Medications guide

Metformin

Metformin is a prescription medicine widely used to help lower blood sugar in adults and some children with type 2 diabetes. This guide gives general information only.

Last updated 3 sources

At a Glance

  • Metformin belongs to a drug class called biguanides.
  • It is one of the most commonly prescribed medicines for type 2 diabetes.
  • It works mainly by reducing the amount of sugar the liver releases into the blood.
  • Stomach upset, such as diarrhea or nausea, is among the most commonly reported side effects.
  • A rare but serious possible effect is lactic acidosis, a buildup of acid in the blood.
  • It is usually used alongside healthy eating and physical activity.

What It Is

Metformin is an oral medicine used to manage blood sugar levels in type 2 diabetes. It has been in use for decades and is available as regular and extended-release tablets, as well as a liquid form.

Unlike some other diabetes medicines, metformin on its own rarely causes low blood sugar (hypoglycemia). It is not used to treat type 1 diabetes or diabetic ketoacidosis.

General mechanism

Metformin lowers blood sugar mainly by reducing glucose production in the liver. It also improves the body’s sensitivity to insulin, helping muscle and fat cells take up sugar from the blood, and may modestly reduce sugar absorption from the intestine. Because it does not directly cause the pancreas to release more insulin, it rarely causes low blood sugar when used alone.

Common Terminology

Biguanide
The drug class metformin belongs to; these medicines lower blood sugar mainly by acting on the liver.
Insulin resistance
A state in which the body’s cells do not respond well to insulin, so sugar stays in the blood.
Lactic acidosis
A rare, serious condition where lactic acid builds up in the blood faster than it can be removed.
Extended-release
A tablet designed to release medicine slowly over time rather than all at once.
Hemoglobin A1C
A blood test that reflects average blood sugar over roughly the past two to three months.
Hypoglycemia
Blood sugar that is lower than normal.

General Uses

Metformin is prescribed for a small number of well-established purposes.

  • Managing blood sugar in type 2 diabetes in adults.
  • Managing blood sugar in type 2 diabetes in some children, in certain formulations.
  • Use together with other diabetes medicines or insulin when a clinician decides that is appropriate.
  • It is sometimes prescribed off-label for other conditions, such as polycystic ovary syndrome (PCOS), at a clinician’s discretion.

Common Side Effects

Side effects vary from person to person. Common ones reported in general references include:

  • Diarrhea, nausea, or upset stomach.
  • Gas, bloating, or stomach discomfort.
  • A metallic taste in the mouth.
  • Reduced appetite.
  • Lower vitamin B12 levels with long-term use, which clinicians may check over time.

Common Warnings and Precautions

Medication labels and health agencies describe several general warnings.

  • The FDA label includes a boxed warning about lactic acidosis, a rare but serious condition. Symptoms described include unusual muscle pain, trouble breathing, unusual sleepiness, stomach pain, and feeling cold or dizzy.
  • The risk of lactic acidosis is described as higher with significant kidney problems, liver problems, heavy alcohol use, serious infection, dehydration, or certain heart conditions.
  • Kidney function is typically checked before and during treatment.
  • Clinicians may pause metformin around certain imaging tests that use iodinated contrast dye or around surgery; these decisions are made by the care team.
  • Low blood sugar is more likely when metformin is used together with insulin or certain other diabetes medicines.

How It Works and How It Is Generally Used

This section describes general mechanism and use only; it does not include dosing information.

  • Metformin reduces the amount of glucose (sugar) the liver produces.
  • It helps the body’s muscle and fat tissue respond better to insulin.
  • It may slightly reduce how much sugar is absorbed from the gut.
  • It is taken by mouth, and the specific product, amount, and schedule are set by the prescriber and shown on the prescription label.
  • It is generally used as part of a broader plan that includes diet, activity, and regular monitoring.

Questions Often Discussed With a Clinician or Pharmacist

People often bring up topics like these when talking about metformin with a healthcare professional.

  • How kidney function is monitored and why it matters.
  • What stomach side effects are common and which symptoms are concerning.
  • Whether vitamin B12 levels should be checked.
  • How metformin fits with other medicines, supplements, and alcohol use.
  • What happens with metformin around scans that use contrast dye or around surgery.

Questions to Ask a Healthcare Professional

General questions people often find useful to bring to an appointment:

  • What is metformin meant to do in my overall diabetes plan?
  • Which side effects are common, and which ones should be reported right away?
  • How will my kidney function be checked?
  • Are there any interactions with my other medicines or with alcohol?
  • Should my vitamin B12 level be monitored?
  • How will we know if the medicine is working?

Sources

This guide was written in plain language from the following authoritative references.

  1. Metformin — MedlinePlus (U.S. National Library of Medicine)
  2. Metformin — NHS (UK National Health Service)
  3. Diabetes Medicines — MedlinePlus (U.S. National Library of Medicine)

Last updated October 8, 2026. FreeDoc guides are general educational information and are not a substitute for care from a qualified professional.