Aspirin-d4 is intended for use as an internal standard for the quantification of aspirin (Item No. 70260) by GC- or LC-MS. Aspirin is a non-selective, irreversible COX inhibitor. The IC50 values for ovine COX-1 and -2 are 0.75 and 1.25 mM, respectively. Aspirin acetylates COX-1 at Ser530 and COX-2 at Ser516 resulting in irreversible enzyme inhibition.