Drugs commonly used to treat Parkinson's disease may raise the risk of so-called impulse control disorders, according to a new review.
These disorders include compulsive gambling, compulsive shopping and/or hypersexuality.
That increased risk was seen in a fresh review of a decade's worth of U.S. Food and Drug Administration (FDA) records.
"What we have here is a striking example of a major problem in drug safety," said study author Thomas Moore, a senior scientist with the Institute of Safe Medication Practices and a lecturer in epidemiology and biostatistics at George Washington University, in Washington, D.C. "And that is the issue of how drugs can sometimes provoke psychiatric side effects that actually make people behave in extremely destructive and abnormal ways."
Moore and his colleagues reported their findings online Oct. 20 in the journal JAMA Internal Medicine.
According to the American Parkinson Disease Association, these medicines are part of a powerful and effective class of drugs known as dopamine receptor agonists. These medications work by mimicking the activity of the brain chemical dopamine.
The study's authors stressed that these medicines are widely used to treat a range of illnesses, including Parkinson's, restless leg syndrome and patients diagnosed with high levels of the hormone prolactin -- which can cause impotence in men and can prevent pregnancy in women.
More than 2 million such prescriptions were filled during the last three months of 2012 alone, the researchers noted.
To explore any risk for the development of impulse control issues, the study authors analyzed FDA data on 2.7 million domestic and foreign adverse drug events reported between 2003 and 2012.
Of these, 1,580 events were specifically identified as involving what investigators categorized as "serious" impulse control scenarios, with about half involving American patients. The remaining cases involved patients in 21 other countries.
Nearly 45 percent of these events (710) were linked directly to the use of any of six different dopamine receptor agonist medications, with most involving male patients (more than 65 percent) at an average age of 55. The other 55 percent of cases were associated with the use of other medications, the study found.
More than 60 percent of the time, the impulse control cases involving dopamine receptor agonists were patients struggling with Parkinson's, while nearly a quarter involved patients with restless leg syndrome, the researchers said.
Breaking it down, the team identified 628 instances of pathological gambling, 465 cases of hypersexuality, and 202 examples of compulsive shopping.
"I should say that the risk for these events does not appear to be rare at all," said Moore. "These psychiatric side effects appear to occurring among at least 10 percent of patients.
If you compare that with, say, the risk for suicide among patients who take antidepressant drugs, this is much higher. It's an astronomical rate, in terms of drug adverse event risk. And frankly I think I'm being conservative."
Joshua Gagne, an assistant professor of medicine with Harvard Medical School and Brigham and Women's Hospital in Boston, agreed that the increased risk could exceed 10 percent.
"The problem is that there is certainly a lot of underreporting of the problems we're talking about," he explained. "I suspect many affected patients never tell anyone. It's easy to imagine that a patient would be ashamed if they're losing a lot of money gambling or doing things that they generally don't want their family to know about, which suggests many may be keeping this issue a secret, leading us to underestimate the dimension of the situation."
Gagne, who wrote a commentary accompanying the new analysis, said a lot more research will be needed before the true risk is clearly defined.
"The thing to know," he added, "is that there aren't many treatment options for Parkinson's, for example, that work as well as these drugs. They're really a cornerstone of treatment, and it's important to have them as options.
"But while they should continue to be used, it's equally important to know what the risks are," Gagne said. "So, while more research is done, we need to encourage physicians and pharmacists and patients to discuss the concern, and to help patients be more open about it when these issues take hold."