CYP2C9 rs1057913: What Your Genotype Means
The rs1057913 variant is a specific genetic change located within the CYP2C9 gene. It is associated with the CYP2C9*4 allele, which may reduce the body's ability to metabolize certain medications, potentially affecting how they are cleared from the system.
What each genotype means
Normal enzyme activity
This genotype represents the most common form of the CYP2C9 enzyme, often referred to as the wild-type or *1/*1 genotype. Individuals with this profile typically metabolize medications like phenytoin at a standard rate. Always discuss your medication dosing and any potential side effects with your clinician or pharmacist.
This is the most common genotype observed globally, though frequencies vary significantly by ancestry.
Reduced enzyme activity
This genotype includes one copy of the variant allele, which may be associated with reduced metabolic clearance of certain drugs, including phenytoin. Research indicates that individuals with this profile may experience higher serum drug levels compared to those with the common genotype. Please consult your healthcare provider or pharmacist to discuss how this may influence your specific medication management.
This genotype is rare and observed sporadically in European populations.
Significantly reduced enzyme activity
This genotype indicates the presence of two copies of the variant allele, which is associated with a substantial reduction in the metabolic clearance of substrate medications such as phenytoin. Because this can lead to higher drug concentrations in the blood, it is important to discuss your treatment plan and potential dosing adjustments with your clinician or pharmacist. This variant is rare, and clinical evidence regarding its specific impact remains limited.
This genotype is extremely rare and is observed only sporadically in specific populations.
Understanding the rs1057913 Variant
The rs1057913 variant is a single-nucleotide polymorphism (SNP) found in the CYP2C9 gene. In the context of pharmacogenomics, this variant is categorized as a coding change that defines the CYP2C9*4 allele. Genetic variants like this are studied because they can alter the structure or function of the proteins they encode. When a variant occurs in a gene responsible for drug metabolism, it can lead to differences in how quickly or slowly an individual processes specific therapeutic substances. Because rs1057913 is linked to reduced metabolic activity, researchers monitor it to understand its potential impact on drug safety and efficacy. It is important to note that this variant is considered rare, and its presence is typically identified through specialized genetic testing rather than routine clinical screening.
The Role of the CYP2C9 Gene
The CYP2C9 gene provides instructions for making an enzyme that belongs to the cytochrome P450 family. These enzymes are primarily located in the liver and are essential for the metabolism of a wide variety of substances, including approximately 15% of all clinically used drugs. By breaking down these compounds, the CYP2C9 enzyme helps the body eliminate them. Medications that rely on this enzyme for clearance include phenytoin, an anti-epileptic drug, and warfarin, an anticoagulant. When the CYP2C9 gene carries variants that decrease enzyme function, the body may struggle to clear these medications at a normal rate. This can lead to higher concentrations of the drug in the bloodstream, which may increase the risk of side effects or toxicity if the dosage is not appropriately adjusted by a healthcare professional.
Research and Clinical Evidence
Scientific research into CYP2C9 variants has established that genetic differences are a major factor in how patients respond to drugs with a narrow therapeutic index. While variants like *2 and *3 are the most widely studied in European populations, the *4 allele associated with rs1057913 is recognized for its potential to impair metabolic clearance. Studies have shown that individuals with reduced-function alleles often require lower maintenance doses of medications like phenytoin to maintain safe serum levels. However, the evidence strength for rs1057913 specifically is considered limited compared to more common variants. Because drug response is complex and influenced by other factors like diet, age, and concurrent medications, genetic testing results should always be interpreted by a clinician. There is no evidence linking this variant to disease risk outside of its impact on drug metabolism.
What You Can Do With This Information
If you have received genetic testing results indicating you carry the rs1057913 variant, it is important to discuss these findings with your doctor or pharmacist. This information is a tool for personalized medicine, helping clinicians make more informed decisions about medication selection and dosing. You should never change your medication dosage or stop taking a prescribed drug based on genetic test results without professional medical guidance. Your healthcare provider can evaluate your entire clinical picture, including other medications you may be taking, to determine if a dose adjustment is necessary. Genetic information is just one piece of the puzzle; your doctor will combine it with clinical monitoring to ensure your treatment remains safe and effective. Always keep your medical team updated on any genetic testing you have completed.
How common is this variant?
The rs1057913 variant is very rare and has been observed sporadically, primarily in specific populations such as those of Japanese ancestry.
Frequently asked questions
What does it mean to be a 'poor metabolizer'?
A poor metabolizer is an individual whose genetic makeup results in significantly reduced or absent activity of a specific enzyme. This can cause medications to stay in the body longer than intended, potentially increasing the risk of side effects.
Does this variant cause epilepsy?
No, the rs1057913 variant is not known to cause epilepsy or any other disease. It is only relevant in the context of how your body processes medications used to treat conditions like epilepsy.
Should I get tested for this variant?
Genetic testing for pharmacogenomics is usually recommended by a doctor if you are starting a medication that is known to be affected by CYP2C9, such as phenytoin or warfarin. You should discuss the necessity of such testing with your healthcare provider.
Can I change my dose if I have this variant?
You must never change your medication dosage on your own. Any adjustments to your treatment plan must be made by your doctor, who will consider your genetic results alongside your clinical symptoms and blood test results.
Sources & further reading
Educational information only, last refreshed 10/17/2026. 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.