CHICAGO, June 2 (Xinhua) -- People who take drugs that suppress the immune system are less likely to develop Parkinson's disease, revealed a study from Washington University School of Medicine in St. Louis.

When analyzing millions of medical records and developed an algorithm to predict which people would be diagnosed with the disease, the researchers found that people with several types of autoimmune diseases, including ulcerative colitis, were less likely to be diagnosed with Parkinson's than the general population.

They noted that many autoimmune diseases do have one thing in common: they are treated with drugs that dampen immune activity.

The researchers then analyzed Medicare Part D prescription drug data on 48,295 people diagnosed with Parkinson's in 2009 and 52,324 people never diagnosed with Parkinson's, and identified 26 commonly prescribed immunosuppressant drugs, representing six classes of medications.

They found that people taking drugs in either of two classes were significantly less likely to develop Parkinson's than those taking no immunosuppressant drugs.

People taking corticosteroids such as prednisone were 20 percent less likely to be diagnosed with Parkinson' s, while those on inosine monophosphate dehydrogenase inhibitors were about one-third less likely.

When the researchers included specific autoimmune diseases in their analysis, the calculated risks didn't change.

The findings suggest that tamping down immunity with drugs may keep Parkinson's disease at bay. But doing so also makes people more susceptible to infectious diseases and cancer.

"What we really need is a drug for people who are newly diagnosed, to prevent the disease from worsening," said Brad Racette, a professor of neurology and the study's senior author. "It's a reasonable assumption that if a drug reduces the risk of getting Parkinson's, it also will slow disease progression, and we're exploring that now."

Parkinson's, a neurodegenerative disease characterized by tremors, slow movements, stiffness and difficulty walking, affects about a million people in the United States.

The findings were published on May 31 in Annals of Clinical and Translational Neurology.