The molecular identification of a second isoform of cyclooxygenase-2 (COX-2) led to a major investment by several pharmaceutical companies in the development of selective inhibitors. The central tenets of the rationale for developing selective COX-2 inhibitors are that prostaglandins that contribute to inflammation are derived from COX-2, whereas prostaglandins that are involved in normal physiological processes are derived from the constitutively expressed isoform COX-1. There is now considerable evidence that COX-2 is actually expressed constitutively in many tissues and performs important physiological functions. Thus, suppression of COX-2 with selective inhibitors should not be expected to be without some adverse consequences. Moreover, there is strong evidence that COX-1 contributes to inflammation and pain, so selective inhibition of COX-2 will not necessarily produce the same degree of efficacy that is seen with mixed inhibitors of COX-1 and COX-2.