Russ Kelly hadn't felt this good in years. Which was strange, because to any casual observer the guy's life was falling apart.

He had just begun a three-pills-a-day regimen of Mirapex to alleviate some early symptoms of Parkinson's disease, and almost overnight, he remembers, "there was a clear change in my actions. In the cartoon there'd be the good angel on your right shoulder and the Devil on your left shoulder, arguing with each other. 'Do it!' 'Don't do it!' Usually the angel wins out. But the Devil takes over sometimes." In the case of Russ Kelly—a loyal husband, dutiful father, and stalwart employee in the technology sector—the Devil started riding shotgun in his subconscious.

Instead of having one or two beers after a round of golf with friends, the man would suck down six or seven. Instead of engaging in polite chitchat with comrades, he became antagonistic—"just arguing for the sake of arguing," as he puts it. When evening fell, Kelly would get itchy to bolt from his condo in New Jersey; he'd drive to Atlantic City and gamble away tens of thousands of dollars in a single night. Meanwhile, his libido went off the charts. The urge to bed women became overwhelming. "I was driven by need," he says. "That was a big part of it. It was like: Go for it." Eventually his marriage started to give way and all the signs seemed to indicate that his life was in a downward spiral, except for one crucial detail: He was relishing the ride. "It was exhilarating," he says. "Like in Top Gun, when Tom Cruise says 'I feel the need—the need for speed,' and he gets into his jet fighter and he's strapped to an engine—that's what it's like. You're rocketing through the air. It's that excitement we all want in our lives, but we realize as responsible human beings that we need to reel that feeling back."

What Kelly didn't know—years later, a doctor figured it out and immediately took him off Mirapex—was that he was among the 15 to 20 percent of the population that's susceptible to the more sordid side effects of dopamine agonists. These prescription drugs, which restore the brain's natural neurotransmitter wash of dopamine, can improve motor functions in people struggling with Parkinson's (and the mystery of restless leg syndrome). But in certain patients, the dopamine bath appears to affect something else—the part of the brain that tends to hold a few of our feral impulses in check. "We've had patients with gambling problems who have essentially gone bankrupt," says Daniel Weintraub, an associate professor of psychiatry at the University of Pennsylvania. For some, drugs like Mirapex and Requip seem to zoom in on a hidden vice—compulsive eating, porn viewing, skirt chasing, Web surfing, or shopping, for instance—and magnify it. "Patients compare it to losing the superego," says Melissa Nirenberg, a neurologist at the New York-Presbyterian/Weill Cornell Medical Center. "People still often feel like they shouldn't be doing these things, but they can't stop."