CHICAGO (Reuters) - More than half the people participating in a study of hair-pullers got help for their compulsion from an over-the-counter supplement called N-acetylcysteine, researchers said on Monday.

The study, published in the Archives of General Psychiatry, suggests the inexpensive supplements might compete with prescription drugs for some obsessive-compulsive type disorders.

Estimates of how many people suffer from chronic hair-pulling, or trichotillomania, vary between one and seven out of every 200 people. The compulsion can create bald spots, and anxiety if sufferers resist the urge to yank out hair. Some pull out the hair of others.

Antidepressants and other drugs are generally not helpful.

Researcher Jon Grant and colleagues at the Minnesota School of Medicine in Minneapolis tested N-acetylcysteine, an amino acid available in health food stores and also sold by prescription for other purposes.

It has been shown to have a some benefits for those suffering from obsessive-compulsive disorder, bipolar disorder and schizophrenia. It is also sold by prescription as a nasal spray to help with mucus buildup, to counteract an overdose of the painkiller acetaminophen and other uses.

But its impact on brain function likely accounts for the benefits for hair-pullers, the researchers said. It appears to boost the supply of glutamate, a message-carrying chemical or neurotransmitter.

The researchers conducted a 12-week trial among 50 people with trichotillomania, giving an initial daily dose of 1,200 milligrams of N-acetylcysteine and then doubling it after six weeks if no improvement occurred.

"Fifty-six percent of patients were 'much or very much improved' with N-acetylcysteine use compared with 16 percent taking placebo," they wrote in their report.

No participants reported adverse effects.

As for the 44 percent of people who did not respond to the treatment, the researchers said there are types of hair-pulling that may respond better to other drugs and to talk therapy.