Pubmed, the Cochrane Central Register of Controlled Trials, and EMBASE databases (∼ May 2013) were systematically searched. 
Pubmed
Pubmed, the Cochrane Central Register of Controlled Trials, and EMBASE databases (∼ May 2013) were systematically searched. 
The Cochrane Central Register of Controlled Trials
Pubmed, the Cochrane Central Register of Controlled Trials, and EMBASE databases (∼ May 2013) were systematically searched. 
EMBASE databases
Odds ratios (OR) and associated 95% CI were retrieved by using random-effect model according to heterogeneity. 
random-effect model