CYP2C9 rs9332238: What Your Genotype Means
The genetic variant rs9332238 is a single nucleotide polymorphism located in the CYP2C9 gene on chromosome 10. It defines the CYP2C9*8 allele, a reduced-function variant that leads to an amino acid substitution in the hepatic CYP2C9 cytochrome P450 enzyme. Research primarily links this variant to decreased clearance and lower maintenance dosing requirements for anticoagulant medications like warfarin, particularly in individuals of African ancestry.
What each genotype means
Typical enzyme activity
You carry two reference copies of the CYP2C9 gene at this locus, which corresponds to the standard allele rather than the reduced-function CYP2C9*8 allele. This genotype is associated with normal baseline clearance of CYP2C9 substrates such as warfarin and phenytoin, assuming no other variants are present. Always consult your doctor or pharmacist before making any changes to your medication regimen.
Carried by the vast majority of individuals globally, including more than 85% of people of African ancestry and over 99% of people of European and Asian descent.
Decreased enzyme activity
You carry one copy of the CYP2C9*8 allele, which is associated with moderately reduced CYP2C9 enzyme clearance. Published pharmacogenomic research links this genotype to increased sensitivity and lower maintenance dose requirements for medications metabolized by CYP2C9, notably warfarin. Please discuss this finding with your healthcare provider or pharmacist, as clinical dosing guidelines often recommend adjusted dosing to prevent adverse drug effects.
Carried by approximately 10% to 15% of individuals of African and African American ancestry; it is rare (less than 1%) in European and Asian populations.
Significantly reduced enzyme activity
You carry two copies of the CYP2C9*8 allele, a genotype associated with significantly diminished CYP2C9 metabolic activity. Individuals with this genotype typically break down certain medications, such as warfarin or phenytoin, considerably more slowly and may face higher risks of drug buildup or toxicity at standard doses. It is strongly advised that you discuss dosing adjustments and personalized monitoring with your prescribing clinician or pharmacist.
Rare globally, found in fewer than 1% of individuals of African descent and essentially absent in European and Asian populations.
Genetic Architecture and Molecular Impact
The single nucleotide polymorphism designated rs9332238 is situated within the CYP2C9 gene located on chromosome 10q23.33. At this precise genomic locus, a single base substitution occurs (specifically described at the cDNA level as c.449G>A), leading to a missense alteration where an arginine residue is replaced by histidine at amino acid position 150 (p.Arg150His). In standard pharmacogenetic star-allele nomenclature curated by PharmVar and PharmGKB, this sequence alteration defines the CYP2C9*8 haplotype. The CYP2C9 gene codes for cytochrome P450 2C9, an abundant hepatic enzyme responsible for the phase I oxidation of numerous clinically essential pharmaceutical compounds. When the alternate allele is present, the structural stability and catalytic efficiency of the resulting enzyme are diminished relative to the reference enzyme (*1). This alteration lowers clearance rates for substrate drugs, thereby altering systemic drug exposure.
Role of CYP2C9 in Drug Metabolism
Cytochrome P450 2C9 is one of the most vital drug-metabolizing enzymes in the human liver, responsible for processing approximately 15 to 20 percent of all clinically prescribed medications that undergo phase I hepatic biotransformation. Notable substrates include narrow therapeutic index medications such as the oral anticoagulant S-warfarin and the anticonvulsant phenytoin, as well as multiple widely used nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen, celecoxib, and meloxicam. Under normal physiological conditions, wild-type CYP2C9 enzymes rapidly hydroxylate these molecules into inactive forms for renal excretion. When enzyme function is impaired by genetic variations such as CYP2C9*8, circulating active drug molecules persist in the bloodstream for a significantly extended duration. This altered pharmacokinetics increases the likelihood of cumulative exposure and exaggerated drug responses, emphasizing the central importance of CYP2C9 genetic variation in individual drug clearance.
Clinical Evidence and Anticoagulation Research
The primary clinical focus for rs9332238 has centered on anticoagulant therapy, particularly S-warfarin dosing. Historical pharmacogenetic dosing algorithms initially evaluated only the European-prevalent CYP2C9*2 and *3 variants, often failing to accurately predict drug responses in individuals of diverse backgrounds. Subsequent clinical pharmacokinetic studies demonstrated that the CYP2C9*8 allele independently reduces oral S-warfarin clearance, meaning carriers generally achieve effective anticoagulation at lower maintenance doses than standard non-carriers. Clinical organizations such as the Clinical Pharmacogenetics Implementation Consortium (CPIC) officially designate CYP2C9*8 as a decreased-function allele and include it in genotype-guided dosing guidelines for warfarin. Although evidence for CYP2C9*8 is categorized as limited relative to the decades of data amassed for *2 and *3, clinical trials confirmed that accounting for African-prevalent alleles (*5, *6, *8, and *11) is essential to prevent systematic drug overdosing in diverse populations.
