It leaves you tan and sexually excited, and it works for both men and women. No, it's not a tropical vacation; it's the Barbie drug, as some have dubbed Melanotan II. Researchers looking for a sunless way to trigger tanning came across the compound in 1991. Men who took the drug got tan and reported side effects in their shorts. Later tests on lab rats showed that, unlike Viagra, this miracle pill was not just for males: female lab rats that took it showed more courting behavior. The reason: Melanotan works on the brain center that handles sexual arousal. But the drug companies are seeking approval for Melanotan one use at a time. The University of Arizona, which developed the drug, has licensed its findings to two firms. Epitan, in Australia, is testing a form of the drug it hopes will tan without titillating, while Palatin, in the U.S., is developing a version intended for sexual stimulation. (Guess which will get more attention.) Palatin will soon release the results of its first test with women, and chief financial officer Steve Wills says, provocatively, "I don't think the results will be received unfavorably."