Merck & Co had hoped that new data from the APPROVe study of its Vioxx painkiller would help it fight the thousands of lawsuits brought by patients claiming injury from the drug. But the analysis seems to be causing more pain than relief.

The latest interpretation of the data seems to suggest that, contrary to the position held by Merck and indeed the US Food and Drug Administration (FDA), Vioxx can increase the chances of a heart attack or stroke after as little as four months’ treatment. Prior to this, the perceived wisdom was that patients had to be taking the drug for 18 months or more to be at risk.

Merck withdrew Vioxx from the market in September 2004 after a study showed the drug doubled the chances of heart attack and stroke in patients who took it for this period of time or more.

Merck first unveiled the results of an extension phase of the APPROVe last week, claiming that they showed clearly that once a patient stopped taking the drug their elevated cardiovascular risk tailed off. This would mean that people claiming injury by the drug, but who suffered a thrombotic event after the discontinued therapy, could find it harder to prove Vioxx was responsible.

Of the six Vioxx lawsuits that have reached verdicts, Merck has lost three, with multi-million dollar jury awards in each case.

But, within 24 hours of making the data public Merck’s conclusions were challenged by cardiologists, including Steve Nissen of the Cleveland Clinic in the USA, who pointed to a non-significant 67% increase in thrombotic risk compared to placebo in the post-treatment period. This trend might indicate an increase in risk that would only be revealed by a larger, more statistically powerful dataset, they suggested.

In the latest development, Nissen and other eminent cardiologists, including Curt Furberg of Wake Forest University School of Medicine, claim the APPROVe extension phase shows that cardiovascular risk becomes elevated four to six months after patients start on Vioxx. Not so, according to Merck, whch said in a statement that “the new APPROVe data do not establish that the risk for Vioxx starts earlier than had been previously reported.”

The experts, first cited by National Public Radio in the USA, reportedly drew their conclusions on the new data from a 107-page report on APPROVe that Merck has submitted to the FDA. The agency said it was reviewing the document and would not comment either way until that process had been completed.

The claims follow another study, published earlier this month by researchers at McGill University in Canada, which suggested that the highest period of risk after during Vioxx treatment was the first two weeks after the drug was first taken. This study also suggested that the risk returned to normal levels within one month of stopping treatment.