Population Distribution and Ancestral Patterns
Unlike CYP2C9*2 and CYP2C9*3, which predominate in populations of European and Central or South Asian descent, rs9332238 displays pronounced ancestral specificity. The variant A allele is predominantly observed in individuals of African ancestry and African Americans, where its minor allele frequency (MAF) typically ranges between 0.05 and 0.08 (roughly 5% to 8%). Multiple population surveys indicate that approximately 1 in 11 to 1 in 12 African Americans carries at least one copy of the CYP2C9*8 allele. In stark contrast, this variant is exceedingly rare or virtually absent in European, East Asian, and Indigenous American cohorts, where allele frequencies remain well below 0.1%. These disparities highlight why ancestral inclusivity in pharmacogenomic research is vital, as testing panels restricted to European marker alleles frequently misclassify non-European individuals as normal metabolizers.
Translating Pharmacogenomic Results into Practice
Discovering an altered genotype at rs9332238 does not diagnose any disease, nor does it guarantee that an individual will experience adverse reactions when prescribed a CYP2C9 substrate. Pharmacogenomics represents an assessment of statistical probability regarding drug breakdown rather than a deterministic outcome. Furthermore, overall drug clearance is influenced by multiple interacting non-genetic factors, including body mass index, age, kidney function, dietary vitamin K intake, and concurrent medications that either inhibit or induce cytochrome P450 enzymes. Individuals who learn their genotype should never adjust, initiate, or discontinue any prescription medication independently. Instead, patients should share formal pharmacogenetic reports with their physician, cardiologist, or clinical pharmacist, who can synthesize genetic data with clinical parameters to personalize therapeutic regimens safely.
How common is this variant?
The rs9332238 variant allele occurs at a frequency of approximately 5% to 8% in African and African American populations, whereas it is exceptionally rare (well below 0.1%) among individuals of European and East Asian descent.
Frequently asked questions
What does a positive rs9332238 test result mean for my health?
Having the rs9332238 variant does not cause an illness or medical disorder on its own. Instead, it indicates that your liver may process specific medications, such as certain blood thinners and anti-inflammatory drugs, more slowly than average. You should discuss your result with your healthcare provider to ensure safe and effective medication choices.
How does CYP2C9*8 affect warfarin dosing?
The CYP2C9*8 allele reduces the clearance rate of S-warfarin, meaning the drug stays active in your system for longer. Clinical studies have shown that individuals carrying this allele typically require lower daily doses to achieve safe anticoagulation. When initiating treatment, doctors use this information to choose appropriate starting doses and avoid elevated bleeding risks.
Why is rs9332238 considered an ancestry-specific variant?
Large population genetics surveys reveal that rs9332238 is predominantly found in people of African ancestry, where roughly 5% to 8% carry the variant allele. By contrast, it is almost entirely absent in populations of European or East Asian descent. Because early genetic panels focused mostly on European alleles, this important variant was historically overlooked.
Should I change my medication dose if I have the GA or AA genotype?
No, you should never alter, stop, or adjust your medication dosages based solely on direct-to-consumer genetic data. Many factors beyond genetics, such as diet, age, and organ function, dictate appropriate medication therapy. Always bring your pharmacogenetic report to your doctor or pharmacist so they can evaluate your comprehensive health profile.
Sources & further reading
Educational information only, last refreshed 9/14/2026. Not medical advice — these associations describe population statistics, not individual predictions.
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Rare variant in the CYP2C9 gene potentially impacting enzyme activity and drug metabolism; clinical significance requires further validation.
Coding variant conferring the CYP2C9*3 allele (often mislabeled as *4 in older literature) with decreased enzymatic activity affecting clearance of drugs such as warfarin and phenytoin.
Missense mutation defining the CYP2C9*5 allele that dramatically lowers metabolic activity towards oral anticoagulants and anti-epileptic medications.
A rare variant in the CYP2C9 gene that may impact the metabolism of various clinical drugs.
The CYP2C9*2 variant (p.Arg144Cys) results in reduced enzymatic activity, significantly impacting the metabolism and required dosing of substrates such as warfarin and irbesartan.
Defining the CYP2C9*3 (p.Ile359Leu) allele, this variant markedly decreases enzymatic activity, requiring reduced dosages for warfarin and phenytoin to prevent toxicity.