Metformin

What is metformin?

Metformin is an oral antihyperglycemic medicine in the biguanide class. In current U.S. prescribing information, metformin is indicated to improve glycemic control in people with type 2 diabetes, alongside diet and exercise. It lowers both fasting and after-meal glucose without stimulating the pancreas to produce more insulin. ^[1]

Metformin is prescribed worldwide, but the exact approved indications, formulations, strengths, age limits and dosing instructions depend on the country and product. For example, current NHS information also describes metformin use for gestational diabetes and PCOS/PMOS, while U.S. labels cited here focus on type 2 diabetes. ^[4]

What is metformin used for?

Type 2 diabetes

Type 2 diabetes is the core use. Metformin may be used alone or with other glucose-lowering medicines, including insulin. Modern treatment guidelines increasingly choose therapy according to cardiovascular, kidney, weight and other clinical factors rather than assuming the same first-line combination for everyone. NICE's 2026 update, for example, recommends modified-release metformin together with an SGLT-2 inhibitor for many adults starting treatment, while making different recommendations for particular comorbidities and levels of kidney function. ^[5]

Gestational diabetes

Metformin is used for diabetes that develops during pregnancy in some health systems. Whether it is appropriate depends on local guidance and the individual's clinical circumstances. ^[4]

PCOS

Metformin is also used in polycystic ovary syndrome (PCOS) in selected patients. International evidence-based PCOS guidance supports metformin for metabolic outcomes in adults with PCOS, particularly when BMI is 25 kg/m² or higher, and describes additional uses in adolescents and fertility care. In some jurisdictions this is an off-label use. ^[7]

Weight loss

Weight reduction can occur during metformin treatment, and weight effects may matter when diabetes medicines are selected. However, the U.S. metformin labels cited on this page do not approve metformin as a general weight-management medicine. It should not be self-started solely for weight loss. ^[5]

How metformin works

Metformin lowers glucose through several complementary effects. U.S. prescribing information describes reduced glucose production by the liver, reduced intestinal glucose absorption and improved insulin sensitivity, which increases peripheral glucose uptake and use. Insulin secretion itself does not increase. ^[1]

This is one reason metformin on its own is not usually a major cause of hypoglycaemia. The risk of low blood sugar becomes more important when metformin is combined with insulin or an insulin secretagogue such as a sulfonylurea. ^[1]

Forms and strengths

Metformin is available as immediate-release tablets, extended- or modified-release tablets, and in some markets oral liquid or powder formulations. Product-specific strengths differ. Current U.S. labels cited here include immediate-release tablets of 500 mg, 850 mg and 1,000 mg, while the current GLUMETZA extended-release label lists 500 mg and 1,000 mg tablets. ^{[1] [2]}

Do not assume that two products with the same total amount of metformin are interchangeable in how they are taken. Release profile, strength, combination ingredients and local product instructions matter.

How to take metformin

Metformin is usually taken with food to improve gastrointestinal tolerability. Immediate-release products may be taken more than once daily, while some extended-release products are taken once daily. Extended-release tablets should generally be swallowed whole rather than crushed, cut or chewed. ^{[2] [3]}

If you miss a dose

General patient information advises taking the missed dose when you remember unless it is nearly time for the next one, and not taking a double dose. Some extended-release labels instead instruct patients to resume the next scheduled dose without doubling. Follow the leaflet for your own formulation. ^{[2] [3]}

Vomiting, diarrhoea, fever or dehydration

Illness that causes dehydration can change the safety of metformin. NHS guidance advises contacting a pharmacist or doctor if vomiting, diarrhoea or fever creates a risk of dehydration, or following the sick-day instructions already provided by the clinical team. ^[4]

Side effects

Common side effects

Gastrointestinal effects are the most common. Depending on the product and study, these include diarrhoea, nausea or vomiting, abdominal discomfort, gas, indigestion and reduced appetite. Taste changes and headache can also occur. Taking metformin with food and increasing the dose gradually are commonly used strategies to improve tolerability. ^{[1] [3]}

Vitamin B12 deficiency

Metformin can lower vitamin B12 levels during long-term treatment. The UK MHRA now describes reduced B12 as a common adverse effect, with higher risk at larger doses, longer duration of treatment and in people who already have risk factors for B12 deficiency. New neuropathy, marked tiredness, a sore red tongue or megaloblastic anaemia are examples of situations in which B12 testing may be appropriate. ^[6]

Important warnings and precautions

Kidney function

Kidney function matters because metformin is substantially eliminated by the kidneys. Current U.S. labels require kidney-function assessment before treatment and periodically afterwards. They contraindicate metformin when eGFR is below 30 mL/min/1.73 m² and do not recommend starting it when eGFR is between 30 and 45 mL/min/1.73 m². These are U.S. label thresholds; local product information should be checked. ^{[1] [2]}

Iodinated contrast scans

Metformin sometimes needs to be stopped temporarily for CT scans, angiography or other procedures using iodinated contrast. Current U.S. labels specify interruption for certain patients, including some people with eGFR 30–60 mL/min/1.73 m², a history of liver disease, alcoholism or heart failure, or those receiving intra-arterial contrast. Kidney function is rechecked after the procedure before restarting in these situations. ^[1]

