Warfarin is an anticoagulant whose effective dose varies enormously between people. Common variation in VKORC1 — warfarin's direct target — plus CYP2C9 and CYP4F2 explains a large share of that variability.
Warfarin is an anticoagulant — a medicine that reduces the blood's tendency to form clots. It is the most widely used oral anticoagulant in the world and is prescribed to treat and to prevent clotting disorders. It is also notoriously difficult to dose. Warfarin has what pharmacologists call a narrow therapeutic index: the gap between too little effect and too much is small. Too little and a dangerous clot can form; too much and bleeding risk rises. Because of that, people taking warfarin have their clotting measured regularly with a blood test called the INR, and their dose is adjusted based on the result.
What makes warfarin unusual is how differently people respond to it. Two people of similar size and age, aiming for exactly the same INR target, can end up needing doses that differ several-fold. Finding the right dose for an individual is an iterative process that can take weeks to months, and during that period the person is at higher risk of being over- or under-anticoagulated. Complications from warfarin dosing are among the more frequently reported adverse drug events in the United States and a common reason for emergency department visits.
Where the genetics comes in. A meaningful part of that person-to-person variation is inherited. Warfarin works by blocking an enzyme called vitamin K epoxide reductase, which is encoded by the gene VKORC1. That enzyme recycles vitamin K, and vitamin K is needed to finish building several of the body's clotting factors. Block the enzyme and fewer functional clotting factors get made — which is the drug's intended effect.
The variant on this page, rs9923231, sits just upstream of the VKORC1 gene rather than inside the part that codes for the protein. It lies on a stretch of DNA that influences how much VKORC1 protein the cell produces. People who carry one or two copies of the sensitivity-associated version tend to need progressively less warfarin to reach the same INR than people carrying none — in plain terms, there is less of the drug's target to block. This variant differs a great deal in frequency between populations, which is a large part of why average warfarin dose requirements differ between population groups.
VKORC1 is not the only gene involved. CYP2C9 is the liver enzyme that clears the more potent form of warfarin from the body, and reduced-function versions of it are associated with slower clearance and, in published studies, higher bleeding risk and longer time to a stable INR. CYP4F2 affects the vitamin K side of the equation. There is also a variant in the CYP2C gene cluster that has been shown to matter specifically in African American patients. Taken together with known non-genetic factors — things like age, body size, diet and interacting medications — genetics accounts for roughly half of the variability in warfarin dose. MyGeneLog currently reports the VKORC1 variant only; it is one input among several, not a complete picture.
What this does and does not mean. There is a real, well-established clinical guideline in this area, and it is written for prescribers: it describes how genotype information can be fed into validated dosing algorithms when results happen to be available. It is not a self-service tool. A genotype result cannot tell anyone what dose to take, and nothing on this page should be used to start, stop, adjust, or second-guess an anticoagulant. Warfarin dosing is managed with INR monitoring by a clinician, and that remains true regardless of what a genetic report says. If you are taking warfarin and are curious how your genetics fits in, the useful move is to raise it with the prescriber or pharmacist managing your therapy.
One reassuring note: the common variants in VKORC1, CYP2C9 and CYP4F2 that are looked at in warfarin response testing have not been consistently linked to any disease in their own right. They are relevant to how a person handles this particular drug, not markers of illness.
Guideline. Johnson JA, Caudle KE, Gong L, et al. Clinical Pharmacogenetics Implementation Consortium (CPIC) Guideline for Pharmacogenetics-Guided Warfarin Dosing: 2017 Update. Clin Pharmacol Ther. 2017;102(3):397-404 (PMID 28198005). This 2017 update replaces the 2011 CPIC guideline for CYP2C9 and VKORC1 genotypes and warfarin dosing, and remains the current CPIC warfarin guideline; it extends coverage to CYP4F2 and the CYP2C cluster variant rs12777823 and adds recommendations stratified by continental ancestry and for paediatric patients. It applies to patients with an INR target of 2-3.
Mechanism. Warfarin is administered as a racemate; S-warfarin is the more potent enantiomer and is metabolised predominantly by CYP2C9 to 7- and 6-hydroxy metabolites. Its pharmacodynamic target is VKORC1, the vitamin K epoxide reductase that converts vitamin K epoxide back to vitamin K — the rate-limiting step in vitamin K recycling. Inhibition limits the supply of reduced vitamin K available for gamma-glutamyl carboxylase-mediated carboxylation of the vitamin K-dependent clotting factors, reducing the pool of functionally active factors. CYP4F2 acts on the opposite side of the cycle, oxidising vitamin K to hydroxy-vitamin K1 and removing it from circulation within the cycle.
