This information is for people who have type 2 diabetes. It tells you about insulin plus metformin, a treatment used for type 2 diabetes. It is based on the best and most up-to-date research.
Does it work?
Yes. Taking insulin and metformin together works better than taking insulin alone to avoid problems (complications) from diabetes like heart attack, heart failure, and diabetic foot.  It also works better at controlling blood sugar.    But you're more likely to get side effects if you take both treatments. For example, you may feel sick or put on weight.
What is it?
Metformin is one of a group of drugs called biguanides.  Its 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.
Insulin is a hormone that your body uses to keep the amount of glucose (sugar) in your blood steady. Insulin is made by your pancreas, a gland that sits just behind your stomach. It's used for people who have type 2 diabetes that isn't controlled by other treatments. You can take insulin injections or you can use an insulin pump. To read more, see Insulin therapy.
Taking insulin and metformin together aims to combine the benefits of both treatments.
How does it work?
If you have type 2 diabetes, your pancreas doesn't make enough insulin, or the insulin your pancreas makes doesn't work properly. This means you get too much glucose in your blood.
Metformin helps lower the amount of glucose (sugar) in your blood in two ways.
After some years, metformin might not work as well as it used to. If this happens, taking insulin can replace the missing insulin that your body doesn't make any more.
Can it be harmful?
Taking insulin and metformin together is more likely to cause side effects than the individual treatments on their own.   In one study, about 6 in 10 people felt sick or got diarrhoea when they took both treatments. Only about 1 in 10 people just taking insulin got these problems. 
To read more about side effects, see Metformin and Insulin.
How good is the research on insulin plus metformin?
We found good-quality studies (randomised controlled trials) that compared taking insulin plus metformin with taking insulin plus a dummy treatment (a placebo).
The studies found that insulin plus metformin worked better than insulin alone.     People taking metformin also didn't need to take as much insulin.
However, people taking both insulin and metformin were more likely to gain weight and get side effects, such as nausea and diarrhoea.