Metformin vs gliclazide: two type 2 diabetes medicines compared

Metformin and gliclazide treat the same condition but in different ways, and many people take both together. They are not interchangeable. The choice of which to start with, which to add and what to avoid depends on the individual, so the information here is for understanding, not for choosing between them.

In general terms, metformin is a first-line medicine that does not usually cause low blood sugar when taken on its own, while gliclazide is often an add-on that forces the pancreas to release more insulin and so carries a risk of hypos and weight gain.

Type 2 diabetes happens when the body does not make enough insulin or the insulin it makes does not work properly. The NHS says metformin works by lowering your blood sugar level. It is taken as tablets, a liquid or a powder, usually 1 to 3 times a day with food, and may be a long-term treatment.

Gliclazide works by increasing how much insulin your body makes. The NHS says most people take it once or twice a day, after a meal to help avoid stomach problems, at the same time each day, and usually long term. Its dose may be changed after several weeks. Because the effect depends on insulin release, gliclazide can push blood sugar too low, especially if a meal is skipped or delayed.

For metformin, the NHS lists feeling or being sick, diarrhoea, tummy pain, loss of appetite, a change in how things taste, and vitamin B12 deficiency, which can cause tiredness. Most side effects ease after a couple of weeks. Rarely, too much acid in the blood (lactic acidosis) can occur, and the NHS says to call 999 if you think this might be happening. It may not be suitable if you have liver or kidney problems, a severe infection, heart failure or drink heavily, and illness with vomiting or diarrhoea increases the risk of dehydration, so ask a pharmacist or doctor for advice.

For gliclazide, the NHS says a hypo can make you feel dizzy, sick, very hungry or very tired, and that if it happens you should eat or drink something sugary and see your GP if it is frequent. It may not be suitable if you have severe kidney or liver disease or acute porphyria. It can interact with other blood-sugar medicines, with semaglutide or tirzepatide weight-loss medicines, some antibiotics such as clarithromycin, and some blood pressure or heart failure medicines. The NHS also says to follow dietary advice and not skip or delay meals.

The NICE guideline on type 2 diabetes in adults (NG28, last updated February 2026) recommends that adults with type 2 diabetes and no relevant comorbidity are offered modified-release metformin and an SGLT-2 inhibitor, and an SGLT-2 inhibitor alone if metformin is contraindicated or not tolerated. The guideline explains that modified-release metformin is associated with fewer gastrointestinal side effects and is likely to be better adhered to, though standard-release may be preferable for people with swallowing difficulty.

For people who need further medicines to reach individualised targets, NICE lists a sulfonylurea, pioglitazone or insulin-based treatment among options in different groups, and notes that sulfonylureas might increase hypoglycaemic events and weight gain. For people with worsening kidney function it notes that a sulfonylurea could increase hypoglycaemia risk and so would not be a good option for that group. The guideline is for clinicians; decisions are made with your diabetes team.

Do not stop either medicine without advice; the NHS says that if you stop metformin your blood sugar level may start to rise again, and gliclazide should not be stopped unless your doctor tells you to. Mention both to anyone who prescribes you new medicines, including anti-inflammatory painkillers, which the NHS lists as a potential interaction with metformin. Keep a hypo plan if you take gliclazide, especially if you drive.