Metformin is prescribed for a surprisingly wide range of conditions beyond type 2 diabetes, from polycystic ovary syndrome and antipsychotic-induced weight gain to emerging research on aging itself. Originally derived from a compound found in a European herb called goat’s rue, metformin was first used to treat diabetes in 1957, but its ability to influence inflammation, cellular energy use, and hormonal signaling has made it one of the most studied drugs for repurposing in modern medicine.1PubMed. Metformin: historical overview Some of these uses are well-established in clinical practice, while others remain promising but unproven in humans.
Polycystic Ovary Syndrome and Fertility
The most common non-diabetes use of metformin is for polycystic ovary syndrome (PCOS), a hormonal condition that affects roughly one in ten women of reproductive age. PCOS often involves insulin resistance, elevated androgen levels, irregular periods, and difficulty ovulating. Metformin addresses several of these problems at once: studies show it reduces androgen levels, improves insulin sensitivity, helps restore regular menstrual cycles, and can trigger ovulation.2PubMed Central. Role of Metformin in Polycystic Ovary Syndrome (PCOS)-Related Infertility
That said, metformin is not considered the go-to fertility treatment for women with PCOS who are trying to conceive. Professional guidelines note that while metformin increases ovulation rates compared to a placebo, oral ovulation-induction agents like letrozole or clomiphene citrate are considerably more effective at achieving ovulation, pregnancy, and live births.3Fertility and Sterility. Role of metformin for ovulation induction in infertile patients with polycystic ovary syndrome (PCOS): a guideline In practice, many clinicians still prescribe metformin alongside those agents, or on its own when the primary goal is managing insulin resistance and menstrual regularity rather than immediate pregnancy.
Preventing Antipsychotic-Induced Weight Gain
People taking antipsychotic medications for schizophrenia, bipolar disorder, or other psychiatric conditions often gain significant weight, sometimes rapidly. That weight gain brings real metabolic consequences: higher blood sugar, worse cholesterol profiles, and greater cardiovascular risk. Metformin has emerged as the only pharmacological agent with solid evidence for preventing this side effect. When started at the same time as an antipsychotic, metformin can reduce the extent of weight gain by about 4 kilograms compared to controls.4PubMed Central. Metformin for the Prevention of Antipsychotic-Induced Weight Gain: Guideline Development and Consensus Validation
A meta-analysis of twelve studies with over 700 patients confirmed that metformin also helps reverse weight gain that has already occurred, and improves body mass index and insulin resistance compared to placebo.5PubMed Central. Metformin in prevention and treatment of antipsychotic induced weight gain: a systematic review and meta-analysis A separate meta-analysis found benefits not just for weight and BMI, but also for fasting glucose, fasting insulin, triglycerides, and total cholesterol.6Journal of Clinical Psychopharmacology. Metformin for Weight Gain and Metabolic Abnormalities Associated With Antipsychotic Treatment: Meta-Analysis of Randomized Placebo-Controlled Trials This makes metformin one of the more practical tools psychiatrists have for managing a side effect that often drives patients to stop taking their antipsychotic altogether.
Weight Loss and Appetite Suppression
Beyond the psychiatric context, metformin’s weight-loss effects have attracted broader attention. People with type 2 diabetes who take metformin tend to lose a modest amount of weight rather than gain it, which is unusual among diabetes medications. Research has traced much of this effect to a hormone called GDF15. In both mouse models and two independent randomized controlled clinical trials in humans, metformin was shown to raise circulating GDF15, which acts on receptors in the brainstem to reduce food intake and lower body weight.7Nature. GDF15 mediates the effects of metformin on body weight and energy balance When GDF15 activity was absent, metformin lost its effects on appetite and body weight, though it still lowered blood sugar.8PubMed. Metformin-induced increases in GDF15 are important for suppressing appetite and promoting weight loss
In practice, the weight loss from metformin alone is modest compared to newer GLP-1 drugs like semaglutide. But metformin is generic, cheap, widely available, and well-tolerated by most people, which keeps it relevant in weight management conversations, especially for patients who cannot access or afford newer therapies.
