A US federal appeal court has revived lawsuits brought against US drugs multinational Merck & Co by its shareholders for concealing the health risks of its blockbuster arthritis painkiller Vioxx.

The shareholders allege that Merck deliberately concealed the high risks of heart attack, stroke and death among users. Merck faces some 7,000 outstanding lawsuits connected to Vioxx, with estimated potential liabilities around $5 billion.

Vioxx was launched in 1999 and became a best-selling anti-inflammatory drug, used primarily to treat arthritis, earning the drug giant $2.5 billion a year.

In September 2004 Merck finally withdrew Vioxx, which had earned after a study found it could double the risk of heart attacks.

In February 2006 Merck won a major court victory when a US federal jury cleared the drug maker of any responsibility for the death of a 53-year-old man, accepting the drug makers defence that there was no proven link between Vioxx and the death due to heart attack of the patient who had been on the Vioxx for less than a month. (See: Merck wins second Vioxx lawsuit)

The appeals court has sent the lawsuits back to the New Jersey federal judge who had dismissed them in May 2006. It said US district judge Stanley R Chesler should have allowed the plaintiffs to amend their original complaint with additional material.

The shareholders lawyer Darren Roberts said the decision to go back to court was "a tremendous victory for Merck shareholders".

Juries in cases surrounding claims of injury and death have found that Merck failed to provide adequate warnings about the health risks associated with Vioxx.

This was Merck''s second court victory over the controversial drug, which at the time of its withdrawal had annual sales of $2.5 billion in 2003, and the first in a federal court.

In August last year, Merck was ordered to pay out $253 million as damages after a state jury in Texas held the company liable for the death of a man. In November 2005, a New Jersey jury found Merck had given doctors adequate warning about possible health risks and did not commit consumer fraud in marketing the drug.