Surgery and restricted food or fluids

Metformin may also need temporary interruption around surgery or other procedures when food and fluid intake is restricted, because dehydration, low blood pressure and impaired kidney function can increase risk. ^[1]

Liver disease, hypoxia and alcohol

Current U.S. labeling identifies hepatic impairment, hypoxic states such as severe acute heart failure or shock, and excessive alcohol intake as factors that can increase the risk of lactic acidosis. Product labeling advises avoiding excessive alcohol. ^[1]

Metformin interactions

Metformin has clinically important interactions, but a general web page cannot determine whether a specific combination is safe for an individual. Current U.S. labeling highlights several groups:

• Carbonic anhydrase inhibitors such as topiramate can increase the risk of lactic acidosis.
• Medicines that reduce metformin clearance, including examples such as cimetidine, dolutegravir, ranolazine and vandetanib, can increase metformin exposure.
• Insulin and insulin secretagogues can raise the risk of hypoglycaemia when used with metformin.
• Alcohol, particularly excessive intake, can increase risk related to lactate metabolism.

This is not a complete interaction list. A pharmacist or prescriber should review prescription medicines, OTC products and supplements together. ^[1]

Pregnancy, breastfeeding and fertility

Pregnancy

Diabetes itself can create important pregnancy risks, so treatment should not be stopped without clinical advice. U.S. labeling states that available human data are insufficient to determine a drug-associated risk of major birth defects or miscarriage, while published studies have not shown a clear association. Metformin is also used for gestational diabetes in some health systems. ^{[1] [4]}

Breastfeeding

Published studies show that metformin is present in human milk in small amounts, but available data are limited for determining effects on the breastfed infant. Decisions should consider the mother's clinical need for metformin and the benefits of breastfeeding. ^[1]

Ovulation and fertility

Metformin can restore ovulation in some people who were not ovulating, which means an unintended pregnancy can become possible. PCOS guidance also includes metformin in selected fertility-treatment pathways, although other ovulation-induction medicines may be more effective for specific goals. ^{[1] [7]}

Overdose

Metformin overdose can be dangerous because severe lactic acidosis can occur. U.S. labeling notes that metformin is dialyzable and that haemodialysis may be useful when significant overdose or accumulation is suspected. If someone has taken substantially more than prescribed or develops concerning symptoms, urgent medical assessment is appropriate. ^[1]

Sources

This page separates general metformin information from details that depend on a particular product or health system. The following sources support this reference:

1. 1. DailyMed / U.S. National Library of Medicine. Metformin Hydrochloride Tablets, current label page; DailyMed page updated 28 Aug 2025, prescribing information revised May 2025. Official label ↗
2. 2. U.S. Food and Drug Administration. GLUMETZA (metformin hydrochloride) extended-release tablets prescribing information, revised Aug 2026. FDA label ↗
3. 3. MedlinePlus. Metformin Drug Information, U.S. National Library of Medicine. Patient information ↗
4. 4. NHS. Metformin — uses, taking metformin and side effects. NHS medicine page ↗
5. 5. NICE. Type 2 diabetes in adults: management (NG28), last updated 18 Feb 2026. Guideline ↗
6. 6. UK MHRA. Metformin and reduced vitamin B12 levels: new advice for monitoring patients at risk, 20 Jun 2022. Drug Safety Update ↗
7. 7. International Evidence-based PCOS Guideline / Monash University. 2023 recommendations for metformin in PCOS. Guideline resource ↗
8. 8. World Health Organization. ATC classification principles: plain metformin preparations are assigned A10BA02. WHO ATC ↗
9. 9. electronic Medicines Compendium (UK). Current Glucophage / Glucophage SR product listings for metformin hydrochloride. eMC listings ↗
10. 10. SAHPRA (South Africa). Approved GLUCOPHAGE and GLUCOPHAGE XR product information. SAHPRA product information ↗
11. 11. Pharmacy and Poisons Board (Kenya). Product documents for Glucophage and Metfil metformin products. PPB public register ↗
12. 12. NAFDAC Greenbook (Nigeria). Registered metformin hydrochloride products, including Glucophage and Glucophage XR examples. NAFDAC Greenbook ↗
13. 13. Ghana Food and Drugs Authority / registry-derived product records. Metformin registrations include Glucophage, Metformin-Denk and locally registered generic brands. FDA Ghana product register ↗
14. 14. Botswana Medicines Regulatory Authority. BRIMS national product register workflow; Botswana registration data include DIAMET / DIAMET XR metformin products. BoMRA register guidance ↗
15. 15. Government of Mauritius Price Fixing Unit. Pharmaceutical price register entries for Metformin Denk 500 mg, 850 mg and 1 g products. Mauritius pharmaceutical register ↗

This medicine reference is based on trusted medical and regulatory sources and is awaiting clinician review. It is not a substitute for personalized medical advice.