MEDLINE, PubMed, and the Cochrane Central Register of Controlled Trials for studies published through May 22, 2017. 
MEDLINE
MEDLINE, PubMed, and the Cochrane Central Register of Controlled Trials for studies published through May 22, 2017. 
PubMed
MEDLINE, PubMed, and the Cochrane Central Register of Controlled Trials for studies published through May 22, 2017. 
Cochrane Central Register of Controlled Trials