Metformin is a prescription tablet that lowers blood glucose, and it is usually the first medicine prescribed for type 2 diabetes. It does not cause weight gain, and on its own it is unlikely to cause hypos (healthdirect — Metformin). In Australia it is sold as Diabex, Diaformin and many other brands, and only on a doctor's prescription.

How does metformin work?

It lowers blood glucose in two ways: the liver releases less of the glucose it stores, and your cells absorb more glucose from the blood (healthdirect). Metformin is a biguanide; it helps your body make better use of the insulin your pancreas still makes (Diabex CMI). It also lowers LDL ("bad") cholesterol (healthdirect). After starting, it can take up to two weeks for blood glucose to be properly controlled (Diabex CMI).

How is metformin taken, and at what dose?

With food, at the same times each day. Take it during or straight after a meal, which reduces stomach upsets. The usual adult starting dose is 500 mg once or twice a day; your doctor adjusts it to your blood glucose, up to a maximum of 1000 mg three times a day, and older people and people with kidney problems may need less (Diabex CMI). Modified-release (extended-release) tablets, such as Diabex XR, are another form (PBS 9435N). Do not skip meals, and never take a double dose to make up for a missed one (Diabex CMI). Your doctor decides the dose.

What are the side effects of metformin?

Mostly stomach upsets at the start: nausea, vomiting, diarrhoea, stomach pain, loss of appetite and a changed sense of taste. They are generally mild and pass after the first few weeks (Diabex CMI; healthdirect). Two effects need more attention:

  • Lactic acidosis, a build-up of lactic acid in the blood: very rare, but serious and can be fatal. Trouble breathing, feeling weak or unwell, unusual muscle pain, sleepiness, shivering or feeling extremely cold, or a slow heartbeat mean a doctor or the emergency department straight away. The risk is higher with kidney problems, older age and doses above 2 g a day (Diabex CMI).
  • A low vitamin B12 level, found only by blood tests, so your doctor may check it along with your kidneys and liver (Diabex CMI).

Does metformin help with weight, or cause hypos?

It does not cause weight gain, but it is prescribed for blood glucose, not to lose weight. healthdirect says it may help people lose weight, which makes it a good choice for overweight people with the condition; the leaflet calls it especially useful for them, and says not to take it for any other complaint unless your doctor tells you to (Diabex CMI). On its own it does not normally cause hypoglycaemia (low blood glucose); it can when taken with a sulfonylurea, with repaglinide or with insulin (Diabex CMI). Glimepiride is one of the sulfonylureas (healthdirect — Diabetes medicines).

Who should not take metformin, and when is it paused?

People with severe kidney or liver disease should not take it, and it is not taken while breastfeeding (healthdirect). The Diabex leaflet adds metabolic acidosis, heavy or binge drinking, dehydration, a severe infection, a recent heart attack or severe heart failure, and pancreatitis, and says not to take it in pregnancy, when insulin is used instead (Diabex CMI).

It is paused around major surgery and scans that need an injection of iodinated contrast dye, for as long as your doctor decides. Tell your doctor if you become ill, dehydrated or seriously infected: the risk of lactic acidosis rises (Diabex CMI).

What are metformin's brand names in Australia?

Diabex, Diaformin, Formet, Glucobete, Metformin Sandoz and others: standard metformin tablets are on the PBS General Schedule under these names (PBS 2430X), modified-release ones as Diabex XR, Diaformin XR and others (PBS 9435N), and "no matter which brand you are prescribed, metformin works in the same way" (healthdirect). Glucophage, the name our shop's metformin page uses, returns no result on the Australian register and is not among the PBS brands (ARTG: Glucophage; ARTG: metformin).

Can metformin be taken with other medicines?

Yes, but some change how it works. Adults can take it alone, with other blood-glucose tablets or with insulin. The leaflet's interaction list includes other blood-glucose medicines, some blood-pressure and heart medicines, fluid tablets, anti-inflammatory painkillers such as ibuprofen, steroids and topiramate; your doctor may change a dose (Diabex CMI). Semaglutide did not change metformin levels in a clinically relevant way (EMA — Rybelsus), but the Rybelsus tablet is taken alone and other tablets wait: can you eat or take tablets sooner than 30 minutes after Rybelsus? Viagra is on neither medicine's interaction list: can metformin be taken with Viagra?

The short answer

Metformin is the first-line treatment for most people once diet and exercise are not enough: taken with meals, no weight gain, and unlikely to cause hypos on its own. Know the signs of lactic acidosis and go easy on alcohol. If it is no longer enough on its own, your doctor may add another medicine (healthdirect); the options include gliptins such as sitagliptin, sulfonylureas and GLP-1 medicines such as semaglutide (healthdirect — Diabetes medicines). The classes are compared by goal in our guide "Type 2 Diabetes Medicines in Australia".