Dr. Russell Callaghan, a scientist with the Centre for Addiction and Mental Health in Toronto, and colleagues examined almost 300,000 hospital records from California covering 16 years. Patients admitted to hospitals for methamphetamine or amphetamine-use disorders had a 76 percent higher risk of developing Parkinson's disease compared with those with no disorder.
To put the study findings into numbers, if 10,000 people with methamphetamine dependence were tracked for 10 years, 21 would develop Parkinson's, compared with 12 people out of 10,000 from the general population, Callaghan says.
"It is also possible that our findings may underestimate the risk because in California, methamphetamine users may have had less access to healthcare insurance and consequently to medical care," Callaghan, the lead researcher, says in a statement.
The study provides evidence of the association for the first time, even though it has been suspected for 30 years, Callaghan adds.
"It is important for the public to know that our findings do not apply to patients who take amphetamines for medical purposes, such as attention deficit hyperactivity disorder, since these patients use much lower doses of amphetamines than those taken by patients in our study," study co-author Dr. Stephen Kish, a CAMH scientist, says in a statement.