My mother-in-law, who is from The Old Country, says this is a true fact. I thought it was par for the course grandmother nonsense, but a Google gave all sorts of hits, all of them fishy [insert joke here].
Although it would be fun to see someone walking around with an onion tied to his head, apparently the method is to dice the onions and let them sit in the shampoo bottle for a while.
I asked her if had worked, and she promptly said yes. But she hadn’t had the slightest sign of hair loss to begin with.*
At least one seemingly scientific studyconcluded that onion juice had a better success rate than plain water in regrowing hair lost due to Alopecia areata.
This shouldn’t be extrapolated to mean that “onion juice causes increased hair growth” but it’s at least in the ball park.
One note: many cases of Alopecia areata go away after a short time and the hair regrows. A common complication of Alopecia areata is sometimes permanent hair loss, e.g. male-pattern baldness.
My hunch is that the subjects that exhibited increased hair growth from onion treatment were probably also subjects that would have regrown their hair naturally in a short time. Obviously if it were a cure for permanent hair loss it would not be The Old World’s best kept secret. But like many ‘old world’ remedies there might be at least some partial, possibly coincidental but real benefit there. (like chicken soup and the flu)