Many athletes credit drugs with improving their performance, but some of them may want to thank their brain instead. Mounting evidence suggests that the boost from human growth hormone (HGH), an increasingly popular doping drug, might be caused by the placebo effect.

In a new double-blind trial funded by the World Anti-Doping Agency, in which neither researchers nor participants knew who was receiving HGH and who was taking a placebo, the researchers asked participants to guess whether or not they were on the real drug. Then they examined the results of the group who guessed that they were getting HGH when, in fact, they had received a placebo. That group improved at four fitness tests measuring strength, endurance, power and sprint capacity. The study participants who guessed correctly that they were taking a placebo showed much less improvement, according to preliminary results presented at the Society for Endocrinology meeting in June. The researchers are currently analyzing the results of the other participants for future publication.

"This finding really shows the power of the mind," said Ken Ho, an endocrinologist at the Garvan Institute in Sydney, Australia, who led the study. "Many athletes are reaping the benefits of the placebo effect, without knowing whether what they're taking is beneficial or not."

And in fact, HGH may not be helpful at all, reveals a recent review published May 20 in the Annals of Internal Medicine. Endocrinologist Hau Liu of Stanford University and his colleagues looked at 44 studies and found that although HGH did increase athletes' lean body mass, it did not lead to improvements in athletic performance in double-blind trials.

The implications for athletes are serious, according to Ho. Many athletes take a cocktail of supplements, vitamins and drugs, believing that they are enhancing their game. "But it's really the belief in the mind that improves performance" in most cases, Ho says. "Athletes need to be cautious about ‘snake oil' merchants."