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Background/aims: Causes of gastrointestinal bleeding (GIB) in patients on oral anticoagulants (OACs) are not well established. The aims of the study were to compare the causes of GIB in patients on OACs and those not on OAC therapy., Methods: A nationwide study of all GIB events in patients on OACs in Iceland from 2014-2019 was conducted. Bleeding events were obtained through ICD-10 codes and review of endoscopy databases, confirmed by review of medical records. For comparison, patients not on OACs from previous Icelandic population-based studies were used., Results: Among 752 GIB events in 12,005 patients on OACs, 273 (1.9%) had verified upper and 391 (2.7%) had verified lower GIB. For lower GIB, multivariate analysis showed that OAC users were more likely to have colonic polyps (OR 6.6, 95% CI: 2.4 - 17.8, p  < .001) or colorectal cancer (OR 3.7, 95% CI: 2.0 - 7.0, p  < .001) but less likely to have ischemic colitis (OR 0.11, 95% CI: 0.04 - 0.26, p  < .001). For upper GIB, bleeding from mucosal erosions (OR 4.0 95% CI: 2.5 - 7.9, p  < .001) and angiodysplasia (OR 3.6, 95%CI: 1.5 - 8.6, p  = .003) were more common in OAC users., Conclusions: A high proportion of GIB caused by colonic polyps and colorectal cancer among OAC patients indicates that OACs treatment may facilitate cancer diagnosis. The low proportion of ischemic colitis among those on OACs suggests that OACs provide a protective effect against ischemic colitis. OACs seem to increase the bleeding from angiodysplasia and mucosal erosive disease.