Metformin

Metformin is an oral antidiabetic medicine approved to help control blood sugar in people with type 2 diabetes mellitus, always used alongside diet and exercise.

• Metformin hydrochloride tablets and the oral solution (including Riomet) are approved for adults and children age 10 and older with type 2 diabetes.
• Metformin hydrochloride extended-release tablets — including Fortamet, Glumetza, and Metformin Hydrochloride Extended Release — are approved for adults only with type 2 diabetes.

Metformin lowers blood sugar through three actions: it tells the liver to release less glucose into the bloodstream, slows the absorption of sugar from food in the gut, and makes the body's cells more sensitive to insulin so they take in glucose more efficiently. Because it works on these pathways rather than directly stimulating the pancreas, it does not cause blood sugar to drop too low when used on its own.

Metformin is taken by mouth, and the right dose is individualized — there is no single fixed amount that works for everyone. Tablets and oral solution are taken in divided doses with meals during the day. Extended-release tablets are generally taken once daily with the evening meal, and must always be swallowed whole — never crushed, cut, or chewed. Dosing is always started low and increased gradually to minimize stomach upset. Follow your prescriber's schedule closely, and never adjust your dose without guidance.

The most common side effects of metformin are digestive in nature and tend to be most noticeable when first starting or increasing the dose:

These usually improve as your body adjusts. However, seek care right away for symptoms of lactic acidosis (muscle pain, difficulty breathing, unusual sleepiness, stomach pain, or feeling cold) or signs of low blood sugar if you also take insulin or a sulfonylurea.

• Lactic acidosis (rare but life-threatening): Metformin can cause a dangerous buildup of lactic acid in the blood. Early signs include tiredness, muscle aches, stomach pain, trouble breathing, and unusual drowsiness. Stop metformin and seek emergency care immediately if these occur. Risk is highest in people with kidney disease, liver disease, heart failure, low-oxygen conditions, those who drink heavily, and adults 65 or older.
• Kidney function: Metformin is cleared by the kidneys — if kidneys are not working well, the drug can build up to dangerous levels. Your doctor will check kidney function before starting and at least yearly while you take it.
• Vitamin B12 levels: Long-term metformin use can lower vitamin B12, potentially causing anemia or nerve symptoms. Blood checks every 2–3 years are recommended.
• Low blood sugar (hypoglycemia): Metformin alone rarely causes hypoglycemia, but the risk rises when it is combined with insulin or sulfonylureas. Symptoms include shakiness, sweating, and confusion.
• Before procedures: Tell your care team you take metformin before any surgery, significant procedure, or imaging with IV contrast dye — it may need to be temporarily stopped.
• Alcohol: Avoid heavy drinking — alcohol increases the risk of lactic acidosis with metformin.

Several medications and substances can interact with metformin in clinically important ways:

• Carbonic anhydrase inhibitors (topiramate, zonisamide, acetazolamide, dichlorphenamide) — can cause metabolic acidosis and raise lactic acidosis risk when combined with metformin.
• Drugs that reduce metformin's kidney clearance (cimetidine, ranolazine, vandetanib, dolutegravir) — cause metformin to build up in the body, increasing lactic acidosis risk.
• Insulin or sulfonylureas (e.g., glipizide, glyburide) — combining with metformin raises hypoglycemia (low blood sugar) risk; lower doses of insulin or the sulfonylurea may be needed.
• Alcohol — amplifies metformin's effect on lactic acid; heavy drinking should be avoided.
• Iodinated IV contrast agents (used in CT scans and some imaging) — can impair kidney function acutely, causing metformin to accumulate; metformin may need to be paused before and after the procedure.
• Blood sugar-raising drugs (corticosteroids, thiazide diuretics, estrogens, oral contraceptives, phenytoin, thyroid products, sympathomimetics, calcium channel blockers, nicotinic acid, phenothiazines, isoniazid) — may reduce blood sugar control; monitor glucose closely if these are started or stopped.

Not a complete list. The label covers the interactions the manufacturer studied or reported, and our own interaction database does not cover every medicine either. Bring your full medication list, including vitamins and herbal products, to your pharmacist.

• Kidney disease: Metformin is contraindicated if your eGFR (kidney filtration rate) is below 30 mL/min/1.73 m². Starting metformin is not recommended if eGFR is 30–45. Kidney function should be checked before starting and regularly while taking it.
• Liver disease: Avoid metformin if you have significant liver problems — it raises lactic acidosis risk.
• Metabolic acidosis or diabetic ketoacidosis: Metformin is contraindicated in these conditions.
• Allergy: Do not take metformin if you have had a hypersensitivity (allergic) reaction to it.
• Age 65 or older: More careful monitoring of kidney function is needed; start at the lowest effective dose.
• Pregnancy: Poorly controlled diabetes in pregnancy is risky for mother and baby; talk to your doctor about whether to continue metformin — available data have not confirmed a clear risk of birth defects, but evidence is limited.
• Breastfeeding: Metformin passes into breast milk. Discuss benefits and risks with your doctor before continuing while nursing.
• Premenopausal women not currently ovulating: Metformin may trigger ovulation; discuss contraception with your doctor if pregnancy is not desired.
• Children under 10: Safety and effectiveness of metformin have not been established in children younger than 10. Extended-release tablets are for adults only.
• Upcoming surgery or procedures: Tell your care team you take metformin — it may need to be temporarily stopped if you will be fasting or receiving IV contrast dye.

Taking too much metformin can lead to lactic acidosis (dangerous acid buildup in the blood) in a significant number of overdose cases. Low blood sugar has also been reported, though metformin's direct role is uncertain. If an overdose is suspected, seek emergency medical care immediately — hemodialysis (a kidney-filtering machine) can be used to remove metformin from the body and may help reverse symptoms.

Metformin reduces blood sugar by decreasing the liver's output of glucose, slowing glucose absorption from the intestines, and improving the body's sensitivity to insulin — leading to better uptake and use of glucose by muscle and other tissues. It lowers both fasting blood sugar (before meals) and postprandial blood sugar (after meals). Importantly, metformin does not stimulate insulin secretion, so insulin levels may actually decrease over time while blood sugar control improves.

When taken by mouth as a standard tablet under fasting conditions, roughly 50–60% of metformin is absorbed. Steady blood levels are typically reached within 24–48 hours of regular dosing. Metformin is not broken down by the liver and is excreted essentially unchanged by the kidneys, primarily through active secretion in the kidney tubules. The plasma half-life is approximately 6 hours, though the drug's half-life in red blood cells is considerably longer (around 17–18 hours), suggesting the cells act as a secondary storage compartment. In people with reduced kidney function, metformin clears more slowly and can accumulate — which is why kidney function is monitored carefully throughout treatment. Food slightly increases and delays the absorption of Riomet (oral solution).

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