Summary—The discovery of an inducible isoform of cyclooxygenase (COX‐2) requires a refinement of the theory that inhibition of cyclooxygenase activity explains both therapeutic and side effects of non‐steroidal anti‐inflammatory drugs (NSAIDs). Indeed, new pharmacological results suggest that COX‐2 inhibition provides the therapeutic (ie, anti‐inflammatory) activity of NSAIDs, whereas inhibition of constitutive COX‐1 is responsible for their gastric and renal side effects as well as for their antithrombotic activity. However, a role of COX‐1 in inflammation cannot be excluded. Furthermore, the functional relevance of COX‐2 expression and induction in various tissues warrants further investigation. These studies should help in predicting potential adverse effects as well as new indications for selective