Long COVID Prevention
One of the more unexpected findings in recent years came from the COVID-OUT trial, a randomized, quadruple-blind phase 3 study of outpatient COVID-19 treatment. Participants who received metformin early in their infection had a cumulative incidence of long COVID of about 6% by day 300, compared to about 10% in the placebo group. When metformin was started within three days of symptom onset, the risk dropped even further.9The Lancet Infectious Diseases. Outpatient treatment of COVID-19 and incidence of post-COVID-19 condition over 10 months (COVID-OUT): a multicentre, randomised, quadruple-blind, parallel-group, phase 3 trial
Additional evidence supports this pattern. The ACTIV-6 randomized trial found a lower risk of clinician-diagnosed long COVID with metformin compared to placebo, though the confidence intervals were wider and the effect fell just short of conventional significance thresholds.10PubMed. Metformin on the Presence of COVID-19 Symptoms 6 Months after Infection: The ACTIV-6 Randomized Clinical Trial A large population-based cohort study in people with type 2 diabetes also found that metformin use was associated with a lower risk of post-acute symptoms after COVID infection.11PubMed Central. Prior SGLT2 Inhibitor and Metformin Use and Risk of Long COVID in Type 2 Diabetes: A Nationwide Population-Based Cohort Study The mechanism is not entirely clear, but metformin’s anti-inflammatory and antiviral properties are likely involved. This is still an active area of research, and metformin is not currently a standard recommendation for COVID treatment.
The Aging Question
Perhaps the most ambitious claim about metformin is that it could slow aging itself. In laboratory animals, metformin has been shown to delay the onset of age-related diseases and extend lifespan.12PubMed Central. Metformin’s Mechanisms in Attenuating Hallmarks of Aging and Age-Related Disease Reviews of the evidence suggest it works on multiple fronts: improving how cells sense nutrients, enhancing autophagy (the process by which cells clean out damaged components), protecting against DNA damage, and reducing the accumulation of senescent cells that contribute to tissue aging.13Cell Metabolism. Benefits of Metformin in Attenuating the Hallmarks of Aging
Whether this translates to humans is the central unanswered question. The TAME (Targeting Aging with Metformin) trial was designed specifically to test it. The study plans to enroll 3,000 people aged 65 to 79 across roughly 14 centers in the United States, measuring time to a composite outcome that includes cardiovascular events, cancer, dementia, and death. Rather than studying each disease separately, the trial treats aging as the underlying condition.14PubMed Central. Metformin as a Tool to Target Aging The trial was developed in consultation with the FDA to potentially create an entirely new regulatory indication: targeting aging.15PubMed Central. TARGETING AGING WITH METFORMIN (TAME) If successful, it would represent a genuine paradigm shift in how aging is treated medically. Results are still pending.
Cardiovascular Benefits in People Without Diabetes
Metformin’s cardiovascular benefits in people with diabetes are well documented, but researchers have been asking whether these benefits extend to people without diabetes. A scoping review of studies looking at metformin’s effects on endothelial dysfunction (the earliest stage of blood vessel disease) in non-diabetic conditions found a visual trend toward benefit across several markers, including improvements in how blood vessels dilate and reductions in inflammatory molecules that line blood vessel walls.16PubMed. Metformin for endothelial dysfunction in non-diabetic disorders: a scoping review
A Mendelian randomization study took a different approach, using genetic data to simulate what happens when metformin’s drug targets are activated. The analysis suggested that the effects of metformin’s targets could reduce coronary artery disease risk by roughly 38%, along with lowering blood pressure and BMI, even in non-diabetic individuals.17EBioMedicine. Efficacy of metformin targets on cardiometabolic health in the general population and non-diabetic individuals: a Mendelian randomization study This is genetic modeling, not a clinical trial, so it points to plausible benefit rather than proven benefit. Still, the direction of evidence across multiple study types is consistent enough to keep cardiovascular researchers interested.
Neuroprotection and Brain Health
Metformin’s potential effects on the brain are being explored in the context of neurodegenerative diseases. Laboratory and preclinical studies show that metformin can trigger autophagy in neurons, reduce neuroinflammation, and protect against the kind of oxidative damage that accumulates in conditions like Alzheimer’s and Parkinson’s disease.18Frontiers in Aging Neuroscience. Exploring the Pharmacological Potential of Metformin for Neurodegenerative Diseases Separately, research has pointed to metformin’s possible neuroprotective role specifically in cognitive decline associated with type 2 diabetes.19PubMed Central. Neuroprotective Effects of Metformin Through the Modulation of Neuroinflammation and Oxidative Stress
The honest assessment here is that this evidence is still early-stage. Most findings come from cell cultures and animal models, and human trial data specifically testing metformin for dementia prevention is limited. The TAME trial, described above, includes dementia as one of its composite endpoints, so it may eventually provide clearer answers. For now, no one should take metformin specifically to prevent Alzheimer’s based on current evidence.
