Researchers are reporting that genetic testing can lead to safer use of warfarin, a blood thinner that is widely-used for preventing dangerous blood clots. Although effective, the drug can potentially cause life-threatening bleeding and has caused more emergency room visits for older people over the last decade than any other medication. In the new study, researchers investigated whether genetics-based dosing of the drug—based on certain genetic markers found in the blood—could help predict the best outcome for the patient. While studying a group of patients who were starting warfarin for elective hip or knee replacement, the scientists compared outcomes for patients whose doses were based on clinical information alone to those whose doses were based on genetic markers combined with clinical factors. The researchers observed fewer adverse events (such as bleeding and clotting) in the group with genetics-based dosing than the group with clinically guided dosing. Their study, which appeared in JAMA, was funded primarily by NHLBI.