Researchers from University of Pennsylvania’s Perelman School of Medicine found that Vioxx was suppressing the production of enzymes vital for the protection of heart.

Vioxx belonged to a new class of painkiller drugs that worked by blocking the production of a single enzyme, Cox-2. This enzyme was responsible for the production of molecules that caused pain.
However some of the molecules were responsible for protecting the heart, including breaking up clots and promoting blood flow into the organ.
Source-Medindia