A biguanide (first-line type 2 diabetes medicine)
Metformin
The usual first-choice tablet for type 2 diabetes — it lowers blood sugar without causing weight gain and, taken alone, without causing hypoglycaemia.
What is Metformin?
Metformin is the standard first-line medicine for type 2 diabetes. It lowers blood sugar mainly by reducing the amount of glucose the liver releases and helping the body respond better to its own insulin. On its own it does not usually cause low blood sugar (hypos) or weight gain, and it has decades of evidence behind it. Digestive upset is the common early side effect and often settles.
Education and reference only. This is a plain-language guide to Metformin — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Metformin is the most widely used medicine for type 2 diabetes and is normally the first tablet offered when diet and lifestyle changes alone are not enough to control blood sugar. It has a long track record, is inexpensive, and unlike some diabetes medicines it does not usually cause weight gain or, when taken on its own, low blood sugar. It is sometimes also used in other conditions linked to insulin resistance, such as polycystic ovary syndrome. It is taken as a long-term tablet, and a slow-release version is available for people who get digestive side effects.
How it works
Metformin lowers blood sugar in a few ways at once. Its main action is to reduce the amount of glucose the liver makes and releases into the blood, particularly overnight. It also helps the body's muscles and tissues respond better to insulin (reducing insulin resistance) and slightly reduces how much glucose is absorbed from food. Because it does not force the pancreas to release extra insulin, it does not usually cause the blood sugar to drop too low on its own — a key advantage over some other diabetes treatments.
What it treats
Conditions Metformin is used for
Practical use
How to take Metformin
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Take it with or just after a meal to reduce stomach upset; if you are starting it, the amount is usually built up slowly for the same reason.
- Swallow slow-release (SR/MR) tablets whole — do not crush or chew them; you may occasionally see the empty tablet shell in your stool, which is normal.
- Follow "sick-day rules": pause metformin if you become dehydrated from vomiting, diarrhoea or a high fever, and restart once you are eating and drinking normally.
- Tell any hospital or radiology department you take metformin before a scan using contrast dye, or before an operation.
- If you miss a dose, take it with your next meal — do not double up.
Weighing it up
Advantages & disadvantages of Metformin
Advantages
- Does not cause weight gain (and may help with modest weight loss), and on its own does not cause hypoglycaemia.
- Decades of evidence, low cost, and possible cardiovascular benefit.
- Combines well with almost all other diabetes medicines as treatment is stepped up.
Disadvantages
- Digestive side effects are common early on and put some people off (the slow-release form helps).
- Must be paused during dehydrating illness and around contrast scans, and is limited or avoided in significant kidney impairment.
- Can lower vitamin B12 over the long term, needing occasional checks.
Practical use
Good to know
Digestive side effects — nausea, a metallic taste, loose stools or stomach discomfort — are common in the first few weeks and are the main reason people struggle with it. Taking it with or just after food, building up slowly, or switching to the slow-release (SR/MR) form usually helps a great deal. Metformin is best paused during acute illnesses that cause dehydration (vomiting, diarrhoea, fever) — the "sick-day rules" — and around procedures using X-ray contrast dye, because dehydration can, very rarely, allow a serious problem called lactic acidosis to develop. Long-term use can lower vitamin B12, which is checked from time to time.
Who should not take it / use with caution
- People with significantly reduced kidney function (the level of kidney function is checked before starting and monitored, and metformin is reduced or stopped if it falls too low).
- During serious acute illness with dehydration, severe infection, or conditions reducing oxygen supply to tissues, and around major surgery or contrast scans.
- People with significant liver disease or a history of lactic acidosis; alcohol excess adds to the risk.
Monitoring
- Blood sugar control (HbA1c) to guide treatment
- Kidney function (before starting and regularly)
- Vitamin B12 on longer-term use, especially if symptoms suggest it
Side effects
- Very common early on: nausea, metallic taste, reduced appetite, wind, stomach upset and diarrhoea — usually settling with time, food and slow build-up, or the SR form.
- Long-term: it can lower vitamin B12, which is monitored and replaced if needed.
- Very rare but serious: lactic acidosis (deep or rapid breathing, severe drowsiness, muscle pain, feeling very unwell) — seek urgent medical help; this is far more likely if you are dehydrated or your kidneys are struggling.
Key interactions
- Alcohol in excess, and dehydration from any cause, increase the small risk of lactic acidosis.
- X-ray contrast dye and some medicines that affect the kidneys can raise metformin levels — metformin is often paused around these.
- Medicines that raise blood sugar (such as steroids) may reduce its effect, while combining it with insulin or sulfonylureas increases the chance of low blood sugar from those medicines.
Available as: Standard tablets, slow-release (SR/MR) tablets for people who get digestive side effects, and a liquid for those who cannot swallow tablets.
Answers
Metformin: frequently asked questions
Does metformin cause low blood sugar (hypos)?
Taken on its own, metformin does not usually cause hypoglycaemia, because it does not force the pancreas to release extra insulin. The risk of hypos comes when it is combined with insulin or a sulfonylurea — in that case, know the symptoms and how to treat a hypo.
Why does metformin upset my stomach, and what helps?
Nausea, loose stools and a metallic taste are common in the first few weeks. Taking it with or just after food, building the amount up slowly, and switching to the slow-release (SR/MR) form all reduce these effects, which usually settle with time.
Do I need to stop metformin if I am unwell or having a scan?
Yes in certain situations. Pause it if you become dehydrated from vomiting, diarrhoea or a high fever ("sick-day rules"), and tell radiology before any scan using contrast dye. This is to avoid a rare but serious problem called lactic acidosis, which is more likely when you are dehydrated or the kidneys are struggling.
Does metformin help with weight loss?
Metformin is generally weight-neutral and may lead to modest weight loss in some people, which is one reason it is preferred over diabetes medicines that cause weight gain. It is not a weight-loss drug in its own right, and any effect is usually small.
Is Glucophage the same as metformin?
Yes — metformin is the generic (active-ingredient) name and Glucophage is a brand name; both contain the same active ingredient. Glucophage SR is the slow-release version.
Authoritative sources
- BNF: Metformin hydrochloride.
- electronic Medicines Compendium (SmPC): Metformin.
- NICE NG28: Type 2 diabetes in adults – management.
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