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VANCOUVER -- A common insomnia drug may raise the risk of developing Parkinson's disease, according to a meta-analysis presented here.

Although only two studies were of high enough quality to be included in the review, pooled data showed a 2.5-fold increased risk of Parkinson's disease among patients taking zolpidem (Ambien), Salman Hussain, PhD, of Hamdard University in New Delhi in India, .

The hitch was that both of these retrospective, observational studies relied on a health claims database from Taiwan, and thus can't be generalized to other populations, Hussain cautioned.

"We found that there's an increased risk of getting Parkinson's disease among zolpidem users, but to confirm the association and make it robust, more study is needed, Hussain told 口袋女优.

It's not clear as to why zolpidem may raise the risk of developing Parkinson's, he said. It's highly selective for GABA_A receptors, and two decades ago it was actually . But the two studies in the meta-analysis -- one by published in 2014, and the other by in 2015 -- found the sleep aid was associated with a higher risk of Parkinson's.

For their review, Hussain and colleagues searched the PubMed, Cochrane, and Embase databases for studies assessing any potential links between zolpidem and Parkinson's, using the Newcastle-Ottawa Scale to rate study quality.

Ultimately, they came up with only two retrospective, observational studies, both of which relied on the National Health Insurance Research Database from Taiwan.

The Yang study included 101,719 patients treated for 5 years, and the Huang study included 14,805 patients treated for 10 years. There were 42,171 zolpidem users and 809 Parkinson's cases in the Yang study and 2,961 zolpidem users and 157 Parkinson's cases in the Huang study.

Overall, both the Yang study (RR 2.57, 95% CI 2.22 to 2.96) and the Huang study (RR 2.62, 95% CI 2.15 to 3.19) revealed a higher risk of developing Parkinson's among zolpidem users. The Huang study subjects had taken zolpidem for at least 3 months, and the relationship with Parkinson's was more prevalent among those who had depression. In the Yang study, there was a U-shaped relationship between number of doses of zolpidem and Parkinson's risk.

When pooling the data, Hussain and colleagues found a significantly increased risk of developing Parkinson's disease among those taking zolpidem (RR 2.58, 95% CI 2.30 to 2.90).

Although the findings indicate a possible link between zolpidem and development of Parkinson's disease, Hussain urged that more robust evidence is needed, particularly from well-designed randomized controlled trials.