5-ASA is a metabolite and potential pharmacologically active component of sulphasalazine, a drug used in the treatment of Crohn’s disease and ulcerative colitis. However, the mechanism by which this drug works has not been established. In whole blood assays, 5-ASA proves to be a weak, non-selective inhibitor of both COX-1 and COX-2 with IC50 values of 410 and 61 µM, respectively. In ionophore-stimulated colonic mucosal cells, 1 mM 5-ASA does not inhibit PGE2 production, but does reduce LTB4 synthesis approximately 50%. In ionophore-stimulated human leukocytes, 400 µM 5-ASA reduces LTB4 production approximately 20%. 5-ASA does not inhibit 15-hydroxy PGDH at concentrations up to 50 µM.