Fatty Liver Disease
Non-alcoholic fatty liver disease (NAFLD) frequently coexists with insulin resistance, making metformin a natural candidate for treatment. In animal models, metformin reliably reduces fat accumulation in the liver through several pathways. In human patients, the picture is messier. A meta-analysis found that metformin improved liver enzyme levels (ALT and AST), triglycerides, total cholesterol, and insulin resistance in NAFLD patients, though it did not significantly change BMI.20PubMed Central. Effect of metformin on nonalcoholic fatty liver based on meta-analysis and network pharmacology However, a review noted discrepancies across studies regarding whether metformin actually reduces the amount of fat in the liver itself.21PubMed Central. Effects of Metformin on Hepatic Steatosis in Adults with Nonalcoholic Fatty Liver Disease and Diabetes: Insights from the Cellular to Patient Levels
Adding further nuance, a real-world cohort study of patients with type 2 diabetes found that long-term metformin use was not associated with a lower incidence of NAFLD, and some dose categories even showed slightly higher odds of developing fatty liver.22Frontiers in Endocrinology. Correlation between long-term use of metformin and incidence of NAFLD among patients with type 2 diabetes mellitus: A real-world cohort study This likely reflects the complexity of the population being studied (people already at high metabolic risk) rather than a harmful effect of metformin. Current guidelines generally do not recommend metformin as a standalone treatment for NAFLD, though it remains commonly prescribed to these patients for their underlying insulin resistance.
Skin Conditions
Metformin has drawn attention in dermatology, particularly for hidradenitis suppurativa (HS), a chronic inflammatory skin disease involving painful abscesses and scarring, often in the groin, armpits, and under the breasts. Research shows that metformin reduces key inflammatory markers in HS patients. In one study, patients who took metformin for at least six months had significantly lower expression of several inflammatory cytokines in their blood cells compared to patients not taking the drug. In skin tissue taken directly from HS lesions, metformin reduced the secretion of multiple inflammatory molecules.23British Journal of Dermatology. Metformin has anti-inflammatory effects and induces immunometabolic reprogramming via multiple mechanisms in hidradenitis suppurativa Additional work suggests metformin also improves cardiovascular risk markers and insulin resistance in HS patients.24PubMed Central. Metformin Treatment of Hidradenitis Suppurativa: Effect on Metabolic Parameters, Inflammation, Cardiovascular Risk Biomarkers, and Immune Mediators
Beyond HS, researchers have explored metformin’s anti-androgenic, anti-angiogenic, and antioxidant properties as potential treatments for acne vulgaris and rosacea.25PubMed Central. Metformin: A Potential Treatment for Acne, Hidradenitis Suppurativa and Rosacea These uses remain largely theoretical and are not yet part of standard dermatological practice, but they illustrate how broadly metformin’s anti-inflammatory effects reach.
Changes to Gut Bacteria
One of metformin’s more intriguing effects is what it does to the microbiome. People taking metformin tend to have a higher abundance of Akkermansia muciniphila, a bacterium associated with healthy gut lining, along with increased populations of bacteria that produce short-chain fatty acids, molecules important for gut health and immune regulation.26PubMed. Metformin Is Associated With Higher Relative Abundance of Mucin-Degrading Akkermansia muciniphila and Several Short-Chain Fatty Acid-Producing Microbiota in the Gut In mice fed a high-fat diet, metformin treatment both increased Akkermansia levels and boosted the number of mucin-producing goblet cells that line the intestine.27PubMed. An increase in the Akkermansia spp. population induced by metformin treatment improves glucose homeostasis in diet-induced obese mice
This is relevant because some of metformin’s gastrointestinal side effects (bloating, diarrhea, nausea) may actually be the flip side of these microbiome changes. The gut is where metformin concentrates at its highest levels, and reshaping the bacterial community there seems to be part of how the drug works, not just a side effect. Researchers have even proposed that these microbiome shifts contribute to metformin’s glucose-lowering ability, suggesting that the gut plays a bigger role in the drug’s mechanism than previously appreciated.
Polycystic Kidney Disease
Autosomal dominant polycystic kidney disease (ADPKD) is a genetic condition in which fluid-filled cysts gradually enlarge the kidneys and destroy function over decades. A feasibility study tested metformin in ADPKD patients and found that while overall differences in kidney volume and function between metformin and placebo groups were not statistically significant, the numbers trended in the right direction. In patients with the largest kidneys (above 800 mL per meter of height), metformin was associated with significantly less kidney growth compared to placebo.28PubMed Central. Metformin Therapy in Autosomal Dominant Polycystic Kidney Disease: A Feasibility Study The decline in kidney filtration rate was also numerically much smaller in the metformin group, even though those patients had more severe disease at the start. Larger trials are needed, but the drug’s safety profile and low cost make it an attractive candidate for a disease with few treatment options.
