Rosemary oil is probably the most repeated hair tip on the internet in recent years. It works, say one camp. It does not work, say the other. The truth, as usual, is duller and more useful than either.

Luz Aranda, who works at the company behind several haircare brands, states the effect precisely, without promises: rosemary is "particularly associated with stimulating blood circulation in the scalp". That means support for the follicles and a better foundation. "A well-cared-for scalp is the basis for maintaining strong and healthy hair", she says. That is all. Not a word about bringing back lost hair.

And here is the part the internet skips over: rosemary oil will not stop hair loss once it has already started. Hair loss has various causes, from hormonal to medical, and when it persists you go to a specialist, not to a cosmetics shop. A small bottle is not a diagnosis.

The second part of the advice matters more than the first, and almost nobody says it: pure rosemary essential oil does not go directly onto the scalp. "Essential oils are highly concentrated ingredients and must be used appropriately diluted", Aranda says, adding that she would always recommend "a formula developed specifically for the scalp". So: a product in which rosemary is an ingredient, not a bottle of essential oil off the spice shelf.

The application is simple. A small amount on the scalp before washing. A few minutes of gentle massage with the fingertips, in circular motions. Then work it down the length to the ends. Leave it ten minutes, then wash with shampoo and rinse thoroughly.

How often? "Once or twice a week is enough. Consistency matters more", Aranda says. Whether it is morning or evening makes no difference at all.

The word carrying the whole story is "consistency", and that is precisely the word that does not sell well. Which is why the industry around it offers a new product every three months, instead of saying you need to use the same bottle ten times before expecting anything. Advice that requires patience does not make content that gets shared.