VKORC1 rs9923231. Designated c.-1639G>A, a promoter-region variant lying on a haplotype that alters VKORC1 protein expression. Carriage of one or two -1639A alleles is associated with progressively lower warfarin dose requirements relative to -1639G/G homozygotes. Other common VKORC1 SNPs and haplotypes do not add predictive value beyond this one; clinical laboratories commonly genotype either c.-1639G>A or the tightly linked c.1173C>T (rs9934438). Note that VKORC1 is transcribed from the minus strand, so population databases report this position with C/T alleles, the T allele corresponding to -1639A.
Phenotype terminology. CPIC does not assign named metabolizer phenotypes for VKORC1. Laboratories report the genotype directly (G/G, G/A, A/A) with an interpretive comment on warfarin sensitivity; the guideline text refers to VKORC1 -1639 A/A as increased sensitivity. Metabolizer phenotype terminology in this guideline belongs to CYP2C9, where CYP2C9*1/*1 is the normal metabolizer reference and diplotypes such as *2/*3 and *3/*3 are given as examples of poor metabolism. Separately, rare non-synonymous coding variants in VKORC1 confer warfarin resistance with high dose requirements; most commercial platforms do not interrogate them.
Other genes in the guideline. CYP2C9*2 (c.430C>T, p.Arg144Cys, rs1799853) and CYP2C9*3 (c.1075A>C, p.Ile359Leu, rs1057910) are the two most common decreased-function alleles in European-ancestry populations and impair S-warfarin metabolism by approximately 30-40% and 80-90% respectively. CYP2C9*5, *6, *8 and *11 are decreased-function alleles found at highest frequency in African-ancestry populations, where collectively they are more common than *2 and *3; most FDA-approved CYP2C9 assays cover only *2 and *3 and are therefore less informative in those populations. CYP4F2*3 (c.1297G>A, p.Val433Met, rs2108622) has a statistically significant but modest effect, with meta-analyses indicating roughly 8-11% higher dose requirements in A-allele carriers; the association is supported in European and Asian ancestry cohorts but not African ancestry. rs12777823, in the CYP2C cluster near CYP2C18, was identified by GWAS in African Americans and affects warfarin clearance independently of CYP2C9*2 and *3; the guideline advises against applying it in non-African-American individuals.
Variance explained. In European-ancestry patients, common variants in CYP2C9, VKORC1 and CYP4F2 account for up to approximately 18%, 30% and 11% respectively of the variance in stable warfarin dose; because allele frequencies differ, these variants explain less of the dose variability in other ancestry groups. Genetic plus known non-genetic factors together account for about 50% of dose variability.
Population frequency. 1000 Genomes Phase 3 (Ensembl) frequency of the -1639A (sensitivity-associated) allele: EAS 0.885, AMR 0.411, EUR 0.388, SAS 0.145, AFR 0.055. This distribution largely accounts for the observed differences in average warfarin dose requirement between population groups.
Regulatory status. The FDA's Table of Pharmacogenetic Associations lists VKORC1 with warfarin in Section 1 — associations for which the data support therapeutic management recommendations — identifying -1639G>A variant carriers as the affected subgroup and describing the effect as altered dosage requirements, with initial dose selection informed by clinical and genetic factors and subsequent adjustment based on INR.
Incidental findings. No disease has been linked to the common CYP2C9, VKORC1 or CYP4F2 variants interrogated in warfarin response testing independent of drug metabolism and response. Homozygosity for rare VKORC1 coding mutations is a recognised cause of combined deficiency of vitamin K-dependent clotting factors type 2 (VKCFD2), a rare bleeding disorder that responds to oral vitamin K.
No dosing guidance is given on this page. CPIC's recommendations are directed to prescribers and are operationalised through validated pharmacogenetic dosing algorithms alongside INR monitoring.
What a 23andMe/AncestryDNA export or raw VCF can and can't tell you about Warfarin Sensitivity and Anticoagulant Dose Variability comes down to these specific, well-studied positions — not a diagnosis.
