Sign Up for MedicineNet Newsletters. VKORC1, which affects the liver's ability to use and excrete warfarin, and CYP2C9, which helps with the essential blot-clotting vitamin K, were found to be responsible. Researchers from Sweden and the United Kingdom, studying the genetic makeup of more than 1,000 warfarin users, reported finding three genes -- VKORC1, CYP2C9 and CYP4F2 -- with com variants that explain more than 40 percent of the variability in optimal dosing levels. Caldolor Is First Approved Injected Ibuprofen Recession Means Free Rx Drugs for Some Some OTC Sleep, Cold Meds Could Harm Aging Brain Want More News. Food and Drug Administration estimates, about 2 million Americans use the anticoagulant medication to prevent blood clots and reduce the likelihood of a stroke, pulmonary embolism or heart attack. New Anticoagulant Pill Works Well in Trial ADHD Drugs. Though the drug can be very effective, the optimal therapeutic dosage varies as much as 20-fold among individuals, with too high a dose risking severe bleeding and too small a dose failing to protect against the original cardiovascular worry.

Trial and error follo by frequent monitoring of blood levels have been the com method of determining the proper dose for a given person.