“Rapamycin is the most promising anti-aging drug”

The strongest evidence is a mouse programme run at three independent research centres in the United States, part of the National Institute on Aging's Interventions Testing Program. Rapamycin extended both the typical and the maximum lifespan of male and female mice, by 9% in males and 14% in females when measured at the age by which nine in ten animals had died - and it worked even though feeding did not start until the mice were 600 days old and already late in life [1].

That is the most reproducible lifespan result any drug has produced in a mammal, and it is the honest basis for calling rapamycin promising.

Rapamycin works by blocking mTOR, short for mechanistic target of rapamycin, a protein switch that tells cells when to grow and that holds back autophagy, the cell's internal clean-up and recycling process. Because rapamycin is already an approved medicine for other uses, mTOR has become the main target for scientists, doctors and biotechnology companies trying to treat the diseases of ageing [2].

The gap is people. Human trials so far have asked much narrower questions than lifespan - whether an mTOR-blocking drug can safely prevent, delay or treat one specific age-related disease [2] - and the one lifespan-style trial now under way is in dogs rather than humans: TRIAD, the Test of Rapamycin in Aging Dogs.

The first year-long randomised trial in healthy, normally ageing adults has now reported. PEARL, short for Participatory Evaluation of Aging with Rapamycin for Longevity, gave 5 or 10 milligrams of rapamycin once a week, or a dummy pill, for 48 weeks.

Side effects were no more frequent than on the dummy pill, but the trial's main measure - fat stored around the internal organs - did not change, and only a few secondary measures moved, such as lean tissue and self-reported pain in the women taking the 10 milligram dose [3].

So the human evidence to date shows tolerability, not benefit, and certainly not extra years of life.

At the much higher daily doses used to stop the body rejecting a transplanted organ, rapamycin suppresses the immune system by design, and carries known risks including slower wound healing and raised blood fats and blood sugar. The low weekly doses being studied for ageing are a separate, experimental use.

Rapamycin is prescription-only. Taking it outside its approved purpose is a decision for a doctor, not something to start on mouse data alone.