Table 1. The baseline characteristics of included studies.

Author	Year	Study design	Article type	Country	Cancer type	Cancer stage	Standard chemotherapy regimen	ECGO status	Sample size	Age in year (mean/range)	Dose for metformin
Pimentel	2019	Phase II RCT	Full article	Canada	BC	IV	Lines of chemotherapy	0–2	40	56 [39–75]	850 mg, bid
Nanni	2019	Phase II RCT	Full article	Italy	BC	IV	Non pegylated liposomal doxorubicin 60 mg/m2 i.v plus cyclophosphamide 600 mg/m2 i.v	0–2	126	60 [51–66]	1,000 mg, bid
Li	2019	Phase II RCT	Full article	China	NSCLC	IIIB–IV (EGFR mutation)	Gefitinib, 250 mg, qd	0–2	224	59 [32–78]	1,000 mg, bid
Arrieta	2019	Phase II RCT	Full article	Mexico	LA	IIIB–IV (EGFR mutation)	Standard dose EGFR-TKIs	0–2	139	59	500 mg, bid
Marrone	2018	Phase II RCT	Full article	American	NSCLC	IIIB–IV	Standard dose carboplatin paclitaxel and bevacizumab	0–1	25	[37–74]	1,000 mg, bid
Zhao	2017	Phase II RCT	Full article	China	BC	IV	Aromatase inhibitor (letrozole 2.5 mg/d orally or exemestane 25 mg/d orally)	0–1	60	57 [33–73]	500 mg, bid
Sayed	2015	Open label RCT	Full article	Egypt	NSCLC	IV	Gemcitabine and Cisplatin	0–2	30	[18–80]	500 mg, qd
Kordes	2015	Phase II RCT	Full article	Netherlands	PC	III–IV	Gemcitabine and erlotinib	0–2	121	65 [44–79]	500 mg, bid
Reni	2016	Phase II RCT	Full article	Italy	PC	IV	Standard four-drug chemotherapy regimen (cisplatin, epirubicin, capecitabine, gemcitabine)	NR	60	63 [42–75]	2,000 mg, bid
Kim	2019	Phase II RCT	Abstract	Korea	BC	II–III	Neoadjuvant chemotherapy with letrozole	NR	208	NR	2,000 mg, bid

RCT, randomized control trial; BC, breast cancer; NSCLC, Non-small cell lung cancer; LA, lung adenocarcinoma; PC, pancreatic cancer; NR, not reported.