CYP2C9 rs1799853: What Your Genotype Means
The rs1799853 variant is a well-studied genetic change in the CYP2C9 gene that results in reduced enzymatic activity. It is primarily recognized for its role in pharmacogenomics, specifically influencing how the body processes certain medications, including the anticoagulant warfarin.
What each genotype means
Normal enzyme activity
This genotype is considered the reference or normal activity profile for the CYP2C9 enzyme. Individuals with this profile typically metabolize medications like warfarin and certain NSAIDs at standard rates. Please discuss any medication dosing or concerns with your clinician or pharmacist.
This is the most common genotype in most global populations, though frequencies vary by ancestry.
Reduced enzyme activity
This genotype, often referred to as carrying one CYP2C9*2 allele, is associated with a reduction in the activity of the CYP2C9 enzyme. This may lead to slower metabolism of certain drugs, such as warfarin, which can increase sensitivity to the medication. Please consult your healthcare provider or pharmacist to determine if this information is relevant to your current or future medication dosing.
Carried by a significant portion of individuals of European and Near Eastern ancestry, often estimated at 10-20% in these groups, but is rare in East Asian populations.
Significantly reduced enzyme activity
This genotype, often referred to as being homozygous for the CYP2C9*2 allele, is associated with a more pronounced reduction in CYP2C9 enzyme activity compared to those with one copy. This may result in a further decreased ability to metabolize substrates like warfarin, potentially requiring lower doses to achieve therapeutic effects. Always discuss your genetic profile and medication management with your clinician or pharmacist.
This genotype is relatively uncommon, occurring in approximately 1-4% of individuals of European ancestry and is very rare in other global populations.
Understanding the rs1799853 Variant
The rs1799853 variant, commonly referred to as the CYP2C9*2 allele, is a single nucleotide polymorphism (SNP) located within the CYP2C9 gene. In this specific genetic change, a cytosine (C) is replaced by a thymine (T) at a specific position in the DNA sequence. This alteration leads to an amino acid change in the resulting protein, specifically substituting arginine with cysteine at position 144 (p.Arg144Cys). This structural change is significant because it impairs the protein's ability to function efficiently. Because this gene provides instructions for making an enzyme that breaks down various substances, including many prescription drugs, this variant is a focal point in the field of pharmacogenomics. By understanding this variant, researchers and clinicians can better predict how an individual's genetic makeup might influence their response to specific therapeutic treatments.
The Role of the CYP2C9 Gene
The CYP2C9 gene belongs to the cytochrome P450 family, a group of enzymes primarily responsible for metabolizing a wide array of compounds, including toxins, endogenous substances, and approximately 15% of all clinically used drugs. These enzymes are predominantly expressed in the liver, where they perform chemical reactions that make drugs more water-soluble, facilitating their elimination from the body. When the CYP2C9 enzyme is functioning normally, it efficiently clears substrates like warfarin, phenytoin, and certain non-steroidal anti-inflammatory drugs (NSAIDs) from the bloodstream. However, when genetic variants like rs1799853 are present, the enzyme's catalytic activity is reduced. This reduction means that drugs metabolized by this enzyme may remain in the body for longer periods or at higher concentrations than intended. Consequently, individuals carrying this variant may require different dosing strategies to achieve the desired therapeutic effect while minimizing the risk of adverse drug reactions.
Clinical Associations and Evidence
The evidence linking rs1799853 to altered drug metabolism is considered strong, particularly regarding warfarin therapy. Numerous studies have demonstrated that individuals carrying the *2 allele (rs1799853) exhibit decreased metabolism of the more potent S-enantiomer of warfarin. Because this leads to higher drug levels in the blood, carriers often require lower maintenance doses of the medication to achieve the same therapeutic effect as those without the variant. Research has consistently shown that failing to account for this genetic difference can increase the risk of bleeding complications. While the association with warfarin is the most well-documented, the variant's impact on other substrates is also a subject of ongoing clinical research. It is important to note that while genetics is a major factor, other variables such as age, body weight, diet, and concurrent medications also play critical roles in determining the appropriate dose for any patient.
What This Information Means for You
Genetic information regarding rs1799853 is a tool for personalized medicine, but it should never be used to make independent medical decisions. If you have received results indicating you carry this variant, it does not mean you will necessarily experience a negative reaction to a medication; rather, it provides your healthcare provider with additional data to optimize your treatment plan. You should always discuss any pharmacogenomic test results with your physician or a clinical pharmacist. They are trained to interpret these findings in the context of your overall health, current medications, and clinical history. Never stop or change the dosage of a prescribed medication based on genetic test results without consulting your medical team. Your clinician can use this information to adjust your treatment, ensuring that your therapy is both safe and effective based on your unique genetic profile.
How common is this variant?
The rs1799853 variant is common in European populations, with an allele frequency of approximately 8-12%, but it is very rare in East Asian populations.
Frequently asked questions
What is the CYP2C9*2 allele?
The CYP2C9*2 allele is a specific genetic variant (rs1799853) that results in an enzyme with reduced ability to break down certain medications. It is one of the most studied variants in pharmacogenomics.
Does having the rs1799853 variant mean I cannot take warfarin?
No, it does not mean you cannot take warfarin. It simply means your body may process the drug more slowly, so your doctor may need to adjust your dose to ensure it is safe and effective.
Is rs1799853 the only factor in warfarin dosing?
No, warfarin dosing is complex and depends on many factors, including your age, weight, diet, other medications you take, and other genetic variants like those in the VKORC1 gene.
Should I get tested for CYP2C9 variants?
Pharmacogenomic testing is typically ordered by a healthcare provider when it is clinically relevant to your treatment. You should discuss with your doctor whether such testing is appropriate for your specific medical situation.
Sources & further reading
Educational information only, last refreshed 9/15/2026. 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.
Defining the CYP2C9*3 (p.Ile359Leu) allele, this variant markedly decreases enzymatic activity, requiring reduced dosages for warfarin and phenytoin to prevent toxicity.
The CYP2C9*3 allele significantly reduces enzyme activity, necessitating dose adjustments for warfarin and phenytoin.