Evidence is growing that Mirapex and similar drugs used to treat Parkinson’s Disease cause compulsive behavior, including gambling addiction, in some patients. In June, results from the largest study ever to investigate the connection between compulsive behavior and dopamine agonists was presented at the at the International Congress of Parkinson’s Disease and Movement Disorders conference in Chicago. That study found that more than 13 percent of patients taking dopamine agonists, sold under brand names suffer from at least one of four serious behavioral addictions.

For years, people taking Mirapex and similar drugs have complained about problems with compulsive behavior. Most of those complaints involved people who had no history of compulsive behavior before they started dopamine agonist therapy, and most reported that the behavior stopped as soon as they quit using the drugs. Several small studies, including one published in 2005 by Mayo Clinic researchers, found a link between the drugs and compulsive behavior, especially gambling addiction. The drugs’ labeling also includes warnings about possible compulsive behavior.

The study presented in June, which looked at more than 3,000 patients from 46 medical centers in the United States and Canada, found that Parkinson’s patients on dopamine agonists are nearly three times more likely to have at least one impulse-control disorder compared with patients receiving other treatments.

People with Parkinson’s lack dopamine in key areas of the brain. Mirapex and drugs like it are known as a dopamine agonist, and they work by mimicking the effects of this vital hormone and neurotransmitter. Other dopamine agonists include Requip, Parlodel, Dostinex Apokyn and Neupro.

This week, the first of three “bellweather” trials concerning Mirapex and its alleged linkage to compulsive gambling is underway in U.S. District Court in Minneapolis. More than 200 people are suing Boehringer Ingelheim Pharmaceuticals, the maker of Mirapex, over compulsive behavior they say it caused. These first three trials will be used by many legal experts to gauge the strengths and weaknesses of those cases.