Metformin is one of the most commonly prescribed medicines for type 2 diabetes. If you have recently been told you have type 2 diabetes, prediabetes, gestational diabetes or polycystic ovary syndrome, you may hear metformin mentioned early in the conversation.
For many people, metformin is the first diabetes tablet offered because it is well understood, effective, inexpensive and does not usually cause weight gain. It can help lower blood sugar and improve how the body responds to insulin. But like any medicine, it can cause side effects, and it needs to be used safely.
This guide explains what metformin does, who may be prescribed it, how it is usually taken, common side effects, vitamin B12, kidney checks, alcohol, missed doses and when to speak to a GP, pharmacist or diabetes nurse.
For a wider overview of diabetes, read Diabetes Explained: Symptoms, Types, Tests and Treatment in the UK. If you are wondering whether diet and sugar caused diabetes, see Can You Get Diabetes from Eating Too Much Sugar?.
What is metformin?
Metformin is a medicine used mainly to treat type 2 diabetes. It belongs to a group of medicines called biguanides. It is available as standard-release tablets, slow-release tablets and sometimes as liquid medicine.
In the UK, metformin may be prescribed under the generic name “metformin” or under brand names such as Glucophage, Glucient, Metabet, Diagemet or Axpinet. The brand may vary depending on what your pharmacy supplies.
Metformin is most commonly used for type 2 diabetes, but it may also be used in gestational diabetes and polycystic ovary syndrome, also called PCOS. In PCOS, it may be used because insulin resistance can be part of the condition.
Metformin is not insulin. It does not replace insulin, and it is not normally used to treat type 1 diabetes on its own. People with type 1 diabetes need insulin because their body makes little or no insulin.
If you are unsure about the difference between diabetes types, read Type 1 vs Type 2 Diabetes: What’s the Difference?.
How metformin works
Metformin helps lower blood glucose in several ways. Its main effect is to reduce the amount of glucose released by the liver. The liver naturally stores and releases glucose, especially between meals and overnight. In type 2 diabetes, the liver can release too much glucose into the bloodstream.
Metformin also helps the body respond better to insulin. This is important because type 2 diabetes is often linked with insulin resistance, where insulin is present but does not work as effectively as it should.
Metformin may also slightly reduce glucose absorption from the gut and can have effects on appetite and weight in some people. It is not a weight loss drug, but it is generally weight-neutral and may support modest weight loss for some.
Unlike some diabetes medicines, metformin does not usually cause low blood sugar when taken on its own. This is because it does not force the pancreas to release extra insulin. However, the risk of low blood sugar can be higher if metformin is taken with insulin or certain other diabetes medicines.
You may not “feel” metformin working. Many people do not notice an obvious day-to-day effect, especially if they did not have strong symptoms before starting it. That does not mean it is not helping. Its effect is usually seen in blood test results, especially HbA1c.
Who is metformin prescribed for?
Metformin is most often prescribed for people with type 2 diabetes, especially when HbA1c is above target and lifestyle changes alone are unlikely to be enough. It may also be used when someone has type 2 diabetes and is living with overweight or insulin resistance.
It may be considered in:
- type 2 diabetes
- gestational diabetes, if diet and activity changes are not enough
- PCOS, particularly where insulin resistance is suspected
- some people at high risk of type 2 diabetes, depending on clinical judgement
Metformin is not suitable for everyone. Your GP or clinician will consider your kidney function, liver health, alcohol intake, other medicines, age, pregnancy status, symptoms and overall health before prescribing it.
Kidney function is especially important because metformin is cleared from the body through the kidneys. If kidney function is reduced, the dose may need adjustment, extra monitoring, or a different medicine may be more suitable.
If you have been told you have prediabetes rather than diabetes, read What Is Prediabetes and Can It Be Reversed?. Lifestyle changes are usually the first step, but medication may be considered for some people at higher risk.
How and when to take metformin
Always follow the instructions from your GP, diabetes nurse or pharmacist. Metformin is usually started at a low dose and increased gradually. This helps reduce stomach-related side effects.
Standard-release metformin is often taken once, twice or three times a day with meals. Slow-release metformin is usually taken once daily, often with the evening meal, but your instructions may differ.
Taking metformin with food can reduce nausea, stomach pain and diarrhoea. NHS medicines guidance says it is best to take metformin with a meal to reduce side effects.
Do not crush or chew slow-release tablets unless your pharmacist says it is safe. Slow-release tablets are designed to release the medicine gradually.
If you miss a dose, check the patient information leaflet or ask a pharmacist. In general, do not double up to make up for a missed dose unless a healthcare professional specifically tells you to. Taking extra can increase side effects.
