Metformin

The first-line type-2 diabetes drug for 60 years, adopted by the longevity crowd after diabetics on it appeared to outlive non-diabetics in one famous (and heavily criticised) analysis. Cheap, everywhere, and the subject of the long-delayed TAME trial.

Metformin lowers blood sugar mainly by telling the liver to stop overproducing glucose, which it does at night in type 2 diabetes. It also improves how sensitive muscle and fat are to insulin. The molecular story is still argued over — it partially inhibits a step in mitochondrial energy production and activates AMPK, the enzyme that signals low cellular energy, and there is good evidence a large part of its action happens in the gut rather than systemically. Notably it does not force the pancreas to release insulin, which is why it does not cause hypoglycaemia on its own and why it has been safe enough for sixty years of first-line use.

Bulletproof for diabetes. For longevity in healthy people: the Bannister analysis that started it all has known survivorship flaws, mouse data is mixed, and one consistent human finding CUTS AGAINST the hype — metformin blunts part of the adaptation to exercise (VO2max and muscle gains). For someone training hard, it may subtract more than it adds.

• Strong evidence for glycaemic treatment in type 2 diabetes and still very widely used, but modern care is person-centred — GLP-1 agonists, SGLT2 inhibitors, insulin and others may take priority depending on cardiovascular disease, kidney disease, heart failure and weight. It is no longer accurate to call it automatically first-line for everyone.
• Reduces progression from prediabetes to diabetes in the Diabetes Prevention Program, though intensive lifestyle change outperformed it.
• Used in polycystic ovary syndrome for metabolic features. For ovulation induction specifically it is NOT first-line — the international guideline puts letrozole there — and metformin is less effective than dedicated ovulation agents.
• The longevity interest rests on observational data and animal work, not human outcome trials. TAME, the trial designed to test ageing endpoints, has not produced evidence because it has not launched — it remains a proposed study seeking funding, which is different from a trial whose results are pending.
• A genuine complication for the longevity thesis: randomised trials in older adults — 53 in one aerobic-training study and 94 in another resistance-training study — found metformin attenuated the average training response, with substantial variation between individuals. That is a real signal in older adults, not a demonstrated verdict for every healthy person who trains.

Not metabolised — it is cleared unchanged by the kidneys, which is why kidney function governs both dosing and the main safety concern. Half-life around 5 hours, with extended-release forms substantially better tolerated for gastrointestinal side effects. It reliably lowers vitamin B12 over years of use, which is a well-documented and frequently missed cause of neuropathy that gets mistaken for diabetic nerve damage.

GI distress is common and real. B12 depletion with long use (worth testing). Lactic acidosis is rare and mostly a kidney-function story. The quiet cost for this audience is the exercise-adaptation blunting — taking a longevity drug that eats your training response is a bad trade nobody advertises.

• Anyone with significantly impaired kidney function
• Anyone with acute or unstable heart failure, liver failure, or an acute illness causing dehydration
• Anyone who drinks heavily
• Anyone taking it purely for longevity whose main intervention is exercise, given the blunting data

• Iodinated contrast for CT scans — contrast can raise lactic-acidosis risk in susceptible people, but current labelling does not require everyone to withhold it for every scan. It depends on kidney function, how the contrast is given and other risk factors, and the decision belongs to the treating team.
• Alcohol, particularly binge drinking, which raises lactic acidosis risk.
• Other diabetes drugs, especially insulin and sulfonylureas — metformin alone rarely causes hypoglycaemia but the combination can.
• Any acute illness with vomiting, diarrhoea or dehydration — standard sick-day guidance is to pause it.
• Exercise training, in the specific sense that it may blunt adaptations — relevant to anyone taking it for longevity rather than diabetes.

• Kidney function (eGFR) — the parameter that determines whether the drug is safe to take at all.
• Vitamin B12, periodically. Long-term use depletes it and the resulting neuropathy is commonly misattributed.
• HbA1c, where it is being used for glycaemic control.
• Any situation causing dehydration, acute illness or reduced kidney perfusion, which is when the rare lactic acidosis risk becomes real.

No withdrawal syndrome and no taper needed. Blood sugar drifts back to where it would have been within days to weeks, and any gastrointestinal side effects resolve quickly. Anyone taking it for diabetes should not stop without their prescriber, because the underlying condition does not stop. Long-term use can cause B12 deficiency, and it does not simply correct itself on stopping — it may need testing and active replacement, and established nerve damage can persist. That is the argument for testing during use rather than after.

US: prescription-only (not scheduled). Widely prescribed off-label.

Papers about this compound, not proof of the claims above. A title says what was studied, which is sometimes a negative result or a different question entirely.

• Metformin and Combined Oral Contraceptive Pills in the Management of Polycystic Ovary Syndrome: A Systematic Review and Meta-analysis
  J Clin Endocrinol Metab, 2024
• Association of metformin use and cancer incidence: a systematic review and meta-analysis
  J Natl Cancer Inst, 2024
• GLP-1 receptor agonists versus metformin in PCOS: a systematic review and meta-analysis
  Reprod Biomed Online, 2019

• Understanding and overcoming metformin gastrointestinal intoleranceDiabetes Obes Metab · 2017
• Metformin and the gastrointestinal tractDiabetologia · 2016
• Metformin--mode of action and clinical implications for diabetes and cancerNat Rev Endocrinol · 2014
• Metformin: historical overviewDiabetologia · 2017
• Metformin: Is it a drug for all reasons and diseases?Metabolism · 2022
• Metformin-associated lactic acidosis: Current perspectives on causes and riskMetabolism · 2016
• Metformin and Systemic MetabolismTrends Pharmacol Sci · 2020
• A Critical Review of the Evidence That Metformin Is a Putative Anti-Aging Drug That Enhances Healthspan and Extends LifespanFront Endocrinol (Lausanne) · 2021
• Metformin derivatives - Researchers' friends or foes?Biochem Pharmacol · 2023
• Metformin: A Promising Antidiabetic Medication for Cancer TreatmentCurr Drug Targets · 2023
• Metformin - a new approachPediatr Endocrinol Diabetes Metab · 2021
• MetforminEndocrinol Metab Clin North Am · 1997