Metformin received FDA approval in December 1995, making it available in the United States roughly four decades after it entered clinical use in Europe. The delay had nothing to do with the drug being ineffective. Instead, the FDA’s caution stemmed from safety concerns about an older, related drug, and metformin had to undergo extensive scrutiny before regulators were satisfied it could be used safely in American patients. Today it is the most widely prescribed diabetes medication in the world, and its story involves a surprisingly long regulatory journey, a landmark clinical trial, and a growing list of potential uses that go well beyond blood sugar.
Why It Took Decades to Reach the United States
Metformin’s active chemistry traces back to a plant called Galega officinalis, or French lilac, which had been used in European folk medicine for centuries to relieve symptoms now recognized as diabetes. In the early twentieth century, researchers isolated guanidine compounds from the plant and eventually developed synthetic versions called biguanides. Metformin was first described in the scientific literature in 1922 and entered clinical use in France and the United Kingdom in the late 1950s.
The problem for metformin’s American prospects was a sibling drug called phenformin. Another biguanide, phenformin was approved in the US in 1959 but pulled from the market in 1977 after it was linked to a dangerous buildup of lactic acid in the blood. That safety disaster cast a long shadow over the entire biguanide class. Even though metformin had a much lower risk profile, the FDA treated it with deep suspicion. European clinicians had been prescribing it for decades with a strong safety record, but American regulators wanted their own data. After extensive clinical trials and review, metformin was finally approved, entering the US market under the brand name Glucophage. Its ability to lower blood sugar without causing weight gain or increasing the risk of dangerously low blood sugar gradually won over skeptics on both sides of the Atlantic.1PubMed. Metformin: historical overview
The Landmark Trial That Changed Everything
Metformin’s approval rested on standard clinical trial data showing it effectively lowered blood sugar. But the study that truly cemented its reputation came a few years later. The UK Prospective Diabetes Study, known as UKPDS, was one of the largest and longest trials ever conducted in type 2 diabetes. It enrolled thousands of newly diagnosed patients and followed them for over a decade. A specific arm of the trial, published in 1998, looked at metformin in overweight patients and delivered results that went far beyond glucose control.
Compared to conventional treatment, patients on metformin saw a 32% reduction in any diabetes-related complication, a 42% reduction in diabetes-related death, and a 36% reduction in death from any cause. The risk of heart attack dropped by 39%. Metformin also outperformed other intensive glucose-lowering treatments, including insulin and older oral drugs called sulfonylureas, on several of these outcomes.2PubMed. Effect of intensive blood-glucose control with metformin on complications in overweight patients with type 2 diabetes (UKPDS 34) These were not just better blood sugar numbers. These were fewer heart attacks, fewer strokes, fewer deaths. No other diabetes drug at the time had shown anything like it.
A ten-year post-trial follow-up, published in 2008, found that the benefits persisted even after the formal study ended and treatment differences between groups had narrowed. Patients originally assigned to metformin still had a 33% lower risk of heart attack and a 27% lower risk of death from any cause compared to the conventional-treatment group.3PubMed. 10-year follow-up of intensive glucose control in type 2 diabetes That kind of lasting benefit, sometimes called a “legacy effect,” is unusual in drug trials and helped push metformin into the position it holds today as the standard first-line treatment for type 2 diabetes worldwide.
How Metformin Actually Works
For a drug prescribed billions of times, metformin’s mechanism of action is surprisingly complicated and still not fully understood. The traditional explanation is that it reduces the amount of glucose the liver produces and releases into the bloodstream. That is true, but it is far from the complete picture. Over the past decade, researchers have realized that metformin acts through multiple pathways simultaneously, and that the gut plays a much bigger role than anyone initially appreciated.4PubMed Central. The mechanisms of action of metformin
In the gastrointestinal tract, metformin increases glucose uptake by intestinal cells, boosts the secretion of a hormone called GLP-1 that helps regulate blood sugar, and alters the composition of gut bacteria.5PubMed Central. Understanding the action mechanisms of metformin in the gastrointestinal tract It also changes the bile acid pool in the intestine, which can influence how the body processes fats and sugars.6PubMed Central. Metformin and the gastrointestinal tract At the cellular level, the drug affects energy-sensing pathways in ways that differ depending on the dose and how long someone has been taking it. The old simple story of “metformin works by acting on the liver” has been replaced by a picture of a drug that nudges metabolism in multiple organs at once. That complexity is part of why it has so many potential applications beyond diabetes.
