First-line for type 2 diabetes and widely used in PCOS. The gut side effects are largely avoidable with the right approach.
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Metformin is a biguanide. It reduces the amount of glucose the liver releases, improves how well the body responds to its own insulin, and slightly reduces glucose absorption from the gut.
It has been in use since the 1950s, it is inexpensive, and it does not cause hypoglycaemia on its own — which together make it the first-line treatment for type 2 diabetes worldwide.
Always with or immediately after food. On an empty stomach it causes considerably more nausea and diarrhoea.
Start low and increase slowly — typically one tablet daily for a week or two before going up. Most of the gut side effects that make people abandon metformin come from starting at too high a dose too quickly.
Swallow modified-release tablets whole. If standard metformin upsets your stomach, the modified-release form is much better tolerated and is worth asking about rather than giving up on the drug.
If you miss a dose, take the next one as normal.
Gut effects are the main issue: nausea, diarrhoea, abdominal discomfort, wind and a metallic taste. They affect a large minority, are worst at the start, and usually settle within a few weeks. Taking it with food, escalating slowly, and switching to modified-release resolve most cases.
B12 deficiency is the under-recognised one. Long-term metformin reduces B12 absorption, and the resulting deficiency causes fatigue and tingling that can be mistaken for diabetic neuropathy — leading to the wrong treatment. B12 should be checked periodically in anyone on long-term metformin, and frequently is not.
Lactic acidosis is very rare but serious. It occurs almost exclusively where metformin has accumulated because of kidney impairment or dehydration. Deep rapid breathing, severe weakness, muscle pain and vomiting need emergency assessment.
Sick day rules matter and are frequently never given. Stop metformin temporarily if you are vomiting, have significant diarrhoea, or are otherwise dehydrated, and restart when you are eating and drinking normally.
It also needs pausing around scans using iodinated contrast dye.
Before starting and periodically after: kidney function — this is the one that determines whether metformin is safe and at what dose.
HbA1c every three to six months to assess control.
B12 periodically with long-term use.
Interactions: alcohol, iodinated contrast media, diuretics, ACE inhibitors and NSAIDs where they affect kidney function, and steroids which raise glucose.
Yes, where clinically appropriate. Diabetes and PCOS both assess well remotely because they are managed on blood results and history.
What that involves: HbA1c, kidney function and a lipid profile before starting; a discussion of diet and activity that is not a token one, since they remain the foundation of treatment; slow dose escalation; and repeat bloods to confirm effect.
What must still happen in person: diabetic foot checks and retinal screening. Neither can be done remotely by anyone, and both are essential parts of diabetes care. We will tell you what you still need and where it fits rather than leave you assuming a remote review covers everything.
This page is information, not an offer to supply.
Metformin is one of the cheapest medicines in the world. It is long off patent, and the standard tablet costs very little.
On a private prescription the drug cost plus dispensing fee is usually less than the England NHS prescription charge of around £10. NHS prescriptions are free in Wales, and in England people with treated diabetes qualify for a medical exemption certificate — free NHS prescriptions for everything, not just the diabetes drugs. That is worth applying for, and a surprising number of people never do.
The modified-release version costs more and is no more effective, but it causes considerably less diarrhoea and nausea.
That is a fair reason to pay the difference if the standard tablet is making you miserable — and switching to it is a far better answer than abandoning the drug.
Metformin needs no taper and causes no withdrawal. Blood glucose simply drifts back up over weeks.
Stop metformin while you are dehydrated: vomiting, diarrhoea, or a fever where you are not drinking properly. Restart once you have been eating and drinking normally for 24 to 48 hours.
This is not caution for its own sake. Metformin is cleared by the kidneys, and if the kidneys are struggling it accumulates, which is how the rare but serious lactic acidosis happens.
Also pause it before a scan using iodinated contrast dye, and before surgery. The department should tell you — but ask, because they often assume someone else has.
Most people who stop metformin do so because of diarrhoea, and most of those never tried the two things that fix it: taking it with food, and switching to the modified-release preparation.
Start low and increase slowly — going straight to a full dose is the commonest avoidable cause of intolerable side effects.
Metformin usually stays in place when another drug is added rather than being replaced, because it remains first-line and works differently from most alternatives.
Metformin works partly in the gut, so nausea, loose stools and wind are common early on.
Take it with or just after food, start at a low dose, and increase slowly. If that is not enough, the modified-release version is much better tolerated.
On its own, essentially no. Metformin lowers glucose without pushing insulin, so it does not cause low blood sugar the way gliclazide or insulin can.
Combined with those drugs, hypos become possible — the risk comes from the partner drug.
Usually a small amount, a few kilograms at most. It is weight-neutral to mildly helpful rather than a weight loss drug.
Anyone promising more than that from metformin alone is overselling it.
Long-term metformin reduces B12 absorption, and low B12 causes fatigue, pins and needles and poor concentration.
It is very frequently missed, and mistaken for diabetic nerve damage — which matters, because one is reversible and the other largely is not.
Yes, while you are vomiting or have diarrhoea and are not drinking well. Restart after a day or two of normal eating.
Also pause before a contrast scan or surgery.
In moderation. Heavy or binge drinking meaningfully increases the risk of lactic acidosis and should be avoided.
Because PCOS usually involves insulin resistance, and improving that can help cycle regularity and, in some cases, ovulation.
It is not a licensed use, and the evidence is more modest than the internet suggests — worth knowing before you expect too much.
Not necessarily. Substantial weight loss can put type 2 diabetes into remission, and metformin can then be reduced or stopped.
That has to be guided by HbA1c, not by feel.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
August 24, 2026
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