Strange Side Effects Surprise Patients

She says that her patients tend to have gender-based differences in side effects, with women experiencing more compulsive shopping and eating and men turning more to hypersexuality and gambling.

She says she also has patients who would have been overlooked in the study -- a man who plays basketball compulsively for up to 36 hours at a time, and another who compulsively fishes. One spouse of a Parkinson's patient had to put a padlock on the refrigerator because of the compulsive eating, she says.

These impulsive behaviors have "potentially devastating consequences," Nirenberg says. "They have led to marital discord frequently ... shopping and gambling have led to financial losses, and eating has led to weight gain and secondary medical consequences."

But Are They Bad?

Dr. Eric Wassermann, neurologist and researcher at the Neurology Institute at the National Institutes of Health, says that the dopamine levels in the brain are normally lowered when a person experiences negative consequences of a behavior, such as gaining weight when overeating.

However, these medications keep the dopamine levels relatively constant. "If you are flooding the system, it is turned on constantly," Wassermann says. "Probably these addictive behaviors become overly reinforcing, and punishments like losing money don't work."

And certain behaviors stimulated by the drugs, such as increased sexual drive, aren't typically "punished" by drops in dopamine levels, Wassermann says.

"You just keep doing it over and over again and it keeps feeling good," he says.

Robert Simpson, 59, of New York City, is one of Nirenberg's patients who experienced an increased libido from the Mirapex. He began taking the drug in December 2005 and noticed an immediate change.

"I felt good," Simpson says. "Not high, exactly, but I felt a little buzz. It was a new experience. I liked the feeling."

He says he began buying pornography regularly, and masturbating two to three times a day.

"It wasn't actually causing anyone any harm," he says.