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Els, Charl, Tanya D. Jackson, Diane Kunyk, Vernon G. Lappi, Barend Sonnenberg, Reidar Hagtvedt, Sangita Sharma, Fariba Kolahdooz, ja Sebastian Straube. "Adverse Events Associated With Medium‐ and Long‐term Use of Opioids for Chronic Non‐cancer Pain: An Overview of Cochrane Reviews." Cochrane Database Syst Rev 2017.

Els, Charl, et al. "Adverse Events Associated With Medium‐ and Long‐term Use of Opioids for Chronic Non‐cancer Pain: An Overview of Cochrane Reviews." Cochrane Database Syst Rev 2017.