People with this variant -- nearly half of Americans of European ancestry -- were less likely than those without it to have resumed smoking six months after treatment with bupropion, reported the study the journal Biological Psychiatry.

Rachel F. Tyndale of the University of Toronto and Caryn Lerman of the University of Pennsylvania in Philadelphia had more than 300 smokers randomly assigned to treatment with either bupropion or a placebo for 10 weeks. Study participants were genetically tested for the presence of a variant form of the CYP2B6 gene,

"This study is part of our ongoing commitment to develop more accurate and personalized approaches to medicine," Dr. Elias A. Zerhouni, director of the National Institutes of Health, in Bethesda, Md., said in a statement.

"This kind of genetic research is helping us to better understand why some people respond to certain smoking cessation treatments and others don't."