Scientists have recently begun to quantify the behavioral changes associated with dopamine agonist drugs. In a study presented in late June at the International Congress of Parkinson's Disease and Movement Disorders conference in Chicago, more than 13 percent of 3,090 Parkinson's patients had a problem with compulsive gambling, buying, sex or binge-eating.
People who were taking either Mirapex or Requip had a two- to three-times greater chance of having one of the four impulse-control disorders.
"These disorders can often be devastating to people's personal lives, financial lives, and even physical health," says Dr. Daniel Weintraub, lead author of the study and assistant professor of psychiatry at the University of Pennsylvania School of Medicine.
Though Weintraub says that the benefits of the drug outweigh the negatives for most patients, he adds that "clinicians do have a responsibility to notify patients that these are potential side effects."
Nirenberg even says she fears that these side effects are more prevalent than the study indicates, because only four of the many types of irregular behaviors were analyzed.
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."
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.
But when he told Nirenberg of this behavior, she immediately took him off the drug, just three months after he had started it.
"I don't know if it was exactly the next day [after I stopped], but whatever was triggering my feelings of sexuality was diminished again," he says.