A new study conducted by scientists at the University of Pennsylvania suggests that arthritis drugs such as Vioxx and Celebrex may increase a user's risk of suffering a heart attack. Lead scientist Dr. Garret Fitzgerald says the drugs may cause a chemical imbalance that prompts blood clots. Although the study examined mice, not people, several leading cardiovascular experts believe the data may help clarify why earlier reports linked the drugs to heart problems.

Fitzgerald explained that the human body contains two enzymes, Cox-1, which causes blood vessels to constrict, and Cox-2, which dilates blood vessels and prevents clotting. Vioxx and Celebrex block Cox-2, an action that may lead Cox-1's effects to cause dangerous blood clots. When researchers mimicked the effects of Vioxx and Celebrex in mice, clotting activity was seen. Doctors say more research is necessary to confirm the study's findings.