Identity
Rosemary Oil is the essential oil obtained by steam distillation of the leaves of Rosmarinus officinalis, now also classified as Salvia rosmarinus. It is a mixture rather than a single molecule, with 1,8-cineole, camphor, borneol, bornyl acetate and the pinenes among its principal constituents, and its composition varies by chemotype and origin.
In cosmetics it has a long and unremarkable history as a fragrance and hair ingredient. Its current prominence comes from somewhere else entirely.
Development & history
Rosemary carried no particular hair reputation until 2015, when a randomised trial compared it directly against minoxidil 2% in androgenetic alopecia. That single paper became the foundation of an enormous volume of subsequent claim-making, and the distance between what it reported and what is now asserted on its behalf is the whole story of this entry.
The trial itself was not small or careless in conception: 100 participants, six months, standardised microphotographic assessment at baseline, three months and six months. Its weakness is architectural rather than sloppy. Both arms received an active treatment. Without an untreated arm, an improvement seen in both groups cannot be attributed to either.
Worth noting from the same report: at the three-month point neither group had changed significantly. The comparison only becomes interesting at six months, in both arms at once, which is exactly the pattern an absent control group leaves unexplained.
Mechanism (as proposed)
Two mechanisms are offered, and both are proposed rather than demonstrated for the oil in humans.
The circulation account holds that rosemary improves microcapillary perfusion of the scalp, which is presented as parallel to one of the routes attributed to minoxidil.
The enzyme account holds that rosemary constituents inhibit 5-alpha reductase, the enzyme producing DHT, making it a mild analogue of a very different class of medicine. The signal for this comes from animal work, and the material used there was a rosemary leaf extract rather than the essential oil. That substitution passes almost unnoticed in popular summaries, and it should not: the mechanism attributed to the oil was observed in a different preparation, in a different species.
Neither account is implausible. Both are currently reasons the result could be true rather than evidence that it is.
Related reading
Our journal article follows this claim back to its source in full detail. For the biology the mechanism claims to act on, see our explanation of DHT and hair loss, and for what the evidence supports in pattern hair loss generally, the hair loss guides.