Long-term oral mesalazine adherence and the risk of disease flare in ulcerative colitis: nationwide 10-year retrospective cohort from the veterans affairs healthcare system

Summary Background Adherence is a major factor in determining disease activity in ulcerative colitis (UC). There are limited data on long-term nationwide adherence levels among patients with UC. Aim To evaluate the long-term adherence levels to oral mesalazine (mesalamine) in the Veterans Affairs (VA) healthcare system, to determine the impact of non-adherence on the risk of flares, and to evaluate the different pharmacy data-based adherence indicators. Methods Nationwide data were obtained from the VA for the period 2001–2011. UC patients who started mesalazine maintenance during the inclusion period were included. Level of adherence was assessed using three different indicators: medication possession ratio (MPR), continuous single-interval medication availability (CSA) and continuous multiple-interval medication gaps (CMG). Cox regression modelling was used to predict disease flares and assess the predictive value of each adherence indicator. Results We included 13 062 patients into the analysis with median follow-up time of 6.1 years. Percentage of patients with high adherence was 47%, 43%, 31% as identified by CSA, MPR and CMG respectively. Low adherers had a significant increase in the risk of flares compared with high adherers (Hazard ratio: 2.8, 1.7 and 1.8, P