Does rapamycin reverse or prevent gray hair?
Cell and animal research shows that rapamycin can activate pigment cells, but there are no studies in healthy humans and the drug has serious side effects. For grey hair it is not a realistic option right now.
Only in mice and cell culture has it been shown that rapamycin can address greying. Studies in healthy humans do not exist.
How does that work? In pigment cells, a signalling protein (mTORC1) can sometimes be turned too far 'on'. This increases oxidative stress inside the cell and reduces the production of the pigment enzyme tyrosinase, causing the cell to make less pigment. Rapamycin turns mTORC1 down and fully restored pigmentation in mice in which mTORC1 was genetically overactive. Researchers observed the same pattern in human pigment cells in culture and in skin tissue from patients with tuberous sclerosis, a rare hereditary condition in which mTORC1 is permanently switched on.1,2
In a separate experiment with cultured human hair follicles, rapamycin stimulated both hair growth and pigmentation, even in already grey and white follicles that still contained a few living pigment cells. The compound increased the production of alpha-MSH, a hormone that activates pigment cells. This is the most direct lead for the question of whether rapamycin can counteract ordinary greying, but it involves tissue outside the body, not people who took the drug.3
Moreover, it does not always work. In an experiment with melanoma cells, rapamycin had no effect whatsoever on pigment production. The effect therefore depends strongly on cell type and circumstances.4 As additional context: in 14 of 71 studied patients with rare white skin patterns, mutations in the mTOR gene were found in skin cells. This confirms that overactive mTORC1 can also cause pigmentation problems in humans outside hereditary diseases, but this condition is very different from ordinary age-related greying.5
Clinical trials in healthy people do not exist. Rapamycin is also a potent drug that suppresses the immune system and has serious side effects. It is entirely unknown whether what works in the laboratory translates to ordinary greying. Rapamycin as a treatment for grey hair is not a realistic option at this time.