People who regularly take the over-the-counter painkiller ibuprofen may have a lower risk of getting Parkinson's disease, a new study says.

Researchers from the Harvard School of Public Health studied more than 136,000 people over six years and asked them about their use of aspirin, ibuprofen and acetaminophen, which are all classified as non-steroid anti-inflammatory drugs, or NSAIDs.

During the study, 293 people developed Parkinson's disease. That's 0.2 percent of the people.

The researchers said in a news release that ibuprofen were 40 percent less likely to develop Parkinson's disease than people who didn't. Also, people who took higher amounts of ibuprofen were less likely to develop Parkinson's disease than people who took smaller amounts of the drug. The results were the same regardless of age, smoking and caffeine intake, they said.

"Ibuprofen was the only NSAID linked to a lower risk of Parkinson's," said Dr. Xiang Gao. "More research is needed as to how and why ibuprofen appears to reduce the risk of Parkinson's disease, which affects up to 1 million people in the United States."

Results of observational studies such as this one are not considered as reliable as true experiments that include a random sample of people and control groups for comparison.

Ibuprofen is often sold under the brand names Advil, Midol and Motrin.