The Warfarin Problem: Why Pharmacogenetic Testing is Necessary

Authored by: Lauren Wilkes

Art by: Edsel Ou

How could we know that two individuals, even seemingly physically similar individuals may have a completely different reaction to the same prescribed drug or drug therapy? What this truly comes down to is how one’s body metabolizes the drug in question. The next question becomes how do we detect or predict, if an individual will metabolize a drug a certain way, or in a way that is different from other individuals receiving the same drug? The field of pharmacogenomics studies the way in which genetic variation in patients affects drug response in the body [1]. This field is vital for identifying genetic markers (DNA sequences with a known physical location on a chromosome) that indicate differences in the way people metabolize drugs, as well as for developing genetic tests that can help predict how patients will respond to these drugs. In so many of the drugs and therapies that are prescribed and provided, even as prominent as chemotherapy, there are cases in which patients have an adverse reaction. These patients may actually have a genetic marker that could have helped identify the risk of said reaction sooner, with premature pharmacogenetic testing. 

While this may seem niche or a worry far from many, this phenomena can be seen in drugs that are much more common than one may think. Warfarin (or Coumadin) is one of the most widely prescribed blood-thinning drugs in the world to treat and prevent thromboembolisms, or in other terms, blood clots that form and block blood flow, as well as break off and travel to block blood flow elsewhere. Warfarin is an anticoagulant that decreases the clotting ability of the blood [2]. As of a 2025 report, Warfarin prescriptions account for approximately eight million prescriptions in the United States annually [3]. 

Warfarin works by antagonizing vitamin K. In the human body, the gene, VKORC1 is a necessary enzyme for activating vitamin K. Warfarin, through its antagonist mechanism, inhibits VKORC1 in order to limit the vitamin K production, which results in less clotting factors being produced. In this way, the decrease in clotting factors thins the blood [4]. The body responds to Warfarin by metabolizing the drug, meaning Warfarin is broken down and processed by the body [5]. Warfarin metabolism is primarily mediated through the CYP2C9 gene [4].

While it is well known and common that there are external factors that respond to Warfarin dosing and treatment such as age, weight, sex, diet, etc., a significant underlying factor is the pharmacogenetic component. The CYP2C9 and VKORC1 genes have multiple variants within the population, making it “polymorphic.” In other words, some individuals may have one variant of VKORC1, while another individual may have a different VKORC1 variant in the same population, despite the genes still having the same function. This is where the importance of pharmacogenetics enters the picture. These variants between the same gene can, in actuality, drastically impact the metabolism response by the body to Warfarin. For example, for the two variants of CYP2C9, Warfarin metabolism is reduced by 40% in patients with one variant and by 90% in those with the other [6].

In reality, patients with a VKORC1 or CYP2C9 variant could have a higher anti-coagulant, or “blood-thinning” response leading to increased bleeding. Thus, patients with CYP2C9 variants or a VKORC1 variant require a lower dose. Both of these are potentially dangerous responses and this is precisely an example of why in 2007, the “US Food and Drug Administration (FDA) required that the warfarin package insert carry information about initial dosing based on CYP2C9 and VKORC1 testing” [7]. Pharmacogenetic testing has the ability to explore the genetic factors in patients that may be pertinent indicators of an adverse or varied metabolic response to certain drugs, such as Warfarin. Having premature testing to observe variant genotypes in patients can significantly impact the decision to prescribe or not prescribe a specific drug to a patient. In this way, pharmacogenetic testing contributes to the appeal and safety that the field of personalized medicine accomplishes. In general, it is observed that patients whose dosage was determined using pharmacogenetic algorithms as opposed to traditional clinical algorithms maintained therapeutic normalized blood clotting levels more consistently. Further, comparatively, patients whose doctor(s) used CYP2C9 and VKORC1 pharmacogenetic testing to determine the proper warfarin dosage had a 31% lower hospitalization rate [8]. 

It would be remiss to not acknowledge the presence of few problems with pharmacogenetic testing. While testing can produce successful results indicating genetic markers, interpretation of said results will always be a viable concern [9]. This being said, a concern such as this is a risk with any form of genetic testing, the results of which physicians are analyzing to determine the best course of action. In the context of drug metabolism, especially with Warfarin, the risk of ignoring variable markers that could have detrimental effects on metabolism makes pharmacogenetic testing something worth requiring and regulating. 

References

1. Loucks, C. M., Groeneweg, G., Roy, C., Lee, D. K., Rieder, M. J., Lebel, D., Ito, S., Ross, C. J., & Carleton, B. C. (2020). Pharmacogenomic testing: Enhancing personalized medication use for patients. Canadian family physician Medecin de famille canadien, 66(4), 241–243.

2. Warfarin (Oral Route) Description and Brand Names - Mayo Clinic. (n.d.). Www.mayoclinic.org. https://www.mayoclinic.org/drugs-supplements/warfarin-oral-route/description/drg-20070945

3. Hong, H., Wilson, A. S., Jones, A. E., Vazquez, S. R., Gilbert, S., Malone, D. C., Chaiyakunapruk, N., King, J. B., Barnes, G. D., Sylvester, K. W., Dube, G., Irving, N. V., Chan, L., Ragheb, B., Delate, T., & Witt, D. M. (2025). Overcoming Barriers to Warfarin Patient Self-Management (PSM) in the US Healthcare System: Implementation Trial Study Protocol. medRxiv : the preprint server for health sciences, 2025.08.18.25333912. https://doi.org/10.1101/2025.08.18.25333912

4. Patel, S., Preuss, C. V., Bhutani, J., & Patel, N. (2024). Warfarin. Nih.gov; StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK470313/

5. Susa, S. T., Preuss, C. V., & Hussain, A. (2023, August 17). Drug Metabolism. Nih.gov; StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK442023/

6. Health, I. of M. (US) R. on T. G.-B. R. for. (2010). Pharmacogenomic Testing to Guide Warfarin Dosing. In www.ncbi.nlm.nih.gov. National Academies Press (US). https://www.ncbi.nlm.nih.gov/books/NBK52750/

7. Rouse, M., Cristiani, C., & Teng, K. A. (2013). Q: Should we use pharmacogenetic testing when prescribing warfarin? Cleveland Clinic Journal of Medicine, 80(8), 483–486. https://doi.org/10.3949/ccjm.80a.12184

8. Lee, M. T., & Klein, T. E. (2013). Pharmacogenetics of warfarin: challenges and opportunities. Journal of human genetics, 58(6), 334–338. https://doi.org/10.1038/jhg.2013.40

9. Mueller, A. (2021, January 27). The problem with pharmacogenetic testing. Stanford Cardiovascular Institute. https://med.stanford.edu/cvi/mission/news_center/articles_announcements/2021/the-problem-with-pharmacogenetic-testing.html