Plavix, a blood thinner sold by Sanofi-Aventis SA of France and Bristol-Myers Squibb Co. of the U. S. was seen to be less effective for people with a common genetic variation. They were more likely to die or have a serious heart problem while taking the drug as the gene variant appears to reduce the function of an enzyme needed to activate the medicine.

The gene in question is known as CYP2C19 and this plays an important part in determining how individuals metabolize commonly prescribed drugs, including treatment for depression, heart disease, high blood pressure and hyperactivity. The mutation is more commonly seen in Asia than it is in the Western population.

Three studies were conducted and one was published in the Lancet and two in the New England Journal of Medicine. The study in the Lancet said the people with the gene variation who had previously had a heart attack had a three times higher risk of a new heart attack or death as compared to people without the gene variation.

In a study conducted between 1996 and 2008, researchers followed 259 men and women who were younger than 45 years of age, had survived a heart attack and were on the drug known generically as clopidogrel for just over a year.

A quarter of the subjects had the gene variant and the researchers found that this appeared to increase the risk of complications and premature death as compared to the people who did not have the gene variation. The researchers also found that stent blockage was six times more likely to occur in people with the variation.

Gilles Montalescot a professor of cardiology at Pitie-Salpetriere Hospital in Paris and colleagues wrote in the journal Lancet, "It's not that Plavix is harmful; it's that it's not effective" for those patients at conventional doses. "These findings need to be independently replicated ... before being extrapolated to older patients or those of non-European ancestry."

A new drug prasugrel by Eli Lilly & Co. is being considered for the treatment of such patients which appears to work through different mechanisms than Plavix and have a less of a dependency on the enzyme controlled gene.

Researchers from Université Pierre et Marie Curie in Paris found that 30% of the population are carriers for one copy of the nonworking version of the gene while only 3 % carry two non working copies. Tabassome Simon, an associate professor of pharmacology said only those with two copies had an increased chance of cardiovascular risk.