Canadian warning adds to Merck's troubles over Vioxx

A new study looks set to pile the pressure on Merck as it fights to limit the financial damage from the withdrawal of its best-selling painkiller, Vioxx.

Canadian scientists are claiming that using Vioxx immediately increased the risk of heart attacks for elderly patients, undermining a key plank of Merck's defence in a slew of lawsuits from users and their families.

Merck withdrew Vioxx in 2004 after finding that long-term use was associated with heart problems, but its lawyers continue to insist that only patients on the drug for more than 18 months were at risk.

In a paper published yesterday in the Canadian Medical Association Journal, researchers at the McGill University Health Center in Montreal found that a quarter of heart attacks suffered by Vioxx users occurred within two weeks of patients starting taking the drug.

"Our previous study on COX-2 inhibitors, which included Vioxx and Celebrex, evaluated whether there was an increased risk of heart attack while taking these medications; the answer was: yes for Vioxx," Linda Levesque, the lead author of the new study, said.

The study looked at the health records of 114,000 senior citizens taking anti-inflammatories in the Quebec region over a three-year period.

The cardiovascular risk peaked on the ninth day after a patient started taking Vioxx, and actually decreased with longer-term use.

Last night Merck said that although had not seen the Canadian research, it believed the clinical trials which found an increased risk after 18 months of use provided stronger evidence than observational studies such as the Canadian research was based on.

Merck is resisting a US-wide settlement to Vioxx lawsuits, which already number 11,500 and is rising. It has lost more cases so far than it has won, as juries in several states found it knew about the risks of the drug for up to four years before taking it off the market in September 2004.

The drug's withdrawal led to the early departure of Merck's chief executive Ray Gilmartin, and his replacement by Richard Clark.

Last month, Merck said it would appeal a $32m (£18m) pay-out ordered by a court in Texas for the family of a man who had a heart attack after taking Vioxx for less than a month.