The Kidney Question and the 2016 Label Change
From the moment metformin was approved, the FDA restricted its use in patients with kidney problems. The logic was straightforward: metformin is cleared by the kidneys, so if kidney function is impaired, the drug could accumulate to dangerous levels and trigger lactic acidosis, the same condition that had killed the related drug phenformin. For two decades, the official labeling used a simple blood test called serum creatinine to define who could and could not take the drug, and the cutoffs were conservative. Many physicians felt the restrictions were too strict, given that the actual rate of lactic acidosis in clinical practice turned out to be very low.
In April 2016, the FDA revised the labeling in a meaningful way. The agency moved from using serum creatinine to a more accurate measure of kidney function called estimated GFR. Under the new rules, metformin could be continued in patients with kidney function as low as a GFR of 30, though the FDA recommended against starting new patients on it when their GFR fell between 30 and 45.7PubMed Central. Changes in metformin use in chronic kidney disease This change opened the door for hundreds of thousands of patients with mild to moderate kidney disease who had previously been told metformin was off-limits. It also reflected the accumulating real-world evidence that the lactic acidosis risk, while real, was far rarer than originally feared.
Vitamin B12 and Long-Term Side Effects
The most common side effects of metformin are gastrointestinal: nausea, diarrhea, stomach cramps, and bloating, especially in the first weeks of treatment. Extended-release formulations were developed partly to reduce these gut complaints, and most people find that the symptoms settle with time. But the longer-term side effect that deserves more attention is vitamin B12 depletion.
Multiple studies have confirmed that metformin interferes with the body’s ability to absorb vitamin B12. In one randomized trial, patients on metformin for about four years had a roughly 7 percentage points higher rate of B12 deficiency compared to placebo, with a number needed to harm of about 14. Patients who developed B12 deficiency also showed elevated homocysteine levels, a marker associated with cardiovascular risk.8BMJ. Long term treatment with metformin in patients with type 2 diabetes and risk of vitamin B-12 deficiency: randomised placebo controlled trial The Diabetes Prevention Program follow-up found a similar pattern: each additional year of metformin use was associated with a 13% increase in the odds of B12 deficiency, and participants with low B12 had higher rates of neuropathy, which is nerve damage that can cause numbness and tingling in the hands and feet.9PubMed Central. Long-term Metformin Use and Vitamin B12 Deficiency in the Diabetes Prevention Program Outcomes Study
The practical takeaway is that if you are on metformin for years, your doctor should periodically check your B12 levels. B12 deficiency is easy to prevent with supplements and easy to miss clinically because its symptoms, including fatigue and nerve problems, overlap with diabetes itself. Many patients and even some physicians do not realize this is a known side effect of the drug.
Diabetes Prevention in People at High Risk
One of the most influential off-label uses of metformin has been in people who do not yet have diabetes but are at high risk of developing it. The Diabetes Prevention Program, a large US trial, randomly assigned people with prediabetes to metformin, an intensive lifestyle program involving diet and exercise, or placebo. The lifestyle group saw a 58% reduction in new diabetes cases, while the metformin group saw a 31% reduction compared to placebo.10PubMed Central. Reduction in the incidence of type 2 diabetes with lifestyle intervention or metformin Lifestyle changes were clearly more powerful overall, but metformin had a stronger effect in certain subgroups: people who were more obese, people with higher fasting glucose levels, younger adults, and women with a history of gestational diabetes.11PubMed Central. Metformin and Type 2 Diabetes Prevention
Long-term follow-up showed that metformin’s preventive benefit faded somewhat over time, declining to about an 18% reduction in diabetes incidence over 15 years compared to placebo.12PubMed Central. Metformin for diabetes prevention: insights gained from the Diabetes Prevention Program/Diabetes Prevention Program Outcomes Study That is still meaningful, but it underscores that metformin is not a substitute for sustained behavioral changes. Current clinical guidelines in the US do recommend considering metformin for diabetes prevention in high-risk adults, particularly those under 60 with a BMI of 35 or higher, but it remains an adjunct to, not a replacement for, lifestyle intervention.
