Introduction Non-steroidal anti-inflammatory drugs (NSAIDs), including aspirin, have detrimental effects on the mucosa of the gastrointestinal tract and may potentially play a role in the aetiology of inflammatory bowel diseases (IBD). The aim of this study was to investigate, for the first time, in a prospective study if aspirin usage is associated with the development of Crohn's disease (CD) and ulcerative colitis (UC).

Methods A total of 138 239 men and women, aged 30–74 years, were recruited between 1993 and 1997 into a prospective cohort study (EPIC European Prospective Investigation into Cancer and Nutrition). Participants were resident in Denmark, Germany or Sweden and completed questionnaires which detailed if they had taken aspirin regularly. Participants were followed up to identify those who developed either CD or UC. Each case was matched with four controls for age, gender and recruitment centre. Odds ratios (OR) were calculated by conditional logistic regression, adjusted for cigarette smoking.

Result A total of 37 participants developed CD and 93 developed UC after a median follow-up time of 4.7 years (range 1.5–9.6 years) and 4.0 years (range 1.7–11.3 years), respectively. The regular intake of aspirin was positively associated with the risk of developing CD (OR 6.84, 95% CI 1.87 to 25.08, p<0.01). In participants who took aspirin, but did not smoke, the odds ratio increased (OR 9.38, 95% CI 2.18 to 40.34, p<0.01) while in those who took aspirin and smoked there was no effect (OR 0.32, 95% CI 0.03 to 3.2). No association was found between aspirin usage and UC (OR 1.11, 95% CI 0.54 to 2.30).

Conclusion The data suggest that aspirin may be involved in the aetiology of CD for which there are plausible mechanisms, although an interaction occurred between aspirin use and smoking. Further epidemiological studies are required to determine if the association is consistent, but suggest aspirin use should be measured in work investigating the aetiology of CD.