Encyclopedia · pathway modulator
Rapamycin (sirolimus)
The longevity-world favourite is a hair paradox: one 2019 mouse study found it starts hair growth, while older and 2013 mouse work found it does not or even delays it, and mTOR-pathway cancer drugs can cause hair loss.
What it is
Sirolimus is a prescription immunosuppressant (transplant, rare diseases) that inhibits mTOR. Any hair use is off-label and experimental. Prescription-only in the US, EU and UK.
How it works
mTORC1 activity changes with the hair cycle. Whether blocking it helps depends on the model: it can trigger autophagy, which is linked to entering the growth phase, but it can also slow cycling.
What the studies show
- In mice, cyclosporin A and FK506 induced growth, but rapamycin did not and blocked their effect. Iwabuchi 1995
- In mice, rapamycin delayed the start of the hair cycle. Kellenberger 2013
- In mice, rapamycin with metformin and alpha-ketoglutarate stimulated growth from rest through autophagy. Chai 2019
- Dissolvable microneedles with rapamycin and EGCG nanoparticles regrew mouse hair faster than no treatment (untreated comparison only). Lin 2022
- Rapamycin-primed stem-cell exosomes gave more follicles and density than unprimed ones in mice. Shrestha 2025
- A meta-analysis of cancer-drug trials found targeted therapies as a group raise alopecia risk (RR 7.9 vs placebo, incidence 14.7%); an mTOR-specific figure was not extracted. Belum 2015
- No human hair trial of rapamycin was found.
How it's used in studies
Topical, microneedle and exosome-priming protocols in mice only.
Side effects and safety
An immunosuppressant with serious infection, metabolic and wound-healing risks; avoid in pregnancy. Off-label "longevity" use is outside the evidence.
Bottom line
Mixed direction in animals and no human data. Do not use it for hair. Discuss with a doctor.
Fun facts
- Three mouse studies give three different answers on whether rapamycin grows, delays or ignores hair cycling.
Studies 6
Iwabuchi T et al. Effects of PPIase inhibitors on hair growth initiation in mouse. J Dermatol Sci. 1995
Result: Cyclosporin A and FK506 induced anagen; rapamycin did not and blocked their effect.
Read the study ↗Kellenberger AJ, Tauchi M. Mammalian target of rapamycin complex 1 (mTORC1) may modulate the timing of anagen entry in mouse hair follicles. Exp Dermatol. 2013
Result: Rapamycin delayed hair-cycle initiation.
Read the study ↗Chai M et al. Stimulation of hair growth by small molecules that activate autophagy. Cell Rep. 2019
Result: Metformin, rapamycin and alpha-ketoglutarate started growth in resting follicles through autophagy; oral alpha-ketobutyrate prevented hair loss in aged mice.
Read the study ↗Lin Y et al. Dissolvable microneedles loaded with rapamycin and EGCG nanoparticles for hair regrowth. Pharmaceutics. 2022
Result: Faster hair regrowth than no treatment (comparison was untreated).
Read the study ↗Shrestha M et al. Rapamycin-primed MSC-derived exosomes. Theranostics. 2025
Result: Rapamycin-primed exosomes gave more follicles and density than naive exosomes.
Read the study ↗Belum VR et al. Alopecia in patients treated with molecularly targeted anticancer therapies. Ann Oncol. 2015
Result: Overall all-grade alopecia 14.7% (RR 7.9 vs placebo); vismodegib 56.9% (95% CI 50.5-63.1); bortezomib lowest at 2.2%.
Read the study ↗Related
- MetforminThe world's most-used diabetes pill woke sleeping hair follicles in mice and was linked to regrowth in 6 of 12 women with a scarring alopecia, but there is no randomized hair trial and it is not a baldness treatment.
- Alpha-ketoglutarate (and alpha-ketobutyrate)Two ordinary body metabolites woke dormant mouse hair follicles in a 2019 study, and oral alpha-ketobutyrate prevented hair loss in aged mice, but there is no human data.
- Cyclosporine (ciclosporin)A transplant and psoriasis immunosuppressant that makes hair grow as a side effect and works off-label in alopecia areata, but is toxic to kidneys and blood pressure.
- SpermidineA natural compound sold as a longevity supplement lengthened human hair follicles grown in a dish at tiny concentrations, but nobody has shown it grows hair on a head.