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I nhibitors selective for prostaglandin G/H synthase-2 (PGHS-2) (known colloquially as COX-2) were designed to minimize gastrointestinal complications of traditional NSAIDs — adverse effects attributed to suppression of COX-1–derived PGE₂ and prostacyclin (PGI₂). Evidence from 2 randomized controlled-outcome trials (RCTs) of 2 structurally distinct selective inhibitors of COX-2 supports this hypothesis. However, 5 RCTs of 3 structurally distinct inhibitors also indicate that such compounds elevate the risk of myocardial infarction and stroke. The clinical information is biologically plausible, as it is compatible with evidence that inhibition of COX-2–derived PGI₂ removes a protective constraint on thrombogenesis, hypertension, and atherogenesis in vivo. However, the concept of simply tipping a “balance” between COX-2–derived PGI₂ and COX-1–derived platelet thromboxane is misplaced. Among the questions that remain to be addressed are the following: (a) whether this hazard extends to all or some of the traditional NSAIDs; (b) whether adjuvant therapies, such as low-dose aspirin, will mitigate the hazard and if so, at what cost; (c) whether COX-2 inhibitors result in cardiovascular risk transformation during chronic dosing; and (d) how we might identify individuals most likely to benefit or suffer from such drugs in the future.