Illicit use of methamphetamine (meth) may lead to a rush of energy, elation, and an invigorated libido. But some meth users may pay a heavy price for these pleasures a few years later by developing Parkinson's disease.

So reported Russell Callaghan, Ph.D., a research scientist at the Center for Addiction and Mental Health in Toronto, and colleagues online July 26 in Drug and Alcohol Dependence.

Since Parkinson's disease is known to arise from impairment or deterioration of dopamine neurons in the brain's substantia nigra, and since studies in lab animals have shown that meth can harm brain dopamine neurons, researchers have long speculated that meth users might develop Parkinson's. Thus Callaghan and his team decided to undertake a study to see if this is the case.

"Given the worldwide prominence of illicit amphetamine-type substance use, I was surprised that no prior human research had followed meth users over time and tracked the incidence of Parkinson's disease," Callaghan told Psychiatric News.

Callaghan and his colleagues used linked statewide California inpatient hospital and death records as the basis of their study. They identified some 40,000 meth users from these records who were aged 39 on average and then followed them for an average of six years to see whether they developed Parkinson's disease.

The researchers also identified approximately 40,000 individuals from California inpatient hospital records who had been hospitalized at the same time as the meth users because of appendicitis and who matched the meth users on age, race, and gender. The researchers followed these control subjects as well for an average of six years to see whether they developed Parkinson's.

Fifty-one individuals in the meth group, but only 29 in the control group, developed Parkinson's disease, indicating a 76 percent increased risk of developing Parkinson's in the meth group.

But if meth use is a risk factor for Parkinson's, then why isn't an epidemic of meth-provoked Parkinson's occurring around the United States where use of that drug is epidemic in some regions?

There are several possible reasons, Callaghan and his team suggested. One is the relatively low incidence of both Parkinson's and meth abuse when looked at on a nationwide basis. Another is that meth abuse by young adults would probably not be considered by clinicians as a possible cause of Parkinson's in middle age. And yet a third hypothesis is that Parkinson's might develop only in individuals who use a lot of high-dose meth over a long period. Indeed, the meth users the researchers followed in their study were moderate to heavy meth users; otherwise, they would not have received a hospital inpatient diagnosis of a meth use disorder.

"There appear to be two main clinical implications of our findings," Callaghan told Psychiatric News. "First, meth users and their caregivers need to know that chronic meth use may pose a risk for the development of Parkinson's disease. However, it is important to note that most Parkinson's disease patients do not develop the disease because of meth use. Secondly, while we note in our paper that our findings may not relate at all to the use of prescription amphetamine-type stimulants (for example, for the treatment of attention-deficit/hyperactivity disorder), our findings do warrant further studies examining the long-term neurological impact of prescription amphetamine-type stimulant use."

Callaghan and a colleague are now working with a team from Sweden in an attempt to replicate their findings. They're using linked Swedish medical record data for that purpose.

The study was funded by the Ontario Ministry of Health and Long-Term Care.

An abstract of "Increased Risk of Parkinson's Disease in Individuals Hospitalized With Conditions Related to the Use of Methamphetamine or Other Amphetamine-Type Drugs" is posted at <www.sciencedirect.com/science/article/pii/S0376871611002766>.