metformin

FDA-Approved Indications:

Type 2 diabetes mellitus — as an adjunct to diet and exercise to improve glycemic control in:

• Adults (immediate-release and extended-release)
• Pediatric patients aged 10 years and older (immediate-release only)

Metformin is recommended as the first-line pharmacologic agent for type 2 diabetes by the American Diabetes Association (ADA), the American Association of Clinical Endocrinology (AACE), and most international diabetes guidelines, due to its efficacy, safety profile, low cost, and cardiovascular benefits demonstrated in the UKPDS.

Off-label uses (clinically practiced but not FDA-approved):

• Polycystic ovary syndrome (PCOS) — improves menstrual regularity, reduces androgen levels, and improves insulin sensitivity; widely used as first-line treatment for metabolic aspects of PCOS
• Prediabetes / prevention of type 2 diabetes — the Diabetes Prevention Program (DPP) trial showed metformin reduced progression from prediabetes to type 2 diabetes by 31% over 3 years; ADA recommends considering metformin for high-risk prediabetes patients
• Gestational diabetes — used in some settings as an alternative to insulin; not FDA-approved but supported by some guidelines
• Antipsychotic-induced weight gain — evidence supports modest benefit in patients on second-generation antipsychotics

Metformin is not approved for type 1 diabetes or diabetic ketoacidosis.

Immediate-Release (IR) Tablets — Glucophage, generic

Starting dose: 500 mg twice daily with meals, OR 850 mg once daily with the morning meal.

Usual effective dose: 1,500–2,000 mg/day in 2–3 divided doses with meals.

Extended-Release (ER) Tablets — Glucophage XR, Glumetza, Fortamet, generic

Starting dose: 500–1,000 mg once daily with the evening meal.

Note: If glycemic control is not achieved on ER formulation, switch to IR which allows higher maximum dose (2,550 mg/day).

Renal Dosing — eGFR Thresholds

Iodinated contrast media: Hold metformin at the time of or prior to iodinated contrast procedures in patients with eGFR 30–60 mL/min/1.73 m², history of liver disease, alcoholism, or heart failure. Reassess renal function 48 hours after the procedure and restart only if renal function is stable.

Administration Instructions

• Always take with meals to minimize gastrointestinal side effects
• Swallow ER tablets whole — do not crush, cut, or chew
• The tablet shell of some ER formulations may appear in stool — this is normal and does not indicate incomplete absorption
• If a dose is missed: take as soon as remembered unless it is almost time for the next dose; do not double doses

Metformin is a biguanide that lowers blood glucose through three complementary mechanisms, all without stimulating insulin secretion:

1. Hepatic glucose production suppression (primary mechanism) Metformin inhibits mitochondrial complex I (NADH dehydrogenase) in hepatocytes, reducing the mitochondrial membrane potential and lowering the ATP/ADP ratio. This activates AMP-activated protein kinase (AMPK), which phosphorylates and inactivates key enzymes in gluconeogenesis (the liver's process of making new glucose from amino acids and glycerol). The net effect is a 25–40% reduction in hepatic glucose output, which directly lowers fasting blood glucose. Metformin also inhibits the mitochondrial glycerophosphate dehydrogenase (mGPD) enzyme, reducing the conversion of lactate and glycerol to glucose.

2. Peripheral insulin sensitization Metformin improves insulin sensitivity in skeletal muscle and adipose tissue by activating AMPK, which increases translocation of the GLUT4 glucose transporter to the cell surface, enhancing glucose uptake independent of changes in insulin secretion. This effect is most pronounced in overweight patients with insulin resistance.

3. Intestinal glucose absorption reduction Metformin mildly slows intestinal glucose absorption by reducing active glucose transport in the small intestine, contributing to lower postprandial glucose excursions. It also increases GLP-1 secretion from intestinal L-cells, which may contribute to its modest appetite-suppressing effect.

Key distinction from insulin secretagogues: Because metformin does not stimulate insulin secretion, it does not cause hypoglycemia as monotherapy and does not promote weight gain — two major advantages over sulfonylureas and insulin.

UKPDS evidence: The UK Prospective Diabetes Study (1998) demonstrated that metformin reduced the risk of any diabetes-related endpoint by 32%, diabetes-related death by 42%, and all-cause mortality by 36% in overweight patients with type 2 diabetes — benefits that exceeded those of sulfonylureas and insulin despite similar glycemic control, suggesting pleiotropic cardiovascular effects beyond glucose lowering.

Key Drug Interactions — Summary Table:

Important: No CYP450 Interactions Metformin is not metabolized by CYP450 enzymes and does not inhibit or induce them. This means metformin has no pharmacokinetic interactions with drugs that are CYP substrates, inhibitors, or inducers (e.g., statins, azole antifungals, macrolide antibiotics) — a significant safety advantage over many other oral antidiabetics.

OCT1 and hepatic uptake: Hepatic uptake of metformin depends on organic cation transporter 1 (OCT1). Drugs that inhibit OCT1 (verapamil, quinidine, trimethoprim) may reduce metformin's glucose-lowering efficacy by limiting its hepatic concentration. This is a pharmacodynamic interaction, not a safety concern.

Pregnancy: Metformin crosses the placenta; fetal exposure is approximately 50% of maternal plasma levels. Insulin remains the preferred agent for glycemic control during pregnancy — it has the longest safety record and does not cross the placenta.

However, metformin is increasingly used in pregnancy based on growing evidence:

• MiG Trial (NEJM 2008): In 751 women with gestational diabetes, metformin was non-inferior to insulin for neonatal outcomes (composite of hypoglycemia, respiratory distress, phototherapy, birth trauma, prematurity, or low Apgar score). Women on metformin had less weight gain and lower rates of severe hypoglycemia.
• PCOS and fertility: Metformin is widely used to induce ovulation in PCOS and is often continued into the first trimester
• Long-term offspring data: Some studies suggest children exposed to metformin in utero have higher BMI at age 4, but clinical significance is uncertain

Recommendation: Insulin is preferred; metformin is an acceptable alternative for gestational diabetes when insulin is declined or not tolerated. Discuss risks and benefits with each patient.

Breastfeeding: Metformin is present in breast milk at low levels (infant dose ~0.11–0.25% of maternal weight-adjusted dose). The American Academy of Pediatrics considers it compatible with breastfeeding. No adverse effects have been reported in breastfed infants. Monitor infant for diarrhea or poor feeding.

Pediatric Use (≥10 years): FDA-approved for type 2 diabetes in patients aged 10 years and older (immediate-release only — ER not approved in pediatrics). Starting dose: 500 mg twice daily with meals; maximum dose: 2,000 mg/day. Safety and efficacy not established in children under 10 years.

Geriatric Use (≥65 years): Elderly patients have age-related decline in renal function that may not be reflected in serum creatinine alone (reduced muscle mass → lower creatinine production despite reduced GFR). Always use eGFR — not serum creatinine alone — to assess renal function in elderly patients. Start at lower doses (500 mg once or twice daily) and titrate slowly. Monitor renal function every 3–6 months. Avoid metformin if eGFR <30; use with caution if eGFR 30–45.

Renal Impairment — Summary Table:

Hepatic Impairment: Avoid in patients with significant hepatic disease. Metformin is not hepatically metabolized (excreted unchanged by kidneys), but hepatic impairment impairs the liver's ability to clear lactate, increasing lactic acidosis risk. No specific eGFR-equivalent threshold exists for liver function — clinical judgment is required.