In a retrospective analysis, using metformin for >3 years reduced
stomach cancer risk by 43% in patients with type 2 diabetes who did
not use insulin.
However, the risk of stomach cancer risk was doubled in insulin users
versus nonusers, regardless of metformin use.

The oral antidiabetic drug metformin has demonstrated anticancer
activity in other studies as well.

Metformin slows prostate cancer growth in adjuvant setting (AACR,
103rd Annual Meeting2012),
boosts survival in ovarian cancer(Mayo Clinic,2012),
exhibits antineoplastic effect in patients with thyroid cancer
(J.Clin.Endoerinol Metab.2013).

Kim Y-I et al. Long-term metformin use reduces gastric cancer risk in
type 2 diabetics without insulin treatment: A nationwide cohort study.
Aliment Pharmacol Ther 2014 Apr; 39:854. (http://dx.doi.org/10.1111/apt.12660
)PubMed abstract.