Research-derived gene–drug associations only — not a prescription, dosing guide, or medical advice. Always follow your prescriber's guidance.
| Gene | Drug | What the research shows |
|---|---|---|
| VKORC1 | Warfarin | VKORC1 encodes vitamin K epoxide reductase, the direct target of warfarin. The rs9923231 (c.-1639G>A) variant sits on a haplotype that changes how much VKORC1 protein is produced. Carrying one or two copies of the -1639A allele is associated with progressively lower warfarin dose requirements compared with -1639G/G, and CPIC describes the A/A genotype as increased warfarin sensitivity. CPIC does not define named metabolizer phenotypes for VKORC1 — laboratories report the genotype with a sensitivity interpretation. Common VKORC1 variants account for up to about 30% of the variance in stable dose in European-ancestry patients. No dose guidance is given here; warfarin is managed by a prescriber with INR monitoring. (CPIC Guideline for Pharmacogenetics-Guided Warfarin Dosing: 2017 Update. Johnson JA et al. Clin Pharmacol Ther. 2017;102(3):397-404 (PMID 28198005).) |
| CYP2C9 | Warfarin | CYP2C9 is the main enzyme clearing S-warfarin, the more potent form of the drug. Decreased-function star alleles — most commonly *2 and *3, plus *5, *6, *8 and *11 which are more frequent in African-ancestry populations — reduce that clearance. CPIC classifies CYP2C9 diplotypes into normal, intermediate and poor metabolizer phenotypes. Published studies associate carriage of reduced-function alleles with greater bleeding risk on warfarin and longer time to a stable INR. This gene is part of the CPIC guideline but is not currently reported by MyGeneLog. (CPIC Guideline for Pharmacogenetics-Guided Warfarin Dosing: 2017 Update. Johnson JA et al. Clin Pharmacol Ther. 2017;102(3):397-404 (PMID 28198005).) |
| CYP4F2 | Warfarin | CYP4F2 oxidises vitamin K and removes it from the vitamin K cycle, acting as a counterweight to VKORC1. The CYP4F2*3 variant (rs2108622) has a modest effect, with meta-analyses indicating roughly 8-11% higher dose requirements in carriers. The association is supported in European and Asian ancestry populations but not in African ancestry populations. Included in the CPIC guideline; not currently reported by MyGeneLog. (CPIC Guideline for Pharmacogenetics-Guided Warfarin Dosing: 2017 Update. Johnson JA et al. Clin Pharmacol Ther. 2017;102(3):397-404 (PMID 28198005).) |
Warfarin is an oral anticoagulant — it reduces the blood's tendency to clot. It is used to treat and to prevent thromboembolic disorders, and it is the most widely used oral anticoagulant worldwide. Its effect is monitored with a blood test called the INR, and the dose is adjusted based on those results by the clinician managing therapy.
VKORC1 encodes the enzyme warfarin is designed to block. The rs9923231 variant sits just upstream of the gene, on a stretch of DNA that influences how much of that enzyme gets made. Research consistently finds that people carrying one or two copies of the sensitivity-associated version reach the same INR target on lower warfarin doses than people carrying none. This page describes that association only; it does not provide dose information.
No. The CPIC guideline also covers CYP2C9, the enzyme that clears the more potent form of warfarin, and CYP4F2, which acts on vitamin K; a variant in the CYP2C cluster is relevant specifically in African American patients. Common variants in CYP2C9, VKORC1 and CYP4F2 account for up to roughly 18%, 30% and 11% of the variance in stable dose in European-ancestry patients, and genetics plus known non-genetic factors together explain about half of the variability. MyGeneLog currently reports VKORC1 only.
No. Warfarin has a narrow therapeutic index — too little risks a clot, too much risks bleeding — and dosing is managed with regular INR monitoring by a clinician. The CPIC guideline is written for prescribers and works through validated dosing algorithms that combine genotype with clinical factors. A genetic result is one input among several and is never a dosing instruction. Nothing here is medical advice.
No. CPIC notes that no disease has been consistently linked to the common VKORC1, CYP2C9 or CYP4F2 variants examined in warfarin response testing, independent of drug metabolism and response. They describe how a person handles a specific medication, not an underlying illness. Separately, rare coding mutations in VKORC1 — not this common variant — can cause a rare inherited bleeding disorder.
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