Over time, half of the people taking certain drugs for Parkinson’s disease may develop impulse control disorders such as compulsive gambling, shopping, or eating. This is according to a study published in the June 20, 2018, online issue of Neurology, the medical journal of the American Academy of Neurology.

In Parkinson’s, a vital chemical in the brain called dopamine, which regulates movement, is gradually reduced. Parkinson’s is treated with levodopa, which converts to dopamine in the brain, or with dopamine agonists, which work by activating dopamine receptors.

“Our study suggests that impulse control disorders are even more common than we thought in people who take dopamine agonists,” says study author Jean-Christophe Corvol, MD, of the ICM Brain and Spine Institute—Pitié-Salpêtrière Hospital, Assistance Publique—Hôpitaux de Paris, Sorbonne University in Paris, France, in a release. “These disorders can lead to serious financial, legal, and social and psychological problems.”

The study involved 411 people who had been diagnosed with Parkinson’s disease for 5 years or less who were followed for an average of about 3 years. The participants were asked in interviews about impulse control disorders such as compulsive shopping, eating, gambling, or sexual behaviors.

About 87% of the participants had taken a dopamine agonist at least once. At the beginning of the study, 20% of the participants had an impulse control disorder, with 11% having compulsive or binge eating problems, 9% compulsive sexual behaviors, 5% compulsive shopping, and 4% compulsive gambling. Six percent of people had more than one impulse control disorder.

Of the 306 people who did not have impulse control disorders at the start of the study, 94 people developed a disorder during the study, for an overall 5-year cumulative incidence of 46%. For people who had never taken dopamine agonists the 5-year incidence was 12%, compared to 52% for those who had ever used the drugs. The average annual incidence was 26 per 1,000 person-years in people who never took the drugs, compared to 119 per 1,000 person-years in those who had taken the drugs.

“These disorders can be challenging for neurologists to discover,” says Laura S. Boylan, MD, of New York University in New York, NY, and a fellow of the American Academy of Neurology, who wrote an editorial accompanying the article. “People might be ashamed to tell their doctor about their problems, they may think these issues are not related to their Parkinson’s disease, or they may not even consider the disorders a problem. Plus, as doctors’ time for meeting with each patient gets shorter and shorter, bringing up sensitive issues gets harder and harder.”

The researchers also found that with higher doses of the drugs and taking them for longer periods of time, people were more likely to develop impulse control disorders. The drugs pramipexole and ropinirole were associated with the highest risk of developing the disorders.

A total of 30 people with impulse control disorders who stopped taking dopamine agonists were followed during the study. The disorders stopped over time, with half of the people no longer having issues after a year.

One limitation of the study was that because the participants were relatively young, with an average age of 62, and younger people are more likely to be given dopamine agonists and to have impulse control disorders, it’s possible that the occurrence rate of these disorders could be overestimated.

The study was funded by the French Ministry of Health and the French Drug Agency.