Clinical safety rating: safe
Human studies have proved safety
Decreases hepatic glucose production, decreases intestinal absorption of glucose, and improves insulin sensitivity by increasing peripheral glucose uptake and utilization.
| Metabolism | Not metabolized; excreted unchanged in urine via tubular secretion. |
| Excretion | Renal: 90% unchanged; biliary/fecal: negligible. |
| Half-life | Terminal elimination half-life: 6.2 hours (range 4–9 hours). In renal impairment, half-life prolongs significantly, increasing risk of lactic acidosis. |
| Protein binding | Negligible (<5%); does not bind to plasma proteins. |
| Volume of Distribution | 654 L (approximately 9 L/kg); indicates extensive tissue distribution. |
| Bioavailability | Oral: 50–60% (absolute); decreases with higher doses due to saturable absorption. |
| Onset of Action | Oral: 2 hours for glucose-lowering effect; maximal effect at 4–8 hours. |
| Duration of Action | Oral: 10–16 hours; extended-release formulation allows once-daily dosing. |
| Molecular Weight | 129.16 |
Oral, 500 mg twice daily or 850 mg once daily, increased gradually to 2000 mg daily in divided doses.
| Renal impairment | eGFR 30-45 mL/min: maximum 1000 mg/day; eGFR <30 mL/min: contraindicated. |
| Liver impairment | Avoid in severe hepatic impairment (Child-Pugh class C); no specific dose adjustment defined for mild-moderate impairment. |
| Pediatric use | Children ≥10 years: starting 500 mg once daily, titrate to maximum 2000 mg daily in divided doses; weight-based: 5-10 mg/kg/day, up to 20 mg/kg/day. |
| Geriatric use | Initiate at lowest dose (e.g., 500 mg once daily), titrate slowly; monitor renal function; avoid if eGFR <45 mL/min. |
| 1st trimester | Metformin is generally not recommended during the first trimester due to limited safety data; however, it may be used in women with polycystic ovary syndrome (PCOS) to reduce miscarriage risk. Animal studies have shown some fetal effects, but human data are insufficient. |
| 2nd trimester | Metformin crosses the placenta but has not been associated with major malformations in second trimester exposure. It is used in gestational diabetes when insulin is not an option, though insulin remains first-line. |
| 3rd trimester | Metformin is used off-label in gestational diabetes and has been associated with lower risk of neonatal hypoglycemia and macrosomia compared to insulin. However, there is a potential risk of neonatal hypoglycemia and long-term metabolic effects; close monitoring is advised. |
Clinical note
Widely used for gestational diabetes mellitus (GDM) and type 2 diabetes in pregnancy. Crosses the placenta, with fetal concentrations approximating maternal concentrations. Despite this, no teratogenicity has been demonstrated in large human studies. Long-term follow-up data (MiG TOFU trial) showed increased childhood adiposity in metformin-exposed offspring; clinical significance is uncertain. Many clinicians use it as adjunct or alternative to insulin for GDM.
| Placental transfer |
■ FDA Black Box Warning
Lactic acidosis: metformin-associated lactic acidosis is a rare but serious complication. Risk factors include renal impairment, hepatic disease, acute heart failure, and excessive alcohol intake.
| Common Effects | Fatigue Headache Slow heart rate Dizziness Nausea Breathlessness Rash Diarrhea Depression |
| Serious Effects |
Acute or chronic metabolic acidosis (including diabetic ketoacidosis)Severe renal impairment (eGFR < 30 mL/min/1.73 m²)Acute conditions that may alter renal function (e.g., dehydration, severe infection, shock)Hypersensitivity to metformin or any excipientsSevere hepatic impairmentHistory of lactic acidosis (including metformin-associated lactic acidosis)
| Precautions | Lactic acidosis risk, Hypoglycemia when used with other antidiabetic agents, Vitamin B12 deficiency with prolonged use, Renal impairment, Temporary discontinuation for iodinated contrast imaging, Hepatic impairment, Acute heart failure, Excessive alcohol intake |
Loading safety data…
| Metformin crosses the placenta by active transport via organic cation transporters (OCT1, OCT2, OCT3). Placental transfer is efficient, with fetal concentrations reaching 50-70% of maternal levels. No evidence of accumulation. |
| Breastfeeding | Metformin is excreted into breast milk in low levels (0.18-1.08 mg/kg/day), which is unlikely to cause adverse effects in term infants. It is considered compatible with breastfeeding, but monitor the infant for signs of hypoglycemia or gastrointestinal disturbance, especially if premature or with renal impairment. |
| Lactation Rating | L2 (Safer) for term infants; L3 (Moderately Safe) for preterm or ill infants |
| Teratogenic Risk | Metformin is FDA pregnancy category B. First trimester: No increased risk of major malformations reported in human studies. Second and third trimesters: Use for gestational diabetes and type 2 diabetes is considered relatively safe, with no clear evidence of teratogenicity. However, insulin is preferred for glycemic control in pregnancy. |
| Fetal Monitoring | Monitor maternal blood glucose levels, HbA1c, renal function (serum creatinine, eGFR). In pregnancy, monitor fetal growth and well-being via ultrasound, non-stress tests, and biophysical profile as indicated. Watch for signs of lactic acidosis (rare). Monitor for vitamin B12 deficiency with long-term use. |
| Fertility Effects | Metformin can improve ovulatory function in women with polycystic ovary syndrome (PCOS) by reducing hyperinsulinemia and androgen levels. It may restore fertility and increase the likelihood of pregnancy in anovulatory women. No direct adverse effects on male fertility. |
| Food/Dietary |
| Metformin may interfere with vitamin B12 absorption; ensure adequate intake of B12-rich foods or supplements. Avoid high-fat meals that may delay absorption of the drug. No specific food restrictions, but consistent carbohydrate intake is recommended to prevent hypoglycemia. |
| Clinical Pearls | Metformin is contraindicated in eGFR < 30 mL/min/1.73m2; hold for 48 hours before iodinated contrast studies and restart after 48 hours if renal function stable. Lactic acidosis risk is low but increased in conditions like sepsis, acute heart failure, or hepatic impairment. Titrate slowly over weeks to minimize GI side effects; consider extended-release formulation if intolerance occurs. Not recommended in pediatric patients under 10 years old or in pregnancy. |
| Patient Advice | Take with meals to reduce gastrointestinal side effects like nausea and diarrhea. · Avoid excessive alcohol intake while on metformin due to increased risk of lactic acidosis. · Report symptoms of lactic acidosis: muscle aches, severe weakness, trouble breathing, stomach pain, dizziness, or fast/irregular heartbeat. · Monitor blood glucose regularly as directed and follow a consistent diet and exercise plan. · Do not crush or chew extended-release tablets; swallow whole. |