Polycystic Ovary Syndrome
Metformin has been prescribed for polycystic ovary syndrome, or PCOS, for decades, though the FDA has never specifically approved it for this condition. Because PCOS is closely linked to insulin resistance, a drug that improves insulin sensitivity seemed like a natural fit. Studies have confirmed that metformin can reduce androgen levels, improve menstrual regularity, and restore ovulation in some women with PCOS.13PubMed Central. Role of Metformin in Polycystic Ovary Syndrome (PCOS)-Related Infertility
However, the evidence has also made clear that metformin is not the best first-line option for all PCOS-related problems. For ovulation induction specifically, oral medications like letrozole and clomiphene citrate produce substantially better pregnancy and live-birth rates.14PubMed. Role of metformin for ovulation induction in infertile patients with polycystic ovary syndrome (PCOS): a guideline Metformin is not particularly effective as a standalone treatment for the visible signs of excess androgens like acne and unwanted hair growth, and it is not a reliable weight-loss drug for PCOS.15PubMed Central. Metformin use in women with polycystic ovary syndrome (PCOS): Opportunities, benefits, and clinical challenges Its strongest role in PCOS is probably as an add-on to other treatments in women who have clear insulin resistance, and as a metabolic intervention in patients who also have prediabetes or glucose abnormalities alongside their PCOS.
Antipsychotic-Induced Weight Gain
A less well-known use of metformin is in counteracting the weight gain caused by antipsychotic medications, which is a serious clinical problem. Drugs used to treat schizophrenia, bipolar disorder, and other psychiatric conditions frequently cause substantial weight gain and metabolic disruption. One randomized trial found that combining metformin with a lifestyle intervention reversed weight gain by about 7% over 12 weeks, while metformin alone reversed it by about 5%, compared to a continued 5% weight gain in the placebo group.16JAMA. Lifestyle Intervention and Metformin for Treatment of Antipsychotic-Induced Weight Gain: A Randomized Controlled Trial A meta-analysis confirmed a meaningful weight-reducing effect of metformin in both adults and children taking antipsychotics, with even larger effects in patients who had already gained more than 10% of their body weight.17PubMed. Metformin for weight reduction in non-diabetic patients on antipsychotic drugs: a systematic review and meta-analysis This use is entirely off-label but has gained acceptance among psychiatrists dealing with the metabolic consequences of long-term antipsychotic therapy.
Metformin in the Newer Drug Landscape
When metformin was approved in 1995, the alternatives for type 2 diabetes were limited: sulfonylureas, insulin, and not much else. The landscape has since been transformed by the arrival of several new drug classes, including SGLT-2 inhibitors and GLP-1 receptor agonists, both of which have shown cardiovascular and kidney benefits in their own large trials. Despite this, metformin has held onto its first-line position in most treatment guidelines.
Part of the reason is practical. Metformin is extraordinarily cheap as a generic, it has decades of safety data, and it lowers blood sugar about as effectively as most other monotherapy options. A large systematic review found that reductions in hemoglobin A1c were similar across monotherapies, with the exception of DPP-4 inhibitors, which had smaller effects. Metformin and the newer agents like GLP-1 receptor agonists and SGLT-2 inhibitors also share the advantage of not causing weight gain, unlike sulfonylureas, thiazolidinediones, and insulin.18PubMed. Diabetes Medications as Monotherapy or Metformin-Based Combination Therapy for Type 2 Diabetes: A Systematic Review and Meta-analysis In current practice, metformin often serves as the foundation onto which newer agents are added. Combining an SGLT-2 inhibitor with a GLP-1 receptor agonist on top of metformin, for example, addresses blood sugar from multiple angles while also providing cardiovascular and kidney protection.19PubMed. Combination therapy with SGLT-2 inhibitors and GLP-1 receptor agonists as complementary agents that address multi-organ defects in type 2 diabetes
Some guidelines have started to shift, recommending that patients with established heart disease or kidney disease go straight to an SGLT-2 inhibitor or GLP-1 receptor agonist regardless of whether their blood sugar is controlled on metformin alone. The evidence for cardiovascular benefit with those newer drugs comes from dedicated outcomes trials, whereas metformin’s cardiovascular data, while impressive, comes from a smaller subgroup within the UKPDS. The trend is not that metformin is being abandoned but that it is sharing the stage rather than standing alone on it.
