Every day millions of people take warfarin (Coumadin), this powerful drug is used to control life-threatening conditions. The dose of warfarin or Coumadin that people need is difficult to determine and often it takes several months of weekly clinic visits and blood tests. However, according to the FDA, hemorrhage during warfarin therapy is a leading cause of death in Western countries and related adverse events account for 1 in 10 hospital admissions.
Thankfully, recent discoveries in DNA research make predicting the dose of warfarin a person needs much more accurate, as well as predicting how long it takes them to eliminate the drug. More than half of the population has variations in at least one of the two genes, CYP2C9 and VKORC1, that are tested. These genetic variations are responsible for almost half of the individual variation seen in warfarin dose. Recently CYP4F2 has been added to CYP2C9 and VKORC1 testing as many scientists believe that the use of this testing will dramatically improve warfarin efficacy and safety. [26,27]
Additional Information on Warfarin
An FDA advisory panel provides warfarin/Coumadin guidance based on genetics: 
- Use lower doses of warfarin for patients with genetic variations in CYP2C9 and or VKORC1 that lead to reduced activities.
- Genotyping patients at the beginning of warfarin therapy would reduce adverse events and improve achievement of stable INR.
- Existing evidence of the influence of CYP2C9 and VKORC1 genotypes warrants re-labeling of warfarin to include genetic test information.
WarfarinDosing.org is a free Web site that helps doctors and other clinicians begin warfarin therapy by estimating the therapeutic dose in patients new to warfarin.