OBJECTIVES: To assess adherence to various oral formulations of 5-ASA for the treatment of UC in a UK patient population. METHODS: Dispensed drug data from 10 million UK patients were examined in a retrospective analysis of adherence to oral 5-ASA therapy.  Patients receiving ≥2 dispensed prescriptions of 5-ASA between January 2008 and December 2009 were eligible; those missing age and gender data were excluded.  Patients with a medication possession ratio (MPR) ≥90% were considered adherent.  Patients were grouped by 5-ASA formulation and dose; age- and gender-adjusted adherence rates were calculated and compared using chi-square, 1-sample t-tests, and odds ratio (OR) statistics. RESULTS: A total of 11,608 patients met inclusion criteria.  Mean (SD) age was 51.7 (18.1) years, with an even gender distribution.  Overall, 30.9% of patients were adherent.  More patients were adherent to Mezavant (48.3%) compared with any other formulation: Asacol 800 mg, 40.7%; Mesren 400 mg, 36.7%; Pentasa 1,000 mg, 31.8%; Pentasa 500 mg, 29.7%; Asacol 400 mg, 29.6%; Colazide 750 mg, 27.8% (all comparisons P <0.001).  Mean continuous adherence rates (MPR across multiple refills), adjusted for age and gender, were also higher for Mezavant (87%) compared with the other 5-ASAs: Asacol 800 mg, 82%; Pentasa 1,000 mg, 75%; Mesren 400 mg, 74%; Pentasa 500 mg, 69%; Colazide 750 mg, 68%; and Asacol 400 mg, 66% (P <0.001 vs. other 5-ASAs).  ORs for likelihood of adherence revealed that, compared with Mezavant, patients on any other 5-ASA were less likely to be adherent: Asacol 800 mg, 0.69; Pentasa 1,000 mg, 0.49; Mesren 400 mg, 0.47; Pentasa 500 mg, 0.42; Colazide 750 mg, 0.36; and Asacol 400 mg, 0.36 (all comparisons P <0.05). CONCLUSIONS: Overall adherence with 5-ASA therapies was low.  Patients on once-daily Mezavant demonstrated greater adherence compared with those on other 5-ASA formulations.