How One Drug Does So Many Things
It is reasonable to wonder how a single pill could plausibly affect PCOS, cancer risk, brain health, gut bacteria, and aging all at once. The answer lies in metformin’s mechanism. Rather than hitting one narrow target like most drugs, metformin works through several overlapping cellular pathways. It inhibits mitochondrial respiration, activates an enzyme called AMPK that acts as a cellular energy sensor, and influences additional pathways involving the lysosome and mitochondrial enzymes.29PubMed Central. The mechanisms of action of metformin It also directly reduces the production of reactive oxygen species, the damaging molecules that contribute to inflammation and tissue aging, by interfering with the mitochondrial electron transport chain.30Frontiers in Genetics. Metformin: Activation of 5′ AMP-activated protein kinase and its emerging potential beyond anti-hyperglycemic action
Because energy metabolism, inflammation, and oxidative stress are involved in virtually every chronic disease, a drug that modulates all three has broad potential. That does not mean metformin is a wonder drug for everything. It means its biological reach is unusually wide, which is why so many different medical specialties have reason to study it.
The Exercise Interaction
One finding that surprises people is that metformin may partially blunt the fitness benefits of exercise. A systematic review and meta-analysis found that compared to exercise alone, adding metformin was associated with smaller improvements in peak oxygen uptake (a measure of cardiorespiratory fitness) and reduced blood pressure improvements.31eClinicalMedicine. The effects of metformin and exercise training on cardiorespiratory, blood pressure, and metabolic adaptations across the spectrum of glucose dysregulation: a systematic review and meta-analysis One prospective study found that metformin cut the improvement in VO2max roughly in half compared to exercise alone in people with elevated blood sugar.32PubMed Central. Effects of chronic metformin treatment on training adaptations in men and women with hyperglycemia: A prospective study
This does not mean you should stop exercising if you take metformin, or that metformin cancels out exercise entirely. The blood sugar and lipid benefits were unchanged whether metformin was added or not. But the attenuation of cardiorespiratory fitness gains is worth knowing about, particularly for people who are already active and considering metformin for off-label reasons like longevity. For someone with diabetes who needs both metformin and exercise, the metabolic benefits of the combination still outweigh the modest fitness trade-off.
Vitamin B12 and Other Practical Concerns
Metformin is one of the most tolerable medications in wide use, but it is not without drawbacks. The most clinically significant long-term concern is vitamin B12 depletion. A randomized placebo-controlled trial found that metformin treatment was associated with a roughly 19% decrease in B12 concentration over about four years, with the absolute risk of B12 deficiency about 7 percentage points higher in the metformin group than in the placebo group.33PubMed. Long term treatment with metformin in patients with type 2 diabetes and risk of vitamin B-12 deficiency: randomised placebo controlled trial Data from the Diabetes Prevention Program confirmed that each additional year of metformin use raised the odds of B12 deficiency.34PubMed Central. Long-term Metformin Use and Vitamin B12 Deficiency in the Diabetes Prevention Program Outcomes Study B12 deficiency can cause fatigue, numbness, and cognitive symptoms that mimic other conditions, so periodic monitoring and supplementation are sensible for anyone on metformin long-term.
Gastrointestinal symptoms (nausea, diarrhea, bloating) are the most common short-term side effects and usually improve within weeks, especially with the extended-release formulation. The most serious rare risk is lactic acidosis, a dangerous buildup of lactic acid in the blood. This is almost exclusively a concern in people with significant kidney disease, where metformin clearance is impaired.35PubMed Central. Lactic Acidosis Associated with Metformin in Patients with Diabetic Kidney Disease For people with normal kidney function, lactic acidosis on metformin is exceedingly rare. Kidney function is typically checked before starting the drug and periodically thereafter.
The Off-Label Landscape
Metformin occupies an unusual position in medicine. It has been around for nearly seven decades, costs pennies per pill, is off-patent, and has a safety record built on billions of patient-years of use. Those same qualities that make it appealing for repurposing also mean there is little financial incentive for pharmaceutical companies to fund the large trials needed to earn new official approvals. Most of the non-diabetes uses described here are prescribed off-label, meaning doctors use their clinical judgment to prescribe for a condition outside the drug’s formal indication.
The strength of evidence varies enormously across these uses. PCOS management and antipsychotic-induced weight gain are supported by multiple randomized trials and professional guidelines. Long COVID prevention has one strong randomized trial and growing observational support. Neuroprotection and cancer prevention remain largely preclinical. The TAME aging trial, when it reports results, could reshape the conversation entirely. In the meantime, metformin remains a fascinating case study in what happens when a cheap, safe, widely available drug turns out to do far more than anyone initially expected of it.