Evidence
Rosemary oil: what the study everyone quotes actually found
Rosemary oil is the most-asked-about hair treatment of the last few years, and the question I am asked is always the same: “Is it true it works as well as minoxidil?” The answer is: in one study, in men, over six months, at a modest effect, yes. Everything else you have seen is extrapolation.
The study
In 2015 a group in Iran compared rosemary oil with 2% minoxidil in 100 men with male pattern hair loss, for six months. At three months neither group had improved. At six months both had a small but measurable increase in hair count, and the two groups were not statistically different from each other. The rosemary group had less scalp itching. That is the finding: rosemary oil, in men, matched the weakest strength of minoxidil over six months, with a modest gain.
What it does not tell you
It was not a placebo-controlled trial, so we do not know how much of the gain in either group was the attention, the massage, or the time. It was in men, whose thinning has the same androgen mechanism as women’s but not the same hormonal context. It used 2% minoxidil, which is the weaker of the two strengths. It measured hair count, not thickness or coverage. And it has not been repeated at scale since. One study is a reason to be interested; it is not a body of evidence.
How it might work
Rosemary contains carnosic acid and rosmarinic acid, which in the laboratory improve blood flow to the skin, reduce inflammation, and have a mild effect on the enzyme that converts testosterone to DHT. In mice, rosemary extract partially reverses testosterone-driven hair loss. All plausible; all a long way from a bottle of oil on a human scalp.
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Dilute it. Rosemary essential oil neat on the scalp causes irritation. Two to five drops in a tablespoon of a carrier oil — jojoba, argan, or plain sweet almond — is the usual ratio.
Scalp, not lengths. Massage into the areas that are thinning for four or five minutes; leave for at least half an hour or overnight; wash out. Two or three times a week.
Six months before you judge it. The study saw nothing at three.
Not in pregnancy in the amounts a treatment would use; the evidence is thin but the caution is standard. And patch-test first behind the ear.
The commercial rosemary hair oils are mostly a carrier oil with a little rosemary, and most contain less than the study used. They are pleasant. They are haircare.
Where it fits
Rosemary oil is a mild, cheap, plausible addition. It is not a substitute for finding out why your hair is thinning, it does nothing for a shed with a cause that needs fixing, and it will not on its own restart a follicle that has been quiet for years. For the follicle-level mechanisms most mums’ thinning is driven by — androgen sensitivity, lost oestrogen shielding, missing growth signals — a formulated leave-on treatment does more, and the product guide explains the rules we judge those by. If you enjoy the rosemary ritual, keep it; use it on the evenings you are not applying anything else, and do the massage properly, because the massage may be doing as much as the oil.
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