A random-effect model was performed using STATA to calculate the combined ORs.



STATA
The observational studies that investigated the associations between metformin and the incidence of NDs were searched in MEDLINE, Embase and Cochrane Library databases. 
MEDLINE
The observational studies that investigated the associations between metformin and the incidence of NDs were searched in MEDLINE, Embase and Cochrane Library databases. 
Embase
The observational studies that investigated the associations between metformin and the incidence of NDs were searched in MEDLINE, Embase and Cochrane Library databases. 
Cochrane Library