Metformin is associated with decreased skin cancer risk in Taiwanese patients with type 2 diabetes

[[abstract]]BACKGROUND: Metformin, an antidiabetic drug, is associated with decreased cancer risk, but its effect on skin cancer is unknown. OBJECTIVE: To evaluate skin cancer risk associated with metformin use. METHODS: Matched pairs of 16237 ever and never metformin users were retrospectively enrolled from patients with new-onset type 2 diabetes diagnosed between 1999 and 2005 from Taiwan's National Health Insurance database, and followed until December 31, 2011. Hazard ratios (HRs) were estimated using Cox regression weighted for propensity scores. RESULTS: Skin cancer incidence was 45.59 and 83.90 per 100,000 person-years in ever and never users, respectively (HR 0.540, 95% confidence interval: 0.357-0.819). In ever users, the HRs (95% confidence intervals) for the first (45.83 months) cumulative duration tertiles were 0.817 (0.448-1.489), 0.844 (0.504-1.412), and 0.114 (0.036-0.364), respectively; and 1.006 (0.579-1.748), 0.578 (0.317-1.051), and 0.229 (0.099-0.530), respectively, for the first, second, and third cumulative dose tertiles. HRs were 0.523 (0.175-1.562) for melanoma and 0.496 (0.319-0.772) for non-melanoma skin cancer. LIMITATIONS: Few patients with skin cancer and lack of information on ultraviolet exposure and tumor histopathology. CONCLUSION: Metformin use is associated with decreased skin cancer risk