Metformin for Longevity: What If It Was Never Meant to Work Alone?

The most famous longevity drug has never passed a longevity trial.

Metformin is old, inexpensive and taken by millions of people every day. In monkeys given it for 40 months, the brain measured about six years younger than expected.

No wonder it became the drug the longevity world talks about most.

And yet. Give metformin alone to mice in the most rigorous lifespan test there is, and they do not live longer. Give it to older adults who exercise, and it blunts what the exercise does for them.

So here is a question worth asking. What if the problem is not metformin? What if the problem is asking it to work alone?

How metformin became the longevity drug

Three things built the legend.

People on it seemed to outlive people who were healthy. A 2014 study of British health records reported that people with diabetes taking metformin lived longer than matched people without diabetes. That one finding launched a thousand headlines.

Animals aged more slowly. Worms lived up to 36% longer. Several strains of mice lived 6 to 14% longer. And in 2024, male monkeys given metformin for 40 months showed younger biological ages across their tissues, most of all in the brain.

The biology made sense. Metformin switches on AMPK, the cell's low-fuel sensor, and turns down mTOR, the growth signal that aging researchers most want to quiet. Those are the same two levers that fasting pulls.

On the strength of all this, researchers designed TAME, a trial to follow more than 3,000 older adults on metformin for six years. It is still waiting for funding.

Then the cracks appeared

Put to harder tests, metformin on its own kept coming up short.

The headline finding did not hold. In 2022 Danish researchers repeated the records study with a tighter design, including twins. People with diabetes on metformin did not outlive people without diabetes. They died sooner.

The toughest mouse test said no. The US National Institute on Aging runs the same lifespan experiment in three laboratories at once. Metformin alone did not extend life. But notice what happened in the same study when it was paired with a second drug, rapamycin: lifespan went up robustly.

It got in the way of exercise. In a trial of 53 older adults, everyone did 12 weeks of aerobic training. Half took metformin. In the metformin group, the gains in fitness and insulin sensitivity were blunted, and the improvement in their muscles' mitochondria, the parts of the cell that make energy, was wiped out.

It did not help the frail. A recent trial in frail older adults with muscle loss found no improvement in physical performance.

Read those four results together and a pattern shows. Metformin alone is not the answer. And twice, the clue points the same way: toward the mitochondria, and toward what metformin is combined with.

Metformin has two faces, and NAD+ sits between them

To see why the dose and the company matter, look at what metformin does inside a cell. It does two different things, depending on how much of it is there.

The first face: the switch. At low, everyday concentrations, metformin flips AMPK, the low-fuel sensor. Several studies report that this route raises NAD+ and wakes up the sirtuins, the repair enzymes that NAD+ feeds. This is the metformin the longevity world fell for.

The second face: the brake. At higher concentrations, metformin starts to slow complex I. That is the step in your mitochondria where spent NAD+, called NADH, is recycled back into NAD+. In the liver, metformin also tilts the balance toward the spent form. It is part of how the drug lowers blood sugar. How hard an ordinary prescription presses this brake is still argued over. Some researchers say it only shows at concentrations above what a pill delivers.

In the lab, the brake can empty the tank. In a 2023 study, breast cancer cells exposed to a very high concentration of metformin lost NAD+ day by day. When the researchers topped the cells up with NAD+ first, the cells and their mitochondria held up far better. That was hundreds of times more metformin than a pill puts in your blood, and these were cancer cells in a dish. But it shows which way the brake pulls.

A brake on your mitochondria is useful if you have diabetes. It is a strange thing to want if your goal is to age well. It may be exactly what those older adults ran into when exercise stopped improving their muscles.

So does metformin raise NAD+ or wear it down? Possibly both. It may depend on the dose and on the tissue. And that is the point: no longevity trial of metformin has been designed with NAD+ in mind. They test one drug, alone.

If you want the background on NAD+ itself, we cover it in NAD+ Injections and Aging: What If You Never Ran Low?

The case for a low dose, in good company

Put the pieces together and a different way of using metformin suggests itself. Three ideas, each with something behind it.

1. Keep the switch, skip the brake. In 2022 a study in Nature worked out how low-dose metformin acts. At the concentrations a normal pill produces, it switches on AMPK through a separate doorway in the cell, with no drop in the cell's energy. In worms, the life-extending effect of metformin disappeared when that doorway was removed. The benefit pathway, in other words, does not need the brake.

