To compare the risk of inflammatory bowel disease (IBD) between ever users and never users of metformin.
Patients with newly diagnosed type 2 diabetes mellitus from 1999 to 2005 were enrolled from Taiwan’s National Health Insurance. A total of 340,211 ever users and 24,478 never users who were free from IBD on January 1, 2006 were followed up until December 31, 2011. Hazard ratios were estimated by Cox regression incorporated with the inverse probability of treatment weighting using propensity score.
New-onset IBD was diagnosed in 6466 ever users and 750 never users. The respective incidence rates were 412.0 and 741.3 per 100,000 person-years and the hazard ratio for ever versus never users was 0.55 (95% confidence interval: 0.51-0.60). A dose-response pattern was observed while comparing the tertiles of cumulative duration of metformin therapy to never users. The respective hazard ratios for the first ( 58.3 months) tertiles were 1.00 (0.93-1.09), 0.57 (0.52-0.62) and 0.24 (0.22-0.26). While patients treated with oral antidiabetic drugs (OADs) without metformin were treated as a referent group, the hazard ratios for patients treated with OADs with metformin, with insulin without metformin (with/without other OADs) and with insulin and metformin (with/without other OADs) were 0.52 (0.42-0.66), 0.95 (0.76-1.20) and 0.50 (0.40-0.62), respectively.
A reduced risk of IBD is consistently observed in patients with type 2 diabetes mellitus who have been treated with metformin.

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