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Change Ordered to Warfarin Drug Label

Warfarin Label Says Dosage May Depend on Genetic Test
WebMD Health News
Reviewed by Louise Chang, MD

Aug. 1, 2007 - In a new trend, labels on the clot-fighting drug warfarin will now warn that people with certain gene variations may need lower doses of the drug.

It's the first time that the FDA has asked doctors to consider a patient's genetic makeup when prescribing a widely used drug, says Larry Lesko, PhD, director of clinical pharmacology at the FDA.

"Today is a significant event for those who see the day when medicine is tailored to the genetic makeup of each one of its users," Lesko said at a news conference.

"We are continuing to engage in various projects that will further evaluate the relationship between genetic variability and treatment outcomes in patients," FDA Deputy Commissioner and Chief Medical Officer Janet Woodcock, MD, said at the news conference.

The change in the label for warfarin (originally sold under the brand name Coumadin) does not require that patients get genetic tests. But the new label suggests that genetic testing can help doctors find the correct dosage of the drug.

Each year, about 2 million Americans start taking warfarin to prevent blood clots, heart attacks, and strokes. It is notoriously difficult to find the right warfarin dosage for an individual patient. Factors such as age, diet, use of other medications, and use of vitamins and supplements affect the dosage.

Because of this, warfarin is the second most common drug (after insulin) that sends patients to the emergency room.

Researchers have noticed that about a third of patients who start warfarin need much lower doses than expected. This difference appears to be due to variations in two genes. One of the genes, VKORC1, affects a patient's sensitivity to warfarin. The other gene, CYP2C9, affects how quickly a person clears warfarin from the body.

The new warfarin label suggests that doctors starting a patient on warfarin might want to have that patient tested for variations in these genes. Most major laboratories already offer the test. Lesko says the cost of testing ranges from $125 to $500; a patient would need to be tested only once.

Patients already taking warfarin probably will not need genetic testing if they already have found the warfarin dosage that works best for them.

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