"Although there is no proven treatment, some people might choose to try a 'safe' treatment -- such as a topical steroid -- even in the absence of good evidence of the treatment's benefit," said Dr. Mike Sladden of the University of Tasmania.

One small, unpublished study found hair regrew in almost half of 60 subjects with mild cases who applied onion juice -- an alternative medicine used for infections, Sladden said.

Sladden, co-author of a review of 17 randomized studies that involved 540 patients, published in the Cochrane Library, said reviewers found no treatment resulted in the return of more than half of the missing hair.

"Doctors and patients need to talk openly about the condition and discuss treatments and likely outcomes," Sladden said. "And they need to avoid those treatments which might be harmful."

Most people with alopecia areata develop bald spots before age 20. A suggested cause is the immune system attacking the hair follicle, the reviewers said.