Sold under the trade name Coumadin, it is taken by people who have had blood clots in their legs or lungs, who have suffered strokes while taking aspirin or who are at increased risk of having a stroke because of atrial fibrillation -- an abnormal heart rhythm.
About 2 million Americans start taking warfarin each year. Many continue to take the drug for the rest of their lives.
Use of warfarin is increasing as the population ages. The number of outpatient prescriptions rose from 21 million in 1998 to nearly 31 million in 2004.
Bleeding complications from warfarin are responsible for about 30,000 emergency room visits a year in the United States. Studies have shown that the risk of a serious bleeding episode -- into the brain or intestine, for instance -- is highest soon after treatment has begun.
It is to prevent such contingencies that genetic testing is being suggested. For research has shown that variations in the activity of two genes can greatly increase the potency of warfarin.
One of the genes involved, CYP2C9, has a role in metabolizing, or burning off, the drug. The other, VKORC1, determines a person's sensitivity to the drug's blood-thinning action. The gene profile accounts for about 40 percent of a person's response to the drug, the other variables being age, sex, diet and overall health.
Gene tests are widely available and range from $125 to $500 for the pair.
The tests are useful in helping practitioners decide what dose of warfarin to prescribe to a new patient. People with unusual variants of the genes should be started at lower doses, experts advise.
This is the first time the agency has provided data on the usefulness of genetic tests in the "label," or official instructions, of a popular drug.
"It shows that genomic medicine has moved into the mainstream," said Lawrence J. Lesko, of the agency's Center for Drug Evaluation and Research.