If you cannot tolerate metformin, do not simply stop and ignore diabetes treatment. Speak to your GP or diabetes nurse. Options may include lowering the dose, increasing more slowly, switching to slow-release metformin, or using a different diabetes medicine.
Common side effects of metformin
The most common side effects of metformin affect the stomach and digestion. NHS and NICE list common adverse effects including nausea, vomiting, diarrhoea, stomach pain, loss of appetite and taste disturbance.
Common side effects include:
- feeling sick
- being sick
- diarrhoea
- stomach cramps or abdominal pain
- loss of appetite
- metallic taste in the mouth
- wind or bloating
These side effects are often worse when first starting metformin or after a dose increase. They may settle as the body adjusts.
Practical steps that may help include:
- taking metformin with or after food
- starting at a low dose and increasing slowly
- avoiding very rich, greasy or large meals when starting
- asking about slow-release metformin if symptoms persist
- speaking to your GP or pharmacist before stopping
Do not ignore severe or persistent diarrhoea, repeated vomiting, dehydration or weight loss. These need medical advice, especially if you are older, have kidney problems, or take blood pressure tablets, diuretics or other medicines affected by dehydration.
Vitamin B12 and long-term metformin use
One side effect that is easy to miss is low vitamin B12. This can happen with long-term or high-dose metformin. The MHRA says reduced vitamin B12 levels are now considered a common side effect of metformin, especially in people taking higher doses or using it for a long time. It advises checking B12 levels in people with symptoms of deficiency and considering periodic monitoring in people with risk factors.
Vitamin B12 is important for red blood cells, nerves and brain function. Low B12 can cause symptoms such as:
- unusual tiredness
- breathlessness
- feeling faint
- pins and needles
- numbness or burning sensations
- mouth ulcers or a sore tongue
- memory or concentration problems
- low mood or irritability
These symptoms can overlap with diabetes symptoms or diabetic neuropathy, so it is important not to assume all tingling or numbness is caused by diabetes itself. If you take metformin and develop nerve symptoms, tiredness or anaemia, ask whether B12 should be checked.
People may be at higher risk of B12 deficiency if they take metformin long term, take higher doses, follow a vegan diet, have stomach or bowel conditions, take acid-suppressing medicines long term, or have had certain types of stomach surgery.
For more information, read B12 and Folate Blood Test Results Explained and Numbness and Tingling.
Kidney function, illness and safety checks
Metformin is usually safe for many people, but kidney function matters. Your GP will usually check kidney function with blood tests such as creatinine and eGFR before and during treatment.
If kidney function is reduced, metformin may need a lower dose or closer monitoring. In more severe kidney disease, it may not be suitable. This is because metformin can build up in the body if the kidneys are not clearing it properly.
A rare but serious complication linked with metformin is lactic acidosis. This is very uncommon, but risk is higher in situations such as severe kidney impairment, severe dehydration, severe infection, low oxygen levels, heavy alcohol use or serious liver disease.
Contact a doctor urgently if you feel very unwell while taking metformin, especially with symptoms such as:
- severe weakness
- fast or difficult breathing
- severe drowsiness or confusion
- severe vomiting or diarrhoea
- dehydration
- severe abdominal pain
- feeling cold, dizzy or faint
If you are unwell with vomiting, diarrhoea, fever, dehydration or not eating and drinking properly, ask your GP, pharmacist or diabetes team about sick-day rules. Some medicines, including metformin, may need to be paused temporarily during significant dehydration or serious illness, but this should be guided by a healthcare professional or local sick-day advice.
You may find these guides useful: Kidney Blood Test Results Explained, eGFR and Creatinine Results Explained, Dehydration Symptoms in Adults and Children and Diarrhoea and Vomiting in Adults.
Metformin, weight, alcohol and diet
Metformin is not a substitute for diet and lifestyle changes. It can help lower blood glucose, but food, activity, weight, sleep, blood pressure and cholesterol still matter.
Metformin does not usually cause weight gain. Some people lose a small amount of weight, especially if it reduces appetite or supports better blood sugar control. But it is not a weight loss injection and should not be used casually for weight loss without a medical reason.
Diet still plays a major role in type 2 diabetes management. A diabetes-friendly diet usually focuses on more vegetables, high-fibre carbohydrates, beans, lentils, lean proteins, healthy fats in sensible amounts, fewer sugary drinks and smaller portions of refined carbohydrates.
Alcohol should be used carefully with metformin. Heavy drinking or binge drinking can increase the risk of low blood sugar in some situations and may increase the risk of lactic acidosis, especially if combined with dehydration, liver disease or not eating properly. If you drink, keep within UK low-risk drinking guidelines and ask your GP or pharmacist if alcohol is safe with your overall health and medicines.
