Metformin: uses, side effects and cautions

Metformin belongs to the biguanide group. The NHS describes the job simply: it lowers your blood sugar level. The emc product information adds the detail that it reduces the amount of glucose the liver releases and helps tissues take up glucose, and that on its own it does not stimulate insulin release, so it does not cause hypoglycaemia when used alone.

That last point is a big reason it is a common starting choice. A person newly diagnosed with type 2 diabetes who is given metformin is not usually at risk of the sudden low blood sugars that can happen with sulfonylurea tablets or insulin, although the picture changes when it is combined with those medicines.

The NHS page also notes use in gestational diabetes and in PMOS, and, with insulin, in type 1 diabetes. Whether metformin suits any one person is a prescriber decision.

Metformin comes as tablets, a liquid and a powder that is stirred into water. Tablets are swallowed whole. The NHS advises taking it with or just after food, which is the main trick for reducing stomach upset.

The emc product information describes starting with a low dose and raising it gradually over roughly ten to fifteen days, guided by blood glucose results. That slow build-up is there because the digestive effects are worst at first. Modified-release versions exist (the emc lists Glucophage SR among them) and are taken differently, so always follow the label on your own box.

The exact amount depends on the condition being treated, your age and your kidney function. Follow your prescriber and the leaflet rather than any figure you see online.

According to the NHS, common side effects include feeling or being sick, diarrhoea, stomach pain, loss of appetite and a metallic or altered taste. Many people find these settle within a couple of weeks, though the NHS notes some can last longer. Tell your prescriber if they do not, because a different formulation sometimes helps.

Long-term use can lower vitamin B12. The NHS lists it as a common effect that can show up as tiredness, and the emc product information asks for B12 to be monitored, particularly with prolonged use or other risk factors.

Rare but serious problems need fast action. The NHS describes lactic acidosis (vomiting, stomach pain, muscle cramps, breathing difficulty), anaphylaxis and hepatitis with jaundice. It advises calling 999 for lactic acidosis or anaphylaxis and NHS 111 for other serious effects.

• Call 999: swelling of the throat or tongue, itchy rash with breathing difficulty, or signs of lactic acidosis.

• Call NHS 111: yellowing of the skin or eyes, or other effects you think are serious.

• Speak to your pharmacist or prescriber: side effects that are bothersome but not severe, or that last beyond a few weeks.

The NHS says metformin is not suitable if you have had an allergic reaction or serious side effect to it before, have liver or kidney problems, have a severe infection of the lung or kidney, have heart failure, or drink excessive amounts of alcohol. The emc information treats kidney function as a firm limit: it is contraindicated once kidney filtration falls below a stated threshold.

Illness that causes dehydration matters. The NHS asks you to tell your doctor if you have vomiting, diarrhoea or a fever. The emc information says treatment should be stopped around iodinated contrast scans and at the time of surgery, and restarted only once kidney function is confirmed to be acceptable. Your care team will tell you what applies to you.

The NHS states that metformin passes into breast milk only in tiny amounts and that it can be used in pregnancy, but you should talk to your doctor first. Keeping blood sugar controlled during pregnancy is important for avoiding complications. The emc product information is more cautious about breastfeeding and describes limited data.

On alcohol, the NHS says to try to limit what you drink because alcohol can raise the risk of serious side effects. On driving, metformin on its own does not usually affect it; avoid driving if you feel dizzy.

The NHS names diuretics, NSAID painkillers such as ibuprofen and diclofenac, and some blood pressure medicines as ones that can interact, usually by affecting the kidneys. Tell any prescriber or pharmacist that you take metformin before accepting a new medicine, including over-the-counter pain relief.

If you take metformin once a day and forget, the NHS says take it when you remember unless the next dose is close. If you take it several times a day and the next dose is near, skip the missed one. Never take two doses together.

If your sugar levels rise after stopping, the NHS notes that is expected, so talk to your doctor if you are having problems rather than stopping alone.

Metformin is prescription only. The emc lists Glucophage among its UK products, alongside many generic tablets. If you think a medicine has caused harm, you can report it yourself through the Yellow Card scheme.