Rofecoxib-d5 is intended for use as an internal standard for the quantification of rofecoxib by GC- or LC-MS. Rofecoxib is a non-steroidal anti-inflammatory drug (NSAID) that selectively inhibits COX-2 over COX-1 with IC50 values of 0.018 and >15 μM, respectively, for production of prostaglandin E2 in CHO cells expressing the recombinant human enzymes.1 In vivo, rofecoxib inhibits carrageenan-induced paw edema and hyperalgesia (ID50s = 1.5 and 1.0 mg/kg, respectively), LPS-induced pyresis (ID50 = 0.24 mg/kg), and M. butyricum-induced arthritis (ID50 = 0.74 mg/kg per day) in rats. Formulations containing rofecoxib were previously used in the treatment of pain and arthritis.