What Rapamycin Actually Does — And What We Still Don't Know

Alan Green, MD was the first prescribing physician of rapamycin for healthspan and longevity. A pathologist by training, he opened his practice in Little Neck, New York in 2016 and, over the following years, treated more than 1,500 patients worldwide. His central idea was that aging is not an immutable fact but a process driven in large part by elevated mTOR activity, and that intermittent rapamycin could lower that activity without the side effects of daily immunosuppressive dosing. He favored a weekly pulse — typically 6 mg once a week — over continuous daily use, arguing that daily dosing suppresses both mTORC1 and mTORC2, while a well-spaced dose preferentially touches mTORC1. His reasoning drew on the mTOR theory of aging, but Green was always clear that his evidence was clinical observation, not the kind of randomized trial that would satisfy a medical textbook.

On his website, Green also published a series of clinical observations he noticed in himself and in his patients — a set of notes that will be examined in detail in a future post here.

The compound he prescribed had arrived by a strange path. Rapamycin began as Streptomyces hygroscopicus, pulled from the soil of one of the most remote places on Earth: Easter Island, in 1964, during the Canadian Medical Expedition to Easter Island (METEI). The expedition was conceived and led by Stanley Skoyna, a McGill University surgeon and gastroenterologist, who wanted to study the isolated Rapa Nui population before the island’s airport opened and changed its isolation forever. Georges Nogrady, a microbiologist at the Université de Montréal, was the co-organizer. His soil samples were later given to Ayerst Laboratories in Montreal. From that unlikely voyage came a compound that began as an antifungal, became an immunosuppressant to protect transplanted organs, found its way into cancer therapy in both human and veterinary medicine, and — almost by accident — turned into the most robust pharmacological lever we have on the biology of aging. That last sentence deserves scrutiny, and this essay is an attempt at giving it some.

The mechanism is unglamorous. Rapamycin binds a protein called FKBP12, and the complex inhibits mTORC1, a cellular sensor that decides whether the cell is in a growth-and-build mode or a repair-and-recycle mode. Push mTORC1 down, and autophagy — the cellular composting process — comes up. In organisms from yeast to mice, that shift extends lifespan. It is not that cell growth is bad; it is that growth without pause may not be the only winning strategy. What if a cell that sometimes slows down is, on the whole, a longer-lived cell?

Beginning in 2009, the Interventions Testing Program — a project funded by the NIH and run across three independent U.S. labs — has now replicated a lifespan extension in mice more times, and more cleanly, than any other intervention we have tested. The effect is real. The effect is dose-responsive. The effect survives replication. And the effect happens when rapamycin is administered to old animals.

What we do not have — and I want to say this plainly — is a randomized, placebo-controlled trial in humans showing that intermittent low-dose rapamycin extends healthspan. We have small studies on immune function in older adults. We have observational data from off-label users. We have mechanism. We do not have the outcome trial. Anyone who tells you otherwise is either selling something or hasn't read carefully.

This blog exists in that gap. Not to fill it with certainty, but to sit inside it honestly.