For more on food and diabetes, read Diabetes and Diet: What to Eat and What to Avoid. For weight-related guidance, see How Weight Loss Really Works and Weight Loss Injections in the UK.
Metformin with other diabetes medicines
Metformin is often used on its own at first, but some people need additional medicines if HbA1c remains above target or if there are other reasons to add treatment.
Other diabetes medicines may include:
- SGLT2 inhibitors
- DPP-4 inhibitors
- sulfonylureas
- GLP-1 receptor agonists or related injectable medicines
- insulin
Which medicine is recommended depends on HbA1c, weight, kidney function, heart disease risk, side effects, cost, personal preference and other medical conditions.
Metformin by itself does not usually cause low blood sugar, but the risk of hypos can increase if it is combined with insulin or sulfonylureas. Symptoms of low blood sugar can include sweating, shaking, hunger, anxiety, palpitations, confusion, dizziness and weakness.
If you are prescribed several diabetes medicines, make sure you understand what each one does, when to take it, what side effects to watch for, and whether it can cause hypos. Your pharmacist can be a very useful source of medicine advice.
If you are unsure what your blood sugar results mean, read Understanding Your Diabetes Blood Test Results from Your GP.
When to speak to your GP, pharmacist or diabetes nurse
Speak to a healthcare professional if metformin is causing side effects, if you are unsure how to take it, or if your blood sugar remains above target despite taking it.
You should ask for advice if:
- diarrhoea, nausea or stomach pain does not settle
- you cannot keep tablets down
- you are losing weight unintentionally
- you have symptoms of B12 deficiency
- you develop numbness, tingling or burning pain
- you have kidney disease or your eGFR has changed
- you are dehydrated, vomiting or have severe diarrhoea
- you are planning pregnancy or become pregnant
- you are due a scan or procedure involving contrast dye
- you want to make major diet changes, especially low-carb changes, while on diabetes medicines
Do not stop metformin long term without discussing an alternative plan. If you cannot tolerate it, there are options. Many people do better with slow-release metformin, a lower dose, slower dose increases or a different medicine.
If you are using private healthcare, make sure your NHS GP knows about diabetes medicines prescribed privately. This helps avoid duplication, interactions and gaps in monitoring. You may find Private GP Services in the UK: Costs, Private Health Check Cost in the UK and NHS vs Private Healthcare helpful.
Frequently asked questions
What does metformin do?
Metformin helps lower blood glucose mainly by reducing glucose release from the liver and improving how the body responds to insulin. It is most commonly used for type 2 diabetes.
Is metformin insulin?
No. Metformin is a tablet or liquid medicine. It is not insulin. People with type 1 diabetes need insulin because their body makes little or no insulin.
Does metformin cause weight loss?
Metformin is not a weight loss drug, but it does not usually cause weight gain and some people lose a small amount of weight while taking it.
What are the most common side effects of metformin?
The most common side effects are nausea, diarrhoea, stomach pain, being sick, loss of appetite, wind, bloating and a metallic taste. These often happen when starting treatment or increasing the dose.
How can I reduce metformin stomach side effects?
Taking metformin with food, starting at a low dose, increasing gradually and asking about slow-release metformin can help. Speak to your GP or pharmacist if symptoms persist.
Can metformin cause vitamin B12 deficiency?
Yes. Long-term or high-dose metformin can reduce vitamin B12 levels in some people. Symptoms can include tiredness, breathlessness, pins and needles, numbness, mouth ulcers and memory or mood changes.
Do I need regular blood tests on metformin?
Yes. Your GP will usually monitor HbA1c and kidney function. Vitamin B12 may be checked if you have symptoms of deficiency or risk factors.
Can metformin damage the kidneys?
Metformin is cleared by the kidneys, so kidney function needs monitoring. It is not usually described as damaging the kidneys, but it may not be suitable or may need dose adjustment if kidney function is reduced.
Can I drink alcohol with metformin?
Small amounts may be acceptable for some people, but heavy drinking or binge drinking can be risky, especially with dehydration, liver disease or poor food intake. Ask your GP or pharmacist for personal advice.
What happens if I miss a dose of metformin?
Follow the patient information leaflet or ask a pharmacist. In general, do not take a double dose to make up for a missed dose unless specifically told to by a healthcare professional.
Can I stop metformin if my blood sugar improves?
Do not stop without discussing it with your GP or diabetes nurse. If your HbA1c has improved, your treatment plan may be reviewed, but stopping medicine should be done safely and with monitoring.
Is slow-release metformin better?
Slow-release metformin can be easier to tolerate for some people, especially if standard metformin causes diarrhoea or stomach upset. It is not automatically better for everyone, so ask your GP or pharmacist.
