Compulsive gambling could be a rare and unusual side effect of treatment with a widely used class of Parkinson’s disease drugs, according to a new study.

The trial, conducted by researchers at the Mayo Clinic in the USA and published online on the Archives of Neurology website, identifies 11 Parkinson’s patients who seem to have developed a pathological addiction to gambling while being treated with dopamine agonists, used to treat the tremors associated with the disease.

These drugs, which are usually given in combination with the mainstay Parkinson’s disease treatment L-dopa, include Boehringer Ingelheim’s Mirapex (pramipexole), GlaxoSmithKline’s Requip (ropinirole) and Pfizer’s Dostinex (cabergoline). The study identifies patients treated with dopamine agonists between 2002 and 2004, but Mayo researchers have since found 14 other patients that may have the same side effect.

In addition to the gambling, the report describes other compulsive side effects with the drugs. Six of the patients in the study developed concomitant compulsions, including eating, drinking alcohol and sex. And in 10 cases described with some detail, the compulsive behaviour disappeared once therapy with the dopamine agonist was withdrawn or reduced.

Nine of the 11 patients in the study were taking Mirapex, and the other two were taking Requip. Boehringer Ingelheim recently changed the labelling for its drug to include compulsive disorders among its rare side effects.

The study should have the benefit of bringing the potential side effect to the attention of neurologists, who can then monitor patients for signs of this type of compulsion, said the authors. Also, exploring the mechanism behind it– thought to involve stimulation of the dopamine D3 receptor – could yield valuable insights into how compulsive disorders develop and may be treated.