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.


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.


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.


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 < 0.001 for CSA, MPR and CMG, respectively). Compared with other adherence indicators, CSA offered the best trend in predicting disease flares.


Long-term high-adherence level was lower than previously reported. Adherence was a significant factor in predicting disease flares. Pharmacy adherence indicators may be useful to healthcare providers in identifying patients at high risk of exacerbations.