Table S1.

Search strategy in some databases

Databases	Search strategy
Search strategy in Scopus	(TITLE-ABS-KEY (cancer* OR neoplasia OR tumor* OR malignan*) AND TITLE-ABS-KEY (metformin) AND TITLE-ABS-KEY (diabetes AND mellitus) AND TITLE-ABS-KEY (meta-analysis OR eta-analyses))
Search strategy in PubMed	((((((neoplasms[MeSH Terms]) OR (cancer[Title/Abstract])) OR (malignan*[Title/Abstract])) OR (tumor*[Title/Abstract])) OR (neoplasm[Title/Abstract])) AND ((meta-analysis[Title/Abstract]) OR (Meta-Analysis[Publication Type]))) AND ((metformin[Title/Abstract]) OR (metformin[MeSH Terms]))
Search strategy in Web of Science	TOPIC: (metformin) AND TOPIC: (cancer OR neoplasm OR neoplasia OR tumor) AND TOPIC: (meta-analysis OR meta-analyses) AND TOPIC: (diabetes mellitus)
Search strategy in Cochrane	(“Cancer”):ti, ab, kw AND (“metformin”):ti, ab, kw AND (“diabetes mellitus”):ti, ab, kw AND (“meta analysis”):ti, ab, kw”
Search strategy in Embase	‘malignant neoplasm’:ab, ti AND metformin: ab, ti AND ‘meta analysis’:ab, ti AND ‘diabetes mellitus’:ab, ti