BACKGROUND AND AIM: Oral mesalazine and sulfasalazine (SASP) are key drugs for treating ulcerative colitis (UC). The efficacy of switching from one of the several mesalazine formulations to another is largely unknown. This study assessed the efficacy of switching among three types of mesalazine formulation and SASP for UC therapy. METHODS: UC patients receiving high-dose mesalazine/SASP who switched to other formulations due to disease activity were considered eligible. Efficacy was evaluated 2, 6, and 12 months after switching. RESULTS: A total of 106 switches in 88 UC patients were analyzed. The efficacy at 2 months after switching was observed in 23/39 (59%) cases from any mesalazine formulation to SASP, in 18/55 (33%) cases from one mesalazine to another, and in 2/12 (17%) cases from SASP to any mesalazine formulation. Nine of 43 effective cases showed inefficacy or became intolerant post-switching. Delayed efficacy more than two months after switching was observed in four cases. Steroid-free remission was achieved in 42/106 (39%) cases-within 100 days in 35 of these cases (83%). CONCLUSIONS: Switching from mesalazine to SASP was effective in more than half of cases. The efficacy of switching between mesalazine formulations was lower but may be worth attempting in clinical practice from a safety perspective.