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Immune checkpoint inhibitor induced colitis: A nationwide population-based study. BACKGROUND: ICIs are used in the management of several malignancies. However, they can result in immune-related adverse events, such as colitis. The aim of this study is to obtain an epidemiological survey of patients who develop immune checkpoint inhibitor (ICI)-induced colitis and identify underlying risk factors. METHODS: A cohort study was performed using Explorys, a US-based population database. Our cohort included all patients in a five-year interval on an ICI. We further identified those who developed colitis after initiating an ICI. Demographic data and possible risk factors were assessed. Odds ratios were calculated and multivariable statistical analysis was performed. RESULTS: 3.6% of patients developed ICI-induced colitis. Women [OR: 1.2; 95% CI 1.224-1.231, p <0.001], Caucasians [OR: 2.3; 2.284 - 2.299], individuals older than 65 years [OR: 1.3; 1.319 - 1.326], obese patients [OR: 3.3; 3.273 - 3.302], and those with a history of alcohol abuse [OR: 2.5; 2.485 - 2.523] were more likely to develop colitis. Patients who received Nivolumab [OR: 2.8; 2.563 - 3.022], Ipilimumab [OR: 4.9; 3.937 - 6.061], Pembrolizumab [OR 2.7; 2.463 - 2.868], and Atezolizumab [OR 2.9; 2.430 - 3.388] had an increased odds of developing colitis. The majority of cases were diagnosed in the first 6 months of therapy. CONCLUSIONS: This is the largest study to describe the epidemiology of ICI-induced colitis and it is the first to identify underlying risk factors. Ipilimumab poses the greatest risk for ICI-induced colitis. The risk of colitis should be discussed with all patients prior to initiating an ICI, as it may be a factor in choosing among ICIs.