Accordingly, because they block the COX-1 enzyme as well as the COX-2 enzyme, older NSAIDs such as aspirin, Advil, and Aleve can produce silent, potentially fatal gastric bleeding.This downside to the ???non-selective??? NSAIDs led researchers to look for NSAIDs that would only block the COX-2 enzyme, and this search led to the introduction of the first ???selective??? COX-2 inhibitors???Celebrex