Sitagliptin is a medicine used to treat type 2 diabetes.Type 2 diabetes is a condition where the body does not make enough insulin, or the insulin that it makes does not work properly. This can cause high blood sugar levels (hyperglycaemia).Sitagliptin is prescribed for people who still have high blood sugar, even though they have a sensible diet and exercise regularly.It works by increasing the amount of insulin that your body makes. Insulin is the hormone that controls sugar levels in your blood.Sitagliptin is only available on prescription. It comes as tablets.It also comes as tablets containing a mixture of sitagliptin and another diabetes medicine called metformin. This is known by the brand name Janumet.