About one-third of patients taking warfarin (Coumadin) metabolize the drug differently than expected, which can lead to an inadequate therapeutic response or bleeding complications. Responding to recent genetic research, the FDA has approved new labeling that includes information about the impact of genetic information on a patient's response to warfarin. This is an example of the FDA's "personalized medicine" initiative, which draws on scientific findings to tailor drug therapy to an individual's genetic makeup.

In the case of warfarin, research suggests that some unexpected responses to warfarin are tied to variations in two specific genes, CYP2C9 and VKORC1. Patients with these variations can be identified with genetic testing. The new labeling encourages prescribers to use genetic testing to improve their estimate of a reasonable initial dose for a patient taking warfarin. As therapy continues, the prescriber uses International Normalized Ratio monitoring to assess the patient's response.

Following insulin, warfarin is the second most common drug implicated in ED visits for adverse drug reactions. According to the FDA, genetic testing "may optimize the use of warfarin and lower the risk of bleeding complications from the drug."