Abstract Background Acute severe ulcerative colitis (ASUC) affects approximately 20% of patients with ulcerative colitis (UC). Patients failing initial corticosteroid therapy receive salvage therapy with either infliximab (IFX) or calcineurin inhibitors, cyclosporine (CYS) or tacrolimus (TAC). We aimed to compare the effectiveness and safety of IFX and the oral calcineurin inhibitor, TAC, in this setting. Methods This retrospective cohort study included consecutive patients with ASUC, based on the Truelove and Witts criteria, admitted to Tel Aviv Medical Center between 2017 and 2022 and treated with salvage therapy. Demographic, clinical, and laboratory data were retrieved. The primary endpoint was successful salvage therapy defined as clinical response resulting in discharge without colectomy. Secondary endpoints included the colectomy rates at 90 days and the safety profile of the treatments. Results During the study period 96 patients with ASUC were admitted to our center. Of these, 40 patients received salvage therapy – 21 patients received IFX and 16 received TAC and were included in the analysis; 3 patients received CYS and were excluded from the study. Demographic data are detailed in Table 1. Disease duration was significantly longer in patients receiving TAC (mean (±SD) 9.7 years (±7.1) vs 4.5 (±5.9), p=0.04). In addition, patients receiving TAC had significantly greater prior exposure to advanced therapies (14 (87.5%) vs 6 (28.6%), p<0.001) and 12 (75%) patients had previous exposure to IFX. Baseline markers of disease severity including disease extent, hemoglobin, C-reactive protein (CRP), albumin and body mass index (BMI) were similar between the treatment groups (Table 1). Therapeutic success was similar between treatment groups (IFX: 18 (86%) vs TAC: 15 (94%), p=0.62). The 90-day colectomy rate (IFX: 4 (19%) vs TAC: 2 (12.5%), p=0.68), CRP upon discharge (median (IQR) IFX: 15 mg/L (3.3-27) vs TAC: 11 mg/L (5.4-25, p=0.64) and length of hospitalization (mean (±SD) IFX: 23.5 days (±19.46) vs TAC: 17.56 days (±5), p=0.19) were similar between the groups. Safety profiles were similar between the treatment groups with mild electrolyte disturbances treated conservatively in both groups and no observed infections, deaths or other adverse events. Conclusion This real world, retrospective study shows that IFX and TAC have similar effectiveness and safety in the setting of ASUC. In this study, patients receiving TAC had significantly longer disease duration and advanced therapy exposure than those receiving IFX and still maintained excellent effectiveness highlighting TAC’s value in this setting.