Rochelle M. Long, director of the National Institutes of Health Pharmacogenetics Research Network, said people with the variant produce a defective version of the CYP2C19 enzyme and are less able to activate the drug clopidogrel, also known as Plavix.
Long said Plavix, one of the world's best-selling medicines, prevents blood clots in people with heart disease by keeping platelets from sticking together.
However, the study, published in the Journal of the American Medical Association, found about 30 percent of people respond poorly to the drug and are at increased risk for dangerous events such as strokes and heart attacks.
The researchers used a genome-wide association study in two distinct populations, Pennsylvania Amish and residents of Baltimore, that found a common variant of the CYP2C19 gene is a key determinant of how people respond to Plavix.
The CYP2C19 enzyme chemically modifies the drug inside the body, converting it to the active form, Long said.