Metformin is a biguanide and the cornerstone first-line medication for type 2 diabetes, used by tens of millions worldwide. It lowers blood sugar primarily by reducing the liver's glucose output, not by stimulating insulin release. The most important things to know: it causes GI side effects in many people (especially at first), long-term use can deplete vitamin B12, and it should not be used in people with significant kidney impairment.
What Is Metformin (Glucophage)? Uses, Side Effects & Warnings
Metformin is one of the most widely prescribed medications in the world. Sold under brand names including Glucophage and Glucophage XR (extended-release), metformin belongs to a drug class called biguanides. It has been a cornerstone of type 2 diabetes treatment for decades and is typically the first medication prescribed when lifestyle changes alone are insufficient to control blood sugar. It is available in immediate-release and extended-release tablet forms and comes in several strengths including 500 mg, 850 mg, and 1000 mg tablets.
What Metformin Is Used For
The FDA has approved metformin specifically for the management of type 2 diabetes mellitus in adults and children 10 years of age and older. It is used to improve glycemic (blood sugar) control and is often prescribed alongside diet and exercise modifications as part of a broader diabetes management plan.
Beyond its approved indication, metformin is also widely used off-label for polycystic ovary syndrome (PCOS), where it can help manage insulin resistance and improve hormonal balance and fertility outcomes. It is also used off-label in prediabetes to slow or prevent the progression to full type 2 diabetes, particularly in high-risk individuals. Research into metformin's potential anti-aging and cancer-preventive properties is ongoing but not yet sufficient to support routine clinical use for those purposes.
Metformin is not appropriate for type 1 diabetes, where the pancreas produces little or no insulin and insulin replacement is essential.
How Metformin Works
Metformin works through several complementary mechanisms, all of which reduce blood glucose without directly stimulating the pancreas to release more insulin. Its primary action is in the liver, where it inhibits a process called hepatic gluconeogenesis — the production of new glucose from non-carbohydrate sources such as amino acids. The liver normally releases glucose into the bloodstream continuously, and in people with type 2 diabetes this process is excessively active. Metformin significantly reduces this liver glucose output, lowering fasting blood sugar levels.
Metformin also improves insulin sensitivity in peripheral tissues, particularly skeletal muscle, helping cells take up glucose from the bloodstream more effectively in response to insulin. It slows the absorption of glucose from the gastrointestinal tract and has favorable effects on gut microbiome composition, which may contribute to its glucose-lowering effects. Unlike many other diabetes medications, metformin does not cause weight gain — it is often weight-neutral or may modestly support weight loss, which makes it particularly valuable. It also does not cause hypoglycemia (dangerously low blood sugar) when used alone.
Common Side Effects
- Nausea, vomiting, and stomach upset — the most common side effects, especially when starting therapy
- Diarrhea — affects a significant portion of people, often improving with time
- Abdominal cramping or discomfort
- Metallic taste in the mouth
- Decreased appetite — generally not considered harmful
- Vitamin B12 deficiency — with long-term use; can cause fatigue, nerve symptoms if untreated
GI side effects are substantially reduced with the extended-release formulation. Taking metformin with food also helps minimize stomach upset. Many people find that side effects diminish after the first few weeks of treatment.
Important Warnings
BLACK BOX WARNING: Lactic acidosis — a rare but potentially fatal buildup of lactic acid in the blood. Risk increases significantly in patients with kidney impairment, liver disease, heart failure, or conditions that cause dehydration or reduced oxygen delivery to tissues. Metformin is contraindicated in patients with an eGFR below 30 mL/min/1.73m² and should be used cautiously in those with reduced kidney function. Patients should seek emergency care immediately for symptoms such as muscle pain, weakness, difficulty breathing, stomach pain, nausea, vomiting, dizziness, or feeling cold.
Metformin should be temporarily stopped before surgical procedures involving iodinated contrast dye (such as CT scans with contrast), as contrast agents can impair kidney function and increase lactic acidosis risk. It should be withheld until kidney function is confirmed to be stable afterward.
Heavy alcohol use increases the risk of lactic acidosis and should be avoided by people taking metformin. The drug is not recommended during pregnancy (insulin is preferred for diabetes management during pregnancy) and should be used cautiously in elderly patients who may have reduced kidney function even with normal-appearing creatinine levels.
Key Drug Interactions
Certain medications can increase metformin's blood levels or compound lactic acidosis risk. These include carbonic anhydrase inhibitors (such as topiramate), iodinated contrast media, and drugs that impair kidney function. Alcohol significantly enhances the risk of lactic acidosis. Cationic drugs that are eliminated by the kidneys — including some diuretics and antibiotics — may reduce metformin clearance. Corticosteroids can raise blood sugar and reduce metformin's effectiveness.
Immediate-Release vs. Extended-Release
Immediate-release (IR) metformin is typically taken two to three times daily with meals. Extended-release (ER/XR) formulations are taken once daily, usually with the evening meal, and release the drug gradually over many hours. The extended-release form produces a slower rise in drug levels in the bloodstream, which is the main reason it causes fewer GI side effects. Both forms are equally effective at lowering blood sugar. Note: some extended-release generics were recalled in prior years due to elevated nitrosamine levels; patients should check FDA recall notices or consult a pharmacist if they have concerns.
Frequently Asked Questions
Can metformin be used for type 1 diabetes?
No. Metformin is only approved for type 2 diabetes. Type 1 diabetes requires insulin because the pancreas produces little or no insulin on its own. Metformin's mechanism depends on some residual insulin function, making it ineffective and inappropriate as a standalone therapy for type 1.
What is the difference between metformin and metformin extended-release?
Immediate-release metformin is taken multiple times daily and is more likely to cause GI side effects such as nausea and diarrhea. Extended-release (ER or XR) formulations are taken once daily and release the drug slowly throughout the day, which substantially reduces stomach-related side effects for many people. Both versions lower blood sugar equally well.
Why does metformin sometimes need to be stopped before a CT scan?
Iodinated contrast dye used in CT scans can temporarily impair kidney function. If kidney function drops, metformin can accumulate to higher levels, increasing the risk of lactic acidosis. Standard guidelines recommend holding metformin at the time of contrast administration and restarting only after kidney function is re-confirmed to be normal, typically 48 hours later.
Does metformin cause vitamin B12 deficiency?
Yes, long-term metformin use is associated with reduced vitamin B12 absorption, likely because it interferes with a calcium-dependent mechanism in the gut. Over years, this can lead to B12 deficiency and peripheral neuropathy (nerve damage). Doctors typically monitor B12 levels periodically and may recommend B12 supplementation for patients on long-term metformin therapy, especially those who are already at risk for deficiency.
Need to identify a pill by its imprint, shape, or color?
Use the PillID Pill Identifier →