better to its own insulin.
- It helps control diabetes but does not cure it, and it is used alongside eating, activity and regular checks rather than instead of them.
- Decisions about whether to take it, and how, belong to you and the clinician who prescribes it.
What metformin is
Metformin is a medicine for type 2 diabetes. MedlinePlus, the consumer health service of the US National Library of Medicine, describes it as belonging to a group of medicines called biguanides, and says it can be prescribed on its own or together with other diabetes medicines, including insulin.
The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) says that most people with type 2 diabetes start with metformin. It is taken by mouth. MedlinePlus lists three forms: a standard tablet, an extended-release (long-acting) tablet, and a liquid. Which form someone is given, and the instructions that go with it, are set by the prescriber and printed on the label. This site does not describe amounts or timing, because those depend on the person.
MedlinePlus is also clear about what metformin is not for: it is not used to treat type 1 diabetes, the form in which the body does not make insulin at all.
A little background on type 2 diabetes
To see what metformin is aiming at, it helps to know what goes wrong in type 2 diabetes. NIDDK explains that glucose, the sugar in the blood, is the body's main source of energy and comes mostly from food. Insulin, a hormone made by the pancreas, helps glucose move out of the blood and into cells, where it is used for energy.
In type 2 diabetes, the body either does not make enough insulin or does not use it well, so too much glucose stays in the blood. MedlinePlus and CDC both describe the usual starting point as insulin resistance: cells stop responding normally to insulin, the pancreas makes more insulin to compensate, and over time it cannot keep up. Blood glucose then rises, first into the prediabetes range and later into the diabetes range.
CDC estimates that more than 40 million people in the United States have diabetes, about 1 in 8, and that 90 to 95 percent of them have type 2. NIDDK adds that symptoms often develop slowly over years and can be mild enough to go unnoticed, which is why many people learn they have type 2 diabetes from a routine blood test rather than from symptoms.
Over time, high blood glucose can damage the body. MedlinePlus lists heart disease, stroke, kidney problems, nerve damage and eye problems among the complications that people with diabetes and high blood sugar can develop. Lowering blood glucose is one of the main ways care teams try to reduce that risk.
How it is thought to work
The consumer sources this guide relies on describe metformin's effect in plain terms. Taken together, MedlinePlus and NIDDK describe three effects:
- Less glucose from the liver. The liver stores and releases glucose. Both sources say metformin reduces the amount of glucose the liver makes.
- Less glucose absorbed from food. MedlinePlus says metformin decreases the amount of glucose absorbed from what you eat.
- A better response to insulin. MedlinePlus says it increases the body's response to insulin, and NIDDK says it helps the body use insulin better. This targets the insulin resistance described above.
This guide says "thought to work" on purpose. These are short public summaries written for patients, not a full account of how the medicine acts inside cells, and the heading is a reminder not to read more certainty into them than they offer. If you want a deeper explanation, the prescriber or pharmacist can go through the information that comes with the medicine.
What it does not do
Some limits are stated plainly in the sources and are worth knowing before an appointment.
- It does not cure diabetes. MedlinePlus says metformin controls diabetes but does not cure it, and tells readers not to stop taking it without talking to their doctor, even when they feel well.
- It does not replace the rest of the plan. MedlinePlus's page on diabetes medicines says that people who take them still need healthy eating, physical activity and their other medicines.
- It may not be enough on its own forever. NIDDK and MedlinePlus both say that over time some people need more than one diabetes medicine to manage their blood glucose.
- It is not for type 1 diabetes. As above, MedlinePlus says it is not used to treat type 1 diabetes.
One part of a wider plan
MedlinePlus describes managing diabetes as a combination: medicines, lifestyle changes such as diet, exercise and quitting smoking, and regular blood glucose checks. It says this combination may help people stay healthier and may lower the chance of a heart attack, a stroke, or complications such as kidney failure, nerve damage and eye problems.
NIDDK makes the same point in its overview of type 2 diabetes: managing blood glucose, blood pressure and cholesterol, and quitting smoking if you smoke, are all important, and taking any prescribed medicines is one part of that. CDC adds that people who take insulin or other diabetes medicines still need to eat healthily and stay active.
That is why the other guides on this site cover blood glucose and A1C, side effects and everyday basics: the medicine makes most sense when you can see how it fits beside the numbers and habits your care team will talk about.
What MedlinePlus says to tell your doctor
MedlinePlus lists several things a person should tell their doctor before taking metformin. They are listed here so that you can think about them before an appointment, not so that you can decide anything on your own.
- Any allergy to metformin or to other medicines.
- Every other medicine you take, prescription or not, plus vitamins, supplements and herbal products.
- Kidney disease, liver disease, heart disease, a past heart attack or stroke, and other conditions named in its warning section. These are covered in the guide on serious risks.
- Any past low levels of vitamin B12.
- Pregnancy, plans to become pregnant, or breastfeeding.
- How much alcohol you drink, including occasional heavy drinking.
- Changes in how much you eat or exercise, which can affect blood glucose.
MedlinePlus also suggests keeping a written list of all the medicines and supplements you take and bringing it to every visit. FDA gives the same advice for any medicine. It is a simple habit that makes each of these conversations easier.
Worked example
This example is invented and describes no real person. Someone newly prescribed metformin writes three lines in a notebook before their next visit: the other medicines and supplements they take, the fact that they had a low B12 result some years ago, and a question about what their A1C goal is. Each line comes straight from the lists in the sources above, and each gives the clinician something specific to respond to.
A note on other uses
MedlinePlus notes that metformin may sometimes be prescribed for other purposes and tells readers to ask their doctor or pharmacist about this. This site covers metformin only as a treatment for type 2 diabetes, which is what the public health sources it relies on describe in detail. Whether any other use is appropriate for a particular person is a decision for a prescriber, and this site does not discuss or encourage any such use.