SubEx

Antidiabetic

Metformin

The first-line drug for type 2 diabetes. It also lowers vitamin B12, one extra deficiency for every fourteen treated, and the risk builds with the years.

Findings
5
Best evidence
★★★
Open questions
1

At a glance

Myth check

Metformin is a cheap diabetes pill that may slow ageing itself, which is why biohackers and longevity enthusiasts without diabetes take it off-label to live longer.

The anti-ageing idea grew from observational data: a 2017 pooling of 53 studies found that people with diabetes taking metformin died slightly less often than people without diabetes. That finding has not held up. A Danish registry study, including twins, found that people starting metformin for diabetes died more often than matched people without diabetes, and a 2025 review from the same research group reports that replications have generally failed, that metformin did not lengthen life in mice in a large multi-centre testing programme, and that most major trials in people without diabetes have not shown the benefits expected. In one trial, healthy adults over 65 taking 1,700 mg a day while strength training for 14 weeks gained less lean mass than those on placebo (0.41% against 1.95%). The large trial designed to test metformin against ageing in people without diabetes was, in that 2025 review, still a proposal.

The longevity case rests on observational data from people with diabetes that later replications did not reproduce, and the trial designed to test it in people without diabetes was still a proposal, with no result, as of a 2025 review.

The findingsStrongest first
1↓
Physical & bodyMetformin is what lowers the B12, not the people who take it

Over 4.3 years against placebo, metformin cut B12 by 19 percent and left 7.2 percentage points more people deficient. Because people were randomised, it is the drug doing it.

★★★Well replicated — this finding’s sources average out to meta-analyses, systematic reviews or high-powered trials.Everyone in the trial was on insulin for type 2 diabetes, so the absolute numbers may not carry to metformin alone. Deficiency was called on the blood level alone, not on markers of whether cells were short. The homocysteine rise that would prove a functional cost fell short of significance.Click to keep this openBoost energy
2↓
Physical & bodyThe longer you have been on metformin, the likelier a shortage

At five years, low or borderline B12 was twice as common on metformin as on placebo, 19.1 percent against 9.5, and each extra year raised the odds 13 percent.

★★★Well replicated — this finding’s sources average out to meta-analyses, systematic reviews or high-powered trials.B12 was measured only at years 5 and 13, so nothing is known about the years between. Frank deficiency at 13 years, 7.4 percent against 5.4, did not reach significance. The neuropathy link is an observation within the group, not a demonstration that the low B12 caused it.Click to keep this openBoost energy
3↕
Sexual & reproductiveOn top of metformin, periods got more regular and nothing else did

Six trials in 388 women compared metformin alone with metformin plus inositol. Cycles came more regularly and hair-growth scores fell about a point. Weight, glucose and insulin did not move.

★★★Well replicated — this finding’s sources average out to meta-analyses, systematic reviews or high-powered trials.The cycle-regularity interval's lower bound is 1.01, so one more small trial could take it to nothing. A one-point change on a hair-growth scale may not be visible to the person looking in the mirror. Six trials, 388 women, three to six months, and no fertility or pregnancy outcomes.Click to keep this openBalance hormones
4–
Metabolic & weightWeight, insulin and blood sugar landed the same on either one

Eight randomised trials, 1,088 women: body mass index, fasting insulin, fasting blood sugar, insulin resistance and a ratio between two pituitary hormones all crossed zero.

★★★Well replicated — this finding’s sources average out to meta-analyses, systematic reviews or high-powered trials.A null result is not proof of equivalence. Eight small trials, with moderate inconsistency on three of the five outcomes, are underpowered to rule out a real difference between the two. No risk-of-bias assessment or certainty grading is reported.Click to keep this openBalance hormones
5↑
Sexual & reproductiveThe big win over metformin comes with an interval from 2.31 to 93.58

In a network comparison of 22 trials, the myo-inositol and D-chiro-inositol blend beat metformin on menstrual frequency at odds of 14.70. The interval runs from 2.31 to 93.58.

★☆☆Contested — the evidence disagrees with itself, or the finding carries a caution. A caution or conflict always shows one star, however strong the sources are.CautionA network meta-analysis mostly compares treatments that were never tested against each other directly, so this is an indirect estimate. Twenty-two trials and 1079 women spread across four classes of treatment leaves each comparison thin. Nothing here goes beyond a surrogate: no pregnancy, no live birth.Click to keep this openBalance hormones
Sources7 behind this page

Taken with something else

What the evidence says about combining Metformin with each of these. The marker is how well established the interaction is; a caution is about the hazard, which is a different question.

What we don't know

1 open question