Metformin use and its effect on gastric cancer in patients with type 2 diabetes: A systematic review of observational studies.

Increasing evidence suggests that metformin use is associated with a decreased risk of cancer. The traditional therapies for gastric cancer (GC) are gastrectomy and chemoradiotherapy; however, these therapies may cause certain adverse effects, which affect a patient's quality of life, and the overall survival rate is low. At present, little is known about whether the use of metformin decreases the risk of GC in patients with type 2 diabetes. Therefore, in the present study, a systematic review was performed to analyze the effect of metformin on GC. A literature search was conducted in PubMed, EMBASE, and the Cochrane Library databases for articles published up to June 30th, 2016. The studies that evaluated GC patients treated with metformin and compared them with GC patients treated with other antidiabetic drugs were reviewed. Eligible studies were evaluated using the Newcastle-Ottawa Scale. Adjusted hazard ratio and 95% confidence intervals were determined to evaluate the effect of metformin on GC. From the 422 articles evaluated, 5 studies involving a total of 1,804,479 patients met the inclusion criteria and were qualitatively analyzed. The quality of all selected articles was classified as moderate. These studies reported that the long-term use of metformin was associated with a lower risk of GC compared with the lack of use of metformin or the use of other hypoglycemic drugs. In GC patients with diabetes who were subjected to gastrectomy, the cumulative use of metformin reduced the rates of disease recurrence and of all-cause and cancer-specific mortality. Despite the limited number of studies on this subject, currently available evidence indicates that metformin is associated with a decreased risk of GC and improves survival in patients with type 2 diabetes. However, more well-designed trials are required to elucidate this association.