Background: Metformin's effects on non-cancer skin diseases are rarely investigated.Objective: To investigate immune-mediated (urticaria, allergic contact dermatitis and psoriasis) and androgen-mediated (acanthosis nigricans, hidradenitis suppurativa and acne) skin diseases associated with metformin use.Methods: Metformin initiators (n=234,585) and non-metformin initiators (n=125,921) within the initial 12 months of antidiabetic drugs prescription during 1999-2009 were followed up until December 31, 2011. Cox regression weighted for propensity score was used to estimate hazard ratios for metformin initiators versus non-metformin initiators in intention-to-treat (ITT) and per-protocol (PP) analyses. Results: For immune-mediated skin diseases, hazard ratios were 0.930 (95% confidence interval: 0.920-0.940) and 0.930 (0.918-0.943) in ITT and PP analyses, respectively; and the hazard ratios for each specific outcome were all significantly below unity. For androgen-mediated skin diseases, the ITT and PP hazard ratios were 1.110 (1.060-1.162) and 0.990 (0.935-1.048), respectively; and all hazard ratios were not significant for each specific outcome except for acne in the ITT analysis (hazard ratio: 1.116, 95% confidence interval: 1.064-1.170). Conclusion: Metformin use is associated with a significantly lower risk of immune-mediated skin diseases but a lack of preventive effect on androgen-mediated skin diseases.
