Metformin: what it does, what it's approved for, and what's hype

Metformin is about as old and as ordinary as a prescription drug gets. It's a generic pill that has been sold in the US since 1995, and it's usually the first drug a person with type 2 diabetes is offered. It's also one of the drugs the longevity world talks about most. Off-label means a licensed prescriber can use it for something the FDA label doesn't cover, and every longevity use of metformin is off-label. The gap between those two reputations is what this page sorts out.

Metformin lowers the sugar your liver puts into your blood

Metformin's main job is to make your liver release less glucose. The label puts it in one sentence: metformin “decreases hepatic glucose production, decreases intestinal absorption of glucose, and improves insulin sensitivity by increasing peripheral glucose uptake and utilization” ^[1].

1. Your liver makes less new glucose, especially overnight, so fasting blood sugar drops.
2. Your gut absorbs a little less glucose from food.
3. Your muscles and other tissues take up and use more of the glucose in your blood, which is what “better insulin sensitivity” means.
4. Your pancreas doesn't pump out extra insulin. The label says “insulin secretion remains unchanged.”

That last point is why metformin on its own rarely causes low blood sugar. Drugs that force the pancreas to release insulin can push blood sugar too low. Metformin doesn't work that way, though the label warns the risk rises when it's combined with insulin or a sulfonylurea.

Metformin is approved for type 2 diabetes, and nothing else

The US label approves metformin for one thing: improving blood sugar control in type 2 diabetes, alongside diet and exercise ^[1]. Everything else people use it for is off-label. Some of those uses rest on solid trials. Others rest on hope.

What is metformin used for other than diabetes? In the US, mostly the off-label uses above. Labels also differ by country. The UK's NHS lists using it to help prevent type 2 diabetes, in gestational diabetes, and for symptoms of polycystic ovary syndrome, which the NHS now calls polyendocrine metabolic ovarian syndrome ^[4]. The US label lists none of those.

The big trials show real benefits in diabetes and prediabetes

Two trials carry most of metformin's reputation. UKPDS 34 followed overweight adults with newly diagnosed type 2 diabetes for a median of about 10.7 years. Those who started on metformin had 32% fewer diabetes-related events, 42% fewer diabetes-related deaths and 36% lower all-cause mortality than those on conventional therapy ^[7]. In the Diabetes Prevention Program, metformin cut new diabetes by 31% in adults with prediabetes ^[8].

The DPP result has a second lesson that gets lost. The lifestyle program, aiming for 7% weight loss and 150 minutes of activity a week, beat metformin by a wide margin. To prevent one case of diabetes over three years, about 7 people had to do the lifestyle program, against about 14 taking metformin ^[8].

Does metformin help you lose weight? A little, at most. The label's own trials show small weight changes: in one 29-week trial, people on metformin lost 1.4 pounds on average, and people on placebo lost 2.4 ^[1]. If weight is the main goal, GLP-1 drugs like semaglutide produce far larger losses, which is the point of our metformin vs Ozempic comparison.

Metformin for longevity: promising in theory, unproven in people

Metformin's longevity reputation comes mostly from one kind of evidence: observational studies of people with diabetes. In a large UK cohort, people with type 2 diabetes who started on metformin lived about as long as, or slightly longer than, matched people without diabetes ^[9]. That's interesting, but it can't separate the drug from the kind of people who get prescribed it.

No randomized trial has shown that metformin slows aging or extends life in people without diabetes. A trial called TAME was designed to test it ^[12], and no results have been published. Meanwhile, two trials in older adults found metformin blunted the benefits of exercise ^{[10] [11]}. For someone taking it to stay healthy, that's a real trade-off. The full story is on our metformin and longevity page, and rapamycin is the other off-label longevity drug people often weigh against it.

Stomach side effects are common; lactic acidosis is rare but serious

The most common metformin side effects are diarrhea, nausea or vomiting, gas, weakness, indigestion, abdominal discomfort and headache, each reported in more than 5% of people in the label's trials ^[1]. They tend to show up early. The extended-release form exists partly to make them easier to live with.

The boxed warning is lactic acidosis, a buildup of acid in the blood. It's rare, but the label says postmarketing cases have been fatal. Risk rises with kidney impairment, age 65 or older, contrast imaging, surgery, low oxygen, heavy drinking and liver impairment. Metformin is contraindicated when kidney function (eGFR) is below 30, and in metabolic acidosis ^[1].

Feeling run down on metformin is one of the most common questions people ask. Our guide to metformin and tiredness covers what the label lists, including the B12 link.

Some extended-release lots were recalled in 2020 for an impurity

In 2020 FDA found an impurity called NDMA above the acceptable intake limit in some extended-release metformin lots, and those lots were recalled. FDA said its testing had not found NDMA in immediate-release metformin ^[3]. FDA then asked extended-release makers to test every batch before release.

Metformin comes as immediate-release and extended-release tablets

Metformin comes in two forms. Immediate-release tablets come in 500, 850 and 1,000 mg. Extended-release tablets release the drug slowly and come in strengths that vary by maker ^[1]. Your prescriber chooses the form, the dose and when you take it, and the label adjusts the dose for kidney function.

What does a metformin pill look like? It depends on the maker. Generics come in different shapes and imprints, usually white to off-white. If a refill looks different from your last one, check the imprint with your pharmacist before you take it.

Metformin also comes inside combination pills

Metformin is often paired with a second diabetes drug in a single tablet. One example is Synjardy, which combines empagliflozin, the active ingredient in Jardiance, with metformin ^[6]. Jardiance on its own contains no metformin ^[5]. Combination pills carry the warnings of both drugs, so the metformin boxed warning applies to them too.

Some online weight programs also sell custom combinations that include metformin alongside other drugs. Those are different products with different evidence, and they aren't counted in the metformin prices below. If a program's “metformin” plan lists more than one ingredient, ask exactly what's in it and whether it's compounded.

Kidney function decides who can take metformin

Kidney function is the main gate. Metformin leaves the body through the kidneys, so the label rules it out below an eGFR of 30 and limits starting it when kidney function is already reduced ^[1]. It's also paused around iodinated contrast imaging in some people, and the label lists heavy drinking and liver impairment as risk factors for lactic acidosis.

Age matters too. The label counts age 65 and older as a lactic acidosis risk factor and advises more frequent kidney checks in older adults. None of that makes metformin a risky drug for most people with type 2 diabetes, but it's why a prescriber orders labs before writing it, even for an off-label goal.

What online programs charge for metformin

Metformin itself is a long-off-patent generic. Online programs that prescribe it charge $25–$162 a month, and that figure usually bundles the medication with a visit or a membership. Here's what each program charges.

If you already have a doctor, a prescription filled at a local pharmacy may cost less than any program above. Online programs mainly sell convenience and, for longevity use, a prescriber willing to consider it. Our monthly cost calculator helps compare a plan's billing with a pharmacy price, and the longevity prescription programs board scores the services.

Metformin is prescription-only, in person or online

Metformin isn't sold over the counter in the US; you need a prescription. A primary care doctor can prescribe it, and so can a telehealth clinician after an online visit. A prescriber will usually want recent kidney function and blood sugar results first. For the online route, read how to get a metformin prescription online.

Metformin and GLP-1 drugs are different, and often used together

Metformin and GLP-1 drugs like Ozempic work in different ways. Metformin mainly turns down the liver's glucose output. GLP-1 drugs act on gut-hormone receptors, slow stomach emptying and reduce appetite. Many people with type 2 diabetes take both, and the SURPASS-2 trial of Mounjaro enrolled people already on metformin. Our metformin vs Ozempic comparison walks through the differences.