Vioxx is thought to cause heart attacks and strokes by damaging fatty compounds in the blood called lipids. These damaged compounds then significantly increase the risk of blood clots, which can lead to heart attacks and strokes.
Vioxx side effects such as heart attacks and strokes were first officially observed in the VIGOR (Vioxx GI Outcomes Research) study in 2000. It showed an increased risk of heart attack and stroke for Vioxx takers. Despite this higher observed risk, these results weren’t conclusive because the study was not designed specifically to address the issue of cardiovascular (CV) events with Vioxx. Heart attacks, strokes, and other side effects were not being tested for in the VIGOR study, and there were some conflicting results in other simultaneous studies. It was not until the APPROVe ( Adenomatous Polyp Prevention on VIOXX) study that Vioxx side effects information was conclusive enough to act on.
The APPROVe study showed that for regular users of 25mg (half the maximum dose) of Vioxx, stroke and heart attack risk, along with risk for other serious CV events, nearly doubled after the first 18 months of treatment. Although this is a very significant increase, the majority of people have nothing to worry about because the total percentage of those affected was small.
There is no evidence for continued increased risk after the people stop taking Vioxx, and there is no evidence that abrupt withdrawal from the drug schedule has any negative effects.
Despite this official story, because of many recent lawsuits, information has surfaced that implicates Merck and some physicians in fraud and misrepresentation. It is alleged that Merck knew about Vioxx side effects long before they claimed to and still allowed the product to be marketed and sold to millions of people. See - Why was Vioxx prescribed? Was Vioxx approved by the FDA? What are the adverse side effects of taking Vioxx? Why was it recalled? for a further discussion.