The NDMA Contamination Episode
Starting in late 2019, batches of metformin around the world were recalled after testing detected the contaminant NDMA, a probable carcinogen also found in certain foods and water. The contamination affected primarily extended-release formulations and led to widespread concern among patients who had been taking the drug daily for years.20PubMed. Dispersant-First Dispersive Liquid-Liquid Microextraction (DF-DLLME), a Novel Sample Preparation Procedure for NDMA Determination in Metformin Products The FDA set acceptable daily intake limits for NDMA in metformin products and required manufacturers to test and control for the impurity before releasing batches.
Interestingly, the recalls did not lead to a decline in metformin prescriptions. An analysis of prescription data found that fills for extended-release metformin actually increased by about 9% even as the price dropped by roughly 20% during the recall period.21The American Journal of Managed Care. Safety vs Price in the Generic Drug Market: Metformin The episode highlighted the challenge of maintaining quality in the generic drug supply chain, but it did not dislodge metformin’s central role in diabetes management. For patients, the main lesson was that extended-release formulations needed the same quality scrutiny as any other pharmaceutical product, not that metformin itself was unsafe.
Cancer Risk and Observational Signals
Multiple observational studies and meta-analyses have found that people with diabetes who take metformin develop cancer at lower rates than those on other diabetes treatments. A systematic review and meta-analysis found a 31% reduction in overall cancer risk among metformin users, with the strongest signals for pancreatic cancer and liver cancer, and a trend toward a dose-response relationship.22PubMed. Metformin and cancer risk in diabetic patients: a systematic review and meta-analysis Other analyses have confirmed similar patterns.23PubMed Central. Metformin in cancer prevention and therapy
The caveat here is important: these are observational findings, not results from randomized trials designed to test whether metformin prevents cancer. People who take metformin differ in many ways from people who take other diabetes drugs, and teasing apart those differences statistically is difficult. Laboratory work has identified plausible biological mechanisms by which metformin could slow cancer growth, including its effects on cellular energy pathways, but we are still waiting for prospective trial data to confirm whether the observational signal holds up under rigorous testing. No regulatory agency has approved metformin for cancer prevention, and prescribing it for that purpose alone would be premature.
The Push to Test Metformin Against Aging Itself
Perhaps the most ambitious chapter in metformin’s story is the TAME trial, short for Targeting Aging with Metformin. The trial was designed not to test whether metformin treats any single disease, but whether it can delay the onset of multiple age-related conditions simultaneously: heart disease, cancer, cognitive decline, and death. If it succeeds, the implications go beyond the drug itself. One of the trial’s explicit goals is to establish “aging” as a legitimate indication that the FDA could recognize for drug development.24Cell Metabolism. Metformin as a Tool to Target Aging
The interest in metformin for aging comes from animal studies showing extended lifespan, epidemiological data showing that diabetic patients on metformin sometimes live as long as or longer than non-diabetic controls, and the drug’s known effects on inflammation and cellular stress pathways. Whether any of that translates into a measurable anti-aging benefit in a controlled human trial remains to be seen. TAME has faced funding and logistical delays, and its results are still years away. But the fact that a major clinical trial is even attempting to treat aging as a targetable process, using a cheap generic drug as the test case, speaks to how far metformin’s story has traveled from its origins as a botanical folk remedy for diabetic symptoms in medieval Europe.