Metformin
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Here is a full clinical summary of metformin based on authoritative textbook sources:
Metformin
Class: Biguanide | Drug: Metformin (only biguanide in clinical use)
Mechanism of Action
Metformin works through three complementary actions:
- Reduces hepatic gluconeogenesis - this is the primary mechanism, responsible for lowering fasting blood glucose (excess hepatic glucose output is a key driver of hyperglycemia in type 2 DM)
- Slows intestinal glucose absorption
- Improves peripheral insulin sensitivity - enhances glucose uptake and utilization in muscle and fat
At the cellular level, metformin activates AMPK (AMP-activated protein kinase), leading to mTOR inhibition and reduced mitochondrial complex I activity. Because it does not stimulate insulin secretion, it carries no intrinsic risk of hypoglycemia.
Indications
- Type 2 diabetes mellitus - first-line preferred initial therapy, initiated at diagnosis
- Prediabetes - reduces progression to type 2 DM
- Polycystic ovary syndrome (PCOS) - reduces insulin resistance, improves ovulation (used alone or combined with clomiphene)
- Gestational diabetes - used in some settings (not recommended as first-line by ACOG or ADA)
Pharmacokinetics
| Parameter | Detail |
|---|---|
| Absorption | Well absorbed orally |
| Protein binding | None |
| Metabolism | Not metabolized |
| Elimination | Renal excretion as unchanged drug |
| Dosing | At least twice daily (short duration of action); start 500 mg once/twice daily with meals |
| Max dose | 2,550 mg/day |
Efficacy
- Reduces HbA1c by 1.0-1.5% (11-16 mmol/mol)
- Reduces body weight (suppresses appetite)
- Decreases cardiovascular events and mortality in obese patients with type 2 DM
- Does not cause hypoglycemia as monotherapy; hypoglycemia risk arises only when combined with insulin or insulin secretagogues (e.g., sulfonylureas)
Adverse Effects
| Effect | Details |
|---|---|
| GI (most common) | Nausea, vomiting, diarrhea, bloating, metallic taste - minimize by slow dose titration and taking with meals |
| Lactic acidosis | Rare but potentially fatal; risk rises with renal impairment |
| Vitamin B12 deficiency | Long-term use reduces B12 absorption; monitor levels especially if anemia or peripheral neuropathy present |
| Weight | Weight loss/neutral (not weight gain) |
Contraindications & Precautions
| Situation | Guidance |
|---|---|
| eGFR < 30 mL/min | Contraindicated - risk of lactic acidosis |
| eGFR 30-45 mL/min | Use with caution; dose reduction may be needed |
| IV iodinated contrast | Temporarily discontinue before procedure (risk of acute kidney injury -> lactic acidosis) |
| Acute MI, acute heart failure, sepsis | Discontinue - risk of acute renal failure and lactic acidosis |
| Age > 80 years | Use with caution |
| Alcohol abuse | Use with caution |
| eGFR < 45 mL/min | Avoid in some guidelines |
Key Drug Combinations
Metformin is commonly combined with:
- Sulfonylureas (e.g., Glucovance = glyburide + metformin)
- DPP-4 inhibitors (e.g., Janumet = sitagliptin + metformin; Kombiglyze XR = saxagliptin + metformin)
- SGLT2 inhibitors (e.g., Invokamet = canagliflozin + metformin)
- TZDs (e.g., ACTOplus Met = pioglitazone + metformin)
Monitoring
- HbA1c every 3-6 months
- eGFR/renal function before initiation and periodically
- Vitamin B12 levels periodically with long-term use
Emerging & Other Uses
- Anti-aging research - metformin extends lifespan in mouse models via AMPK/mTOR pathway; the TAME (Targeting Aging with Metformin) trial is investigating this in humans
- Dementia prevention - recent meta-analyses (PMID 39716328 and 40193122) suggest antidiabetic agents including metformin may reduce dementia risk in type 2 DM patients
- Exercise synergy - a 2024 network meta-analysis (PMID 39242178) showed combined exercise + metformin is superior to either alone for glycemic control
Sources: Lippincott Illustrated Reviews: Pharmacology; Goldman-Cecil Medicine International Edition; Tintinalli's Emergency Medicine; Harrison's Principles of Internal Medicine 22E (2025)
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