Metformin and risk of developing nasopharyngeal cancer in patients with type 2 diabetes mellitus

Background: Metformin has anticancer effects, but whether it can reduce the risk of nasopharyngeal cancer (NPC) is not known. Methods: A total of 15,486 ever-users and 15,486 never-users of metformin matched by propensity score were enrolled from among patients with new-onset type 2 diabetes mellitus diagnosed during the period 1999–2005 and who were found in the reimbursement database of Taiwan's National Health Insurance. The patients were followed until December 31, 2011. Hazard ratios were estimated by Cox regression incorporated with the inverse probability of treatment weighting using propensity scores. Results: NPC incidence was 33.41 and 106.04 per 100,000 person-years in ever-users and never-users, respectively. The overall hazard ratio (95% confidence interval) of 0.312 (0.197–0.494) favored a significantly lower risk among metformin ever-users than in never-users. Hazard ratios comparing the first (<26.03 months), second (26.03–58.03 months) and third (>58.03 months) tertiles of cumulative duration of metformin use to never-users were 0.690 (0.389–1.224), 0.187 (0.076–0.463) and 0.168 (0.068–0.415), respectively. A significantly lower risk of NPC was consistently observed among metformin users in subgroup analyses of age (<50 years and ?50 years), sex (men and women), and patients with or without nephropathy, liver diseases or diseases of the esophagus, stomach, and duodenum, respectively. Conclusions: Metformin use is associated with a significantly lower risk of NPC. ? 2018 Elsevier Inc.