LONDON (Reuters) - Young heart attack survivors with a common genetic variation were more likely to die or have a serious heart problem while taking the blood thinner Plavix than those without the genetic variation, researchers said on Tuesday.

The Sanofi-Aventis and Bristol-Myers Squibb Co drug was less effective for these people and also raised the risk of blood clots for those carrying the genetic change who had heart stents implanted, the French team said.

The findings highlight the importance of factors that can affect the response to Plavix but researchers cautioned it was too soon to start genetically testing patients for the variation without further study.

"These findings need to be independently replicated... before being extrapolated to older patients or those of non-European ancestry," Gilles Montalescot of Pitie-Salpetriere Hospital in Paris and colleagues wrote in the journal Lancet.

The researchers looked at a gene known as CYP2C19, which plays a key role in determining how individuals metabolize commonly prescribed drugs, including treatment for depression, heart disease, high blood pressure and hyperactivity.

Their study between 1996 and 2008 followed 259 men and women younger than 45 years old who survived a heart attack and were on the drug known generically as clopidogrel for just over a year.

A little more than a quarter of the volunteers had the genetic variation, about the same level as in Western populations but less than in Asia where the mutation is more common, the researchers said.

The researchers found the genetic variation appeared to increase the risk of complications and premature death compared to people without the change.

When looked at alone, the researchers also found that stent blockage was six times more likely to occur in people with the variation. Stents are wire mesh tubes inserted into coronary arteries that have been cleared of blockages.

Drug releasing versions of stents help prevent vessels from re-clogging.

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