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Adherence is generally associated with improved treatment outcomes. Risk factors for non-adherence must be understood to improve adherence. A systematic review was undertaken to determine which variables were consistently associated with non-adherence to oral medication in inflammatory bowel disease (IBD).The databases EMBASE, Medline, and PsycINFO were searched for titles relating to adherence, medication, and IBD (1980–2008). Primary, quantitative studies were included if they concerned adult patients with IBD, measured adherence to oral medication, and measured characteristics associated with adherence. The resulting 17 papers were independently reviewed by two researchers who also assessed their quality according to pre-defined criteria. The main outcome was the frequency with which demographic, clinical, treatment, and psychosocial variables were found to be statistically significantly associated with non-adherence.Non-adherence rates ranged from 7 to 72%, with most studies reporting that 30–45% of patients were non-adherent. No demographic, clinical, or treatment variables were consistently associated with non-adherence. Psychological distress and patients' beliefs about medications were both related to non-adherence in four out of five studies, and doctor–patient discordance was associated with non-adherence in two out of three studies.This is the largest review of factors associated with non-adherence in IBD. Demographic, clinical, and treatment variables were not consistently associated with non-adherence. Psychological distress, patients' beliefs about medications, and doctor–patient discordance were associated with non-adherence. These findings call into question some of the conclusions of earlier reviews that did not take into account nonsignificant findings. Practical suggestions for gastroenterologists and future research are discussed.