VKORC1 rs9923231: What Your Genotype Means for Warfarin Dosing
The VKORC1 rs9923231 variant is a common genetic change in the promoter region of the VKORC1 gene. It is strongly associated with individual sensitivity to warfarin, a medication used to prevent blood clots, and helps clinicians determine appropriate starting doses.
What each genotype means
Higher warfarin dose requirement
This genotype is associated with higher expression of the VKORC1 enzyme, which may require a higher therapeutic maintenance dose of warfarin. Please discuss your specific medication dosing and management plan with your clinician or pharmacist.
This genotype is most common in individuals of African ancestry and less frequent in East Asian populations.
Intermediate warfarin dose requirement
This genotype represents an intermediate level of VKORC1 enzyme expression compared to the CC and TT genotypes. You may require a moderate therapeutic maintenance dose of warfarin, so please consult your healthcare provider or pharmacist regarding your dosing strategy.
This genotype is found across many global populations, with varying prevalence depending on ancestral background.
Lower warfarin dose requirement
This genotype is associated with reduced expression of the VKORC1 enzyme, which often necessitates a lower therapeutic maintenance dose of warfarin to prevent bleeding complications. It is essential to discuss your medication dosing with your clinician or pharmacist to ensure your treatment is appropriately tailored.
This genotype is highly prevalent in East Asian populations and occurs at lower frequencies in European and African populations.
What is the rs9923231 Variant?
The rs9923231 variant, often referred to in scientific literature as -1639G>A, is a single nucleotide polymorphism (SNP) located in the promoter region of the VKORC1 gene. A promoter region acts as an on-off switch for a gene, controlling how much protein is produced. In this case, the presence of the A allele instead of the G allele alters the efficiency of this switch. This specific change is one of the most well-studied genetic markers in pharmacogenomics, the field of study that examines how genes affect a person's response to drugs. Because this variant sits in a regulatory area, it directly influences the amount of the VKORC1 enzyme available in the body, which in turn dictates how effectively the body processes certain anticoagulant medications.
The Role of the VKORC1 Gene
The VKORC1 gene provides instructions for making the vitamin K epoxide reductase subunit 1 enzyme. This enzyme is essential for the body's recycling of vitamin K, a nutrient that plays a critical role in blood clotting. Warfarin works by inhibiting this enzyme, effectively reducing the amount of active vitamin K available and thereby slowing the blood's ability to clot. When the rs9923231 variant reduces the expression of the VKORC1 gene, the body produces less of this enzyme. Consequently, individuals with this variant require less warfarin to achieve the same therapeutic effect. If the dose is not adjusted based on this genetic sensitivity, there is a higher risk of the medication being too effective, which can lead to an increased risk of bleeding complications.
Research and Clinical Evidence
Research consistently shows that the rs9923231 variant is a major genetic determinant of warfarin dose requirements. Numerous studies have demonstrated that individuals carrying the A allele require lower maintenance doses of warfarin compared to those with the G allele. The evidence for this association is strong and widely recognized in clinical guidelines, such as those provided by the Clinical Pharmacogenetics Implementation Consortium (CPIC). While this variant is a powerful predictor, it is often used alongside other factors, including age, body weight, and other genetic markers like CYP2C9, to calculate personalized dosing algorithms. It is important to note that while the association with dose requirement is well-established, clinical outcomes like bleeding risk are influenced by a complex interplay of genetic and non-genetic factors. Always consult with your healthcare provider or pharmacist to discuss how your genetic information may relate to your specific medication plan.
Understanding Your Results
Genetic testing for rs9923231 provides information about your potential sensitivity to warfarin, but it is not a diagnostic tool on its own. You cannot use this information to self-prescribe or adjust your medication dosage. If you are prescribed warfarin, your clinician will use a variety of clinical tools, which may include genetic testing, to monitor your International Normalized Ratio (INR) and ensure your dose is safe and effective. This variant is one piece of a larger puzzle; your doctor will consider your overall health, other medications you may be taking, and your lifestyle when making treatment decisions. If you have received results regarding this variant, share them with your doctor or a clinical pharmacist who can interpret them in the context of your complete medical history and current treatment needs.
How common is this variant?
The A allele frequency varies significantly by ancestry, occurring in approximately 40% of Europeans, 10% of Africans, and 85-90% of East Asians.
Frequently asked questions
Does having the AA genotype mean I cannot take warfarin?
No, having the AA genotype does not mean you cannot take warfarin. It simply means you may be more sensitive to the drug, and your doctor will likely start you on a lower dose to ensure your blood clotting levels remain in a safe, therapeutic range.
Is rs9923231 the only gene that affects warfarin?
No, other genes, particularly CYP2C9, also play a significant role in how the body metabolizes warfarin. Clinicians often look at a combination of genetic and non-genetic factors to determine the most appropriate dose.
Should I get tested for this variant before starting warfarin?
Pharmacogenetic testing is sometimes used to help guide initial dosing, but it is not required for everyone. You should discuss with your healthcare provider whether genetic testing is appropriate for your specific clinical situation.
Can I change my warfarin dose based on my genotype?
Absolutely not. You should never change your medication dosage without direct instruction from your doctor. Warfarin dosing is complex and requires regular blood monitoring to ensure safety.
Sources & further reading
Educational information only, last refreshed 10/26/2026. Not medical advice — these associations describe population statistics, not individual predictions.
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This VKORC1 variant is a clinically useful marker for sensitivity to warfarin and related vitamin-K-antagonist dosing, with direct implications for medication safety.
Promoter variant in VKORC1 influencing warfarin sensitivity and dosing requirements.
Regulatory variant in the VKORC1 promoter region that significantly influences warfarin dose requirements.
Located in VKORC1, this missense variant (p.Val66Met) is associated with warfarin resistance requiring elevated therapeutic dosages.
Intronic regulatory variant in VKORC1 in high linkage disequilibrium with rs9923231, dictating clinical warfarin dosing requirements.
This variant is a well-established marker in the VKORC1 gene often used in studies regarding warfarin sensitivity and dosing.