2. Supply the NAD+. If more metformin means slower NAD+ recycling, the obvious partner is NAD+ itself. In the lab study above, supplying NAD+ undid much of the damage that a heavy dose of metformin caused. Has anyone tested the pair in animals or people? Not yet. One 2022 study in aging rats gave metformin to one group and an NAD+ booster to another. Both reduced signs of aging in the tissues examined. Nobody gave them together.

3. Pair it with a GLP-1 medication. These two are old partners in diabetes care. In one small study, adding metformin to GLP-1 raised the level of active GLP-1 in the blood by about 70% and lowered blood sugar further than either alone. And remember the mice: metformin did nothing for lifespan alone, and a great deal alongside a second drug.

What we think

Is it for you?

Metformin is a prescription medicine. It is FDA-approved for type 2 diabetes. Using it for healthy aging is an off-label use, and whether it suits you is a decision for the physician who reviews your intake.

Three things to know before you ask.

• Your stomach gets a vote. About 30% of people have diarrhea, nausea or other stomach trouble on metformin. That is the main reason to start low, and the reason we say "for people who can tolerate it".
• It can lower vitamin B12 over time. Long-term users should have their B12 and kidney function checked.
• If you train hard, say so. The exercise finding above is worth a conversation with your physician.

What it costs

Keep reading: NAD+ Injections and Aging: What If You Never Ran Low? · Glutathione Injections: What If You Could Refill Your Master Antioxidant? · Sermorelin and Aging: What If Your Body Only Needed a Nudge? · Benefits of B12 Shots: What If You Are Not Tired, Just Running Low? · Do GLP-1 Medications Slow Aging?

Frequently asked questions

Does metformin slow aging?

In animals, often. Worms and several strains of mice lived longer on it, and male monkeys showed younger biological ages after 40 months. In people it is unproven. A large trial designed to test it is still waiting for funding.

Why has metformin alone disappointed?

In the most rigorous mouse lifespan program it did not extend life on its own, though it did alongside a second drug. A 2022 Danish study found no survival advantage in people. And in older adults it blunted the benefits of exercise.

Does metformin lower NAD+?

It may do both. Several studies report that metformin raises NAD+ by switching on AMPK. At higher concentrations it slows complex I, the step in the mitochondria that recycles NAD+, and in lab studies at very high concentrations it has drained NAD+ from cells. Which effect wins may depend on the dose and the tissue.

Can I take metformin with NAD+?

Nobody has tested the combination in a trial. We think it is a logical pairing, because NAD+ supplies what a higher exposure to metformin may slow down. A physician should review your history first.

Can I take metformin with a GLP-1 medication?

The two have been used together in diabetes care for years. In one small study, adding metformin raised active GLP-1 levels by about 70%. Your physician decides whether the combination suits you.

Why a low dose?

Research published in 2022 found that low, clinically relevant doses of metformin switch on AMPK without lowering the cell's energy. A low dose is also easier on the stomach. Your physician sets the dose.

What are the side effects of metformin?

About 30% of people have stomach side effects such as diarrhea and nausea. Long-term use can lower vitamin B12. Kidney function should be checked.

Sources

• Metformin at the convergence of aging and longevity, Manneh, Alasmar and El Hajj, Aging, 2026. The animal lifespan figures, the monkey study, the 2014 records study and the frailty trial are read through this review.
• Reassessing the evidence of a survival advantage in type 2 diabetes treated with metformin compared with controls without diabetes, Keys and others, International Journal of Epidemiology, 2022.
• Longer lifespan in male mice treated with a weakly estrogenic agonist, an antioxidant, an alpha-glucosidase inhibitor or a Nrf2-inducer, Strong and others, Aging Cell, 2016. The three-laboratory mouse program.
• Metformin inhibits mitochondrial adaptations to aerobic exercise training in older adults, Konopka and others, Aging Cell, 2019.
• Cellular and Molecular Mechanisms of Metformin Action, LaMoia and Shulman, Endocrine Reviews, 2021.
• Ability of metformin to deplete NAD+ contributes to cancer cell susceptibility to metformin cytotoxicity, Zhuang and others, Frontiers in Oncology, 2023.
• Low-dose metformin targets the lysosomal AMPK pathway through PEN2, Xiamen University, a summary of the 2022 study in Nature.
• A comparative study of metformin and nicotinamide riboside in alleviating tissue aging in rats, Geng and others, Life Medicine, 2023.
• Addition of metformin to exogenous glucagon-like peptide-1 results in increased serum glucagon-like peptide-1 concentrations and greater glucose lowering in type 2 diabetes mellitus, Cuthbertson and others, Metabolism, 2010.
• The TAME Trial, American Federation for Aging Research.