Background/Aims: It is unclear whether 5-aminosalicylic acid (5-ASA) formulation is associated with treatment adherence in ulcerative colitis (UC). Thus, we aimed to investigate the adherence rate after switching from 5-ASA tablets to granules. Methods: This prospective study included 121 UC outpatients treated using 5-ASA tablets. They were grouped based on choice: Group 1 (continued with tablets) and Group 2 (switched to granules without regimen change). Group 2 was further divided into Group 3 (returned to tablets) and Group 4 (continued with granules). The patients completed a self-administered questionnaire regarding the treatment. The primary endpoint was change in adherence after switching to granules. Results: Seventy-nine patients continued with tablets, while 42 patients switched to granules. The adherence rate to the tablet was not significantly different between Group 1 and 2 before switching. In Group 2, switching to granules did not affect adherence. However, in Group 4, adherence significantly improved after switching to granules. Group 3 showed no significant change in adherence before and after switching from tablets. Full-time work and difficulty taking tablets were significant predictors of continuing with granules in Group 4. Conclusion: Patients who continued with 5-ASA granules showed significantly increased adherence, suggesting that patient-tailored drug formulations improved treatment adherence.