A quarter of all heart attacks occurring in patients taking Merck & Co’s withdrawn painkiller Vioxx occurred within first two weeks of use, a Canadian study shows.

Merck - and indeed the US Food and Drug Administration - have taken the position that patients needed to be taking Vioxx (rofecoxib) for 18 months or more before the risk of heart attack was elevated.

This has been a pillar of the company’s legal defense in court cases brought by people claiming their heart attack was caused by the product, and the latest study could provide encouragement and ammunition to lawyers who represent short-term users of the drug.

The study, led by Queen's University researcher Professor Linda Levesque, was conducted before Vioxx was voluntarily taken off the market on September 30, 2004, and examined the medical records of 239 people in Quebec who suffered a heart attack while taking Vioxx. It is the first study to specifically address the question of the timing of cardiovascular risk associated with COX-2 inhibitors.

The first two weeks’ of treatment seemed correspond with the period of highest risk, and additional cardiovascular risk actually decreased with longer duration of use. The study also suggests that cardiovascular risk returns to normal within one month of stopping the drug.

"This demonstrates that cardiovascular risks from taking Vioxx may occur much earlier than previously believed," said Prof Levesque.

The study was a follow-up to an earlier trial run by the same team which found an elevated cardiovascular risk with Vioxx but not another drug in the COX-2 inhibitor class, Pfizer’s Celebrex (celecoxib). Once again, in the latest analysis, Celebrex was not linked to an increase in risk.

The survey was conducted with McGill University researchers James Brophy and Bin Zhang, and are published on-line in the Canadian Medical Association Journal.

Last week, a Texas court awarded $32 million in damages to the family of a man who died of a heart attack whilst on Vioxx for less than a month. Earlier in April, a New Jersey court awarded $13.5 million to a man who had a heart attack after taking Vioxx for four years, but dismissed another claim involving a plaintiff who could not prove long-term use of the drug.

Prior to this, Merck lost its first Vioxx injury case, also in Texas, in which a short-term user of the drug was awarded $253 million, capped to $26 million, but then won two other cases involving people who had not been exposed to the product for long.

Merck is still facing roughly 11,500 lawsuits over the drug. In a statement, the company said observational studies such as this one in the CMAJ are considered less scientifically rigorous than clinical trials.