Metformin: what the NHS says to expect when starting it

What is metformin used for?

The NHS describes metformin as a medicine used to treat type 2 diabetes, and says it is usually the first choice prescribed for it. It can also be prescribed to help improve ovulation in polycystic ovary syndrome, which the NHS pages call PMOS. The NHS says it reduces the amount of sugar the liver releases into the blood and helps the body respond better to insulin. The NHS also says it does not usually cause weight gain.

How is metformin taken?

The NHS says metformin comes as two kinds of tablet: standard tablets and slow-release tablets. Standard tablets act quickly and may be taken several times a day, while slow-release ones work gradually, so they are taken less often. The NHS says the prescriber sets the amount, and that when standard tablets are first started the amount is usually increased slowly, which reduces the chance of side effects. It says it is best to take metformin with, or just after, a meal, and that tablets are swallowed whole with water rather than chewed. For diabetes, the NHS says treatment is usually for life, and that stopping suddenly makes blood sugar levels go up. The prescriber's instructions and the leaflet in the pack remain the guide for any one person.

What side effects are common, and do they settle?

The NHS lists feeling sick, being sick, diarrhoea, tummy pain, loss of appetite and a change in taste among the common side effects. It says most of these should ease after a couple of weeks as the body gets used to the medicine, although some can last longer. The NHS says taking the tablets with a meal and increasing the amount slowly can reduce them. If side effects are too troublesome, it says a doctor may suggest slow-release tablets. Diabetes UK also says early stomach-related effects usually settle within a few weeks. For people on standard-release tablets who have these effects, Diabetes UK advises speaking to a GP or diabetes team, who can say whether slow-release tablets might suit them.

What about vitamin B12?

The NHS says that taking metformin for a long time can lead to vitamin B12 deficiency, which can leave a person feeling very tired. Diabetes UK likewise says prolonged use, especially at higher doses, can lower vitamin B12 levels, which matter for red blood cells and nerves. The NHS says a doctor can check B12 levels in the blood, and that vitamin B12 supplements help if levels get too low. Diabetes UK says anyone with symptoms of deficiency should ask their diabetes team or GP about a blood test.

What does the NHS say about illness, dehydration, alcohol and scans?

The NHS says that being ill with vomiting, diarrhoea or a high temperature while taking metformin carries a risk of dehydration. On alcohol, the NHS advises limiting how much is drunk while taking metformin, because alcohol can increase the risk of serious side effects, and says drinking more than its suggested limit can raise the risk of low blood sugar. The NHS also says a doctor may tell a person to stop metformin a few days before an operation or medical tests, and that it may need pausing before scans that use iodine-based dye. Anyone unsure what to do when unwell or before a procedure can ask a GP or pharmacist.

Who may not be able to take metformin?

The NHS says most adults and children aged 10 and over can take metformin, but it is not suitable for everyone. Situations to raise with a doctor before starting include a past allergic reaction to metformin or another medicine, uncontrolled diabetes, liver or kidney problems, a severe infection, heart failure treatment or a recent heart attack, and heavy alcohol use. The NHS says metformin can be taken in pregnancy, alone or with insulin, after checking with a doctor, and that it can usually be taken while breastfeeding because only tiny amounts pass into breast milk. It also says there is no evidence that metformin lowers fertility in men or women.

What if a dose is missed, or serious symptoms appear?

The NHS says not to take two doses to make up for a forgotten one. For side effects that are troublesome, the NHS says a doctor may suggest slow-release tablets, and Diabetes UK points people to a GP or diabetes team. The NHS describes serious side effects as uncommon and lists lactic acidosis (too much acid in the blood), anaphylaxis (a severe allergic reaction) and liver inflammation (hepatitis). It says to call 999 if lactic acidosis or anaphylaxis is suspected, and to call NHS 111 for any other serious side effect. NHS 111 is also the NHS route if more than the prescribed amount has been taken.