What Is Metformin?

Metformin is an oral antidiabetic medication belonging to the biguanide class. It is the first-line pharmacological treatment for type 2 diabetes in most major clinical guidelines, including those from the American Diabetes Association (ADA) and the European Association for the Study of Diabetes (EASD). It has been in clinical use for decades and has an extensive safety record.

Beyond blood sugar control, metformin is also used off-label for conditions such as polycystic ovary syndrome (PCOS) and, in some settings, as part of prediabetes management programs.

How Does Metformin Work?

Metformin works through several complementary mechanisms:

  • Reduces hepatic glucose production: The liver is the primary target. Metformin suppresses gluconeogenesis — the process by which the liver generates new glucose — which is often overactive in type 2 diabetes.
  • Improves insulin sensitivity: It enhances the body's response to insulin in peripheral tissues such as muscle, helping glucose enter cells more efficiently.
  • Slows intestinal glucose absorption: Metformin modestly reduces how quickly carbohydrates are absorbed from the gut after meals.

Importantly, metformin does not stimulate insulin secretion, meaning it carries a very low risk of causing hypoglycemia (dangerously low blood sugar) when used alone.

Dosage and Administration

Metformin is available in immediate-release (IR) and extended-release (XR) formulations. Typical dosing guidelines include:

  • Starting dose: Usually 500 mg once or twice daily with meals, to minimize gastrointestinal side effects.
  • Titration: The dose is gradually increased over several weeks, typically to a maintenance dose of 1,500–2,000 mg per day in divided doses.
  • Maximum dose: Generally 2,550 mg/day for IR formulations; up to 2,000 mg/day for XR.

Taking metformin with food significantly reduces GI discomfort and is strongly recommended.

Common Side Effects

Gastrointestinal side effects are the most frequently reported and include:

  • Nausea and vomiting
  • Diarrhea
  • Abdominal discomfort or bloating
  • Metallic taste in the mouth

These effects are most common when starting the medication or increasing the dose, and they often improve over time. The extended-release formulation is generally better tolerated in patients who experience persistent GI symptoms.

Serious Precautions

Lactic Acidosis (Rare but Serious)

The most serious — though rare — risk associated with metformin is lactic acidosis, a buildup of lactic acid in the bloodstream. Risk is highest in patients with significant kidney impairment, liver disease, heart failure, or those who consume excessive alcohol. Metformin is generally contraindicated when eGFR falls below 30 mL/min/1.73m².

Vitamin B12 Deficiency

Long-term metformin use is associated with reduced absorption of vitamin B12. Patients on metformin — especially those on higher doses for extended periods — should have B12 levels monitored periodically. Symptoms of deficiency include fatigue, numbness, and neurological changes.

Contrast Media and Surgical Procedures

Metformin is typically held before procedures involving iodinated contrast media and before major surgeries, as temporary kidney stress in these contexts can increase lactic acidosis risk. Patients should always inform their medical team that they are taking metformin before any procedure.

Drug Interactions

Metformin has relatively few significant drug interactions, but clinicians should be aware of:

  • Cationic medications (e.g., cimetidine, trimethoprim) — may compete for renal tubular secretion and increase metformin levels.
  • Alcohol — enhances the risk of lactic acidosis and hypoglycemia in combination with other agents.
  • Diuretics and NSAIDs — may impair kidney function and indirectly affect metformin clearance.

Who Should Not Take Metformin?

Metformin is contraindicated in patients with severe kidney disease (eGFR < 30), acute or chronic metabolic acidosis, known hypersensitivity to metformin, and those undergoing certain radiological procedures with iodinated contrast. It should be used cautiously in patients with liver disease or significant heart failure.

Key Takeaways

Metformin remains a cornerstone of type 2 diabetes treatment due to its effectiveness, low cost, good safety profile, and cardiovascular neutrality (and potential benefit in some populations). As with any medication, it should be used under medical supervision with regular monitoring of kidney function and vitamin B12 levels.