This information is for people who have type 2 diabetes. It tells you about metformin, a treatment used for type 2 diabetes. It is based on the best and most up-to-date research.
Does it work?
Yes. If you have type 2 diabetes, taking metformin will probably control your glucose levels better than just eating healthily. Unlike many other diabetes drugs, metformin is less likely to make you gain weight.
What is it?
Metformin is one of a group of drugs called biguanides.  You can take it as a tablet or a liquid. One brand name is Glucophage. You take metformin as tablets or as powder you dissolve in water. You need a prescription from your doctor to get it.
Your doctor might tell you to take metformin on its own or with another drug to lower your blood glucose. For more about this, see Taking two drugs together.
How can it help?
If you take metformin, your blood glucose will be better controlled than if you take a dummy treatment (a placebo). And your blood glucose may be better controlled than if you just watch what you eat. 
Metformin may help lower your haemoglobin A1c level by about one point to two points.     Doctors use the haemoglobin A1c blood test to see how well you are controlling your diabetes.
Over 10 years, your level may be better than if you are just careful about what you eat. 
Metformin seems to control blood glucose levels at least as well as other diabetes drugs, and possibly better than sitagliptin and vildagliptin.  Metformin is also less likely to make you gain weight than thiazolidinediones or sulphonylureas.     
If you are overweight and take metformin to control your diabetes, you may live longer than if you just try to watch what you eat. 
How does it work?
Metformin helps lower the amount of glucose (sugar) in your blood in two ways.
First, it causes your liver to make less glucose. This means that less glucose goes into your bloodstream.
Second, it causes your muscles to use up more glucose from your blood.
Can it be harmful?
Metformin can make you feel sick. Or you might not feel like eating. It can also give you diarrhoea. But if you take metformin with food, these side effects may not happen so often. Or your doctor can start you on a low dose and increase it slowly.
If you take metformin, you may be slightly more likely to get low blood glucose ( hypoglycaemia) than if you just watch what you eat. But you're less likely to get this than if you take a sulphonylurea or insulin.  
Metformin has long been thought to increase the risk of a rare condition called lactic acidosis. When this happens, you get too much lactic acid in your body. This chemical makes you feel sick to your stomach. You might also not feel like eating, have a stomach ache, throw up, lose weight, and feel very tired. Lactic acidosis usually happens in people who have heart or kidney problems or are over 65. However, reviews of the research have found that people who took metformin were no more likely to get lactic acidosis than those taking different drugs for their diabetes.   But we need more research to look specifically at metformin's safety for people with kidney problems and other conditions that raise the risk of lactic acidosis.