A group of Australian consumers afflicted with Parkinson's disease is suing two drug companies, claiming that medication the companies manufacture has caused them to become addicted to sex and gambling.

The suit, filed in Melbourne, concerns the drugs Cabaser and Permax, both used to treat Parkinson's-related tremors. The action claims that the drugs' respective manufacturers, Pfizer and Aspen Pharmacare, breached their duty of care to the plaintiffs.

Specifically, the suit says that the companies failed to adequately research the drugs' possible side effects, failed to warn patients of possible addictive behavior, and failed to take the drugs off the market once the extent of the problem became clear.

Some of the 100 plaintiffs became so addicted to gambling that they lost hundreds of thousands of dollars and, in some cases, their families. While gambling addiction was the most common problem among the class, the Sydney Morning Herald reported that a few exhibited compulsive sexual behavior such as looking at pornography on the internet.

Both Cabaser and Permax are dopamine agonists, meaning that they imitate the effect of dopamine and open dopamine receptors in the brain. Parkinson's disease causes the brain to produce an insufficient amount of dopamine, which it needs in order to control the body's movements, so the drugs essentially pick up the slack.

Unfortunately, dopamine, a chemical precursor to adrenaline, is also the neurotransmitter behind that rush of motivation, energy or attraction that everyone feels on occasion. And scientists believe that that same rush can cause people to engage in risk-taking behavior, including life-threatening addictions.

Indeed, a press release concerning the suit quoted a study published in the medical journal Archives of Neurology as concluding that [d]opamine agonist treatment in PD (Parkinson's Disease) is associated with 2- to 3.5-fold increased odds of having an ICD (impulse control disorder).

The study found that 13.6 of the subject patients were affected by use of dopamine agonists, with five percent becoming addicted to gambling, 3.5 percent to compulsive sexual behavior, 5.7 percent to compulsive buying, and 4.3 percent to binge eating.

The issues raised in the suit have already seen the inside of at least one other courtroom. In July 2008, Minnesota resident Gary Charbonneau won a case in which he claimed that the Parkinson's' drug Mirapex -- also a dopamine agonist -- caused him to develop a gambling addiction. A federal jury gave Charbonneau an $8.2 million award.

And Permax, the drug manufactured by Aspen Pharmacare, has already seen its share of controversy. The Federal Drug Administration (FDA) announced a voluntary recall of the drug in March 2007 after it was discovered that it caused heart valve disease in 25 percent of patients who took it over an extended period of time. As a result, Permax is no longer available in the United States.

Arnold Thomas and Becker, a Melbourne-based law firm, is representing the plaintiffs.