Background and aims Previous studies have suggested a chemopreventive effect of 5-aminosalicylic acid (5-ASA) therapy in patients with inflammatory bowel disease (IBD). This effect has not been reported in IBD patients using thiopurines. We investigated the association between thiopurine or 5-ASA use and the risk of advanced neoplasia (AN), including high-grade dysplasia and colorectal cancer, in a large cohort of patients with IBD in the Netherlands.

Methods PALGA, the nationwide network and registry of histo- and cytopathology in The Netherlands was linked to an anonymised computerised database of a Dutch health insurance company to identify patients with IBD with or without AN. Pharmaceutical data, including type and duration of medication use, were collected between January 2001 and December 2009. Cox proportional hazard regression analysis was used to calculate risk of AN in patients with and without thiopurine or 5-ASA use.

Results A total of 2578 patients with IBD were included. Of these, 973 patients (38%) used 5-ASA, 314 (12%) thiopurines, 456 (18%) both 5-ASA and thiopurines and 835 (32%) none of these drugs. Twenty-eight patients (1%) developed AN during 16 289 person-years of follow-up. Of these, 11 patients (39%) had used 5-ASA, two (7%) thiopurines and one (4%) both drugs. Thiopurine use was associated with a significantly decreased risk of developing AN (adjusted HR 0.10, 95% CI 0.01 to 0.75). 5-ASA therapy also had a protective effect on developing AN, but this was not statistically significant (adjusted HR 0.56, 95% CI 0.22 to 1.40).

Conclusion Thiopurine use protects IBD patients against the development of AN. The effect of 5-ASA appeared to be less pronounced.