Type 2 diabetes not controlled by metformin alone, or where a patient is already taking both medicines separately and can be moved to one tablet. It may also be combined with a sulfonylurea or insulin.

The two work on different sides of the problem. Metformin reduces the glucose the liver releases and helps muscle take up more of it, so the background level falls. Sitagliptin blocks the DPP-4 enzyme, letting the body's own meal-time incretin hormones survive longer and prompt insulin just when food arrives.

Nausea, diarrhoea, stomach pain, loss of appetite and a metallic taste, mostly from the metformin and mostly in the first weeks. Headache and a stuffy nose from the sitagliptin.

Metformin carries a rare risk of lactic acidosis, so it is stopped temporarily during dehydration, serious illness, before surgery, and before scans using iodinated contrast dye. Seek urgent help for severe abdominal pain spreading to the back (possible pancreatitis) or for deep, rapid breathing with muscle cramps and extreme weakness. Kidney function is checked regularly, and long-term metformin can lower vitamin B12. Take the tablets with food. It is a prescription medicine in Pakistan.