CYP2C9: A Key Pharmacogene for Warfarin and NSAIDs

What Is CYP2C9?

CYP2C9 is a gene that encodes one of the most abundant cytochrome P450 enzymes in the human liver. This enzyme is responsible for metabolizing approximately 15% of all clinically used drugs, including the anticoagulant warfarin, several nonsteroidal anti-inflammatory drugs (NSAIDs), and the antiepileptic drug phenytoin.

Genetic variants in CYP2C9 can reduce enzyme activity, meaning your body may clear certain drugs more slowly than expected. This has direct clinical consequences: slower clearance leads to higher drug levels in the blood, which can increase the risk of adverse effects. CYP2C9 is one of the core pharmacogenes analyzed in pharmacogenomics, and its relationship to warfarin dosing is among the best-studied drug-gene interactions.

CYP2C9 and Warfarin

Warfarin exists as two mirror-image forms (enantiomers): S-warfarin and R-warfarin. S-warfarin is 3 to 5 times more potent as an anticoagulant than R-warfarin, and CYP2C9 is the primary enzyme responsible for metabolizing S-warfarin. This means CYP2C9 activity directly controls how quickly the more active form of warfarin is cleared from your body.

Patients with reduced CYP2C9 function metabolize S-warfarin more slowly. The result is higher S-warfarin plasma concentrations at any given dose, leading to a stronger anticoagulant effect and increased bleeding risk. This is why the CPIC warfarin guideline recommends lower starting doses for CYP2C9 intermediate and poor metabolizers.

Warfarin dosing is also influenced by VKORC1, the gene encoding warfarin's drug target. The CPIC guideline for warfarin considers both CYP2C9 and VKORC1 genotypes together when recommending dose adjustments. See the warfarin dosing page for the full clinical picture.

Other Medications Affected by CYP2C9

Beyond warfarin, CYP2C9 metabolizes medications across several drug classes:

• NSAIDs: Celecoxib, flurbiprofen, ibuprofen, and meloxicam. CYP2C9 poor metabolizers may experience elevated NSAID levels, potentially increasing the risk of gastrointestinal and cardiovascular side effects.
• Phenytoin: An antiepileptic drug with a narrow therapeutic index. CYP2C9 poor metabolizers may require lower doses to avoid toxicity. CPIC has published specific dosing guidelines.
• Siponimod: A multiple sclerosis treatment. CYP2C9 poor metabolizers should not use siponimod per FDA labeling, as they cannot adequately metabolize the drug.

Metabolizer Phenotypes

CYP2C9 phenotype assignment follows the CPIC activity score system. Each allele is assigned a functional value, and the combination determines your metabolizer category. Learn more about how pharmacogenomic testing determines your phenotype or read our plain-language guide to metabolizer status.

• Normal Metabolizer (NM): Two normal-function alleles (*1/*1). Standard enzyme activity. Standard drug dosing guidelines apply.
• Intermediate Metabolizer (IM): One normal-function allele plus one reduced-function allele (e.g., *1/*2 or *1/*3). Moderately reduced enzyme activity. Lower warfarin starting doses may be recommended.
• Poor Metabolizer (PM): Two reduced-function alleles (e.g., *2/*3 or *3/*3). Significantly reduced or absent enzyme activity. Substantially lower warfarin doses are recommended, and certain medications may require alternatives.

CYP2C9 poor metabolizer frequency is approximately 1-3% in European populations. Intermediate metabolizers are more common, found in roughly 20-35% of Europeans. Frequencies differ across populations — for example, CYP2C9*3 is less common in African populations, while CYP2C9*8 is more clinically relevant in individuals of African descent.

Common CYP2C9 Variants

• CYP2C9*1: The reference (wild-type) allele with normal enzyme function.
• CYP2C9*2 (rs1799853): A reduced-function allele. The enzyme is produced but with approximately 50% reduced catalytic activity compared to *1. Found in approximately 10-15% of European populations, less common in African and East Asian populations.
• CYP2C9*3 (rs1057910): A reduced-function allele with more significant impact than *2, retaining only about 10% of normal catalytic activity. Found in approximately 5-10% of Europeans and 2-5% of East Asians, rare in African populations.
• CYP2C9*8: A reduced-function allele particularly important in African populations, where it can reach frequencies of 5-10%. Often underrepresented in older studies that focused primarily on European populations.
• CYP2C9*11: Another reduced-function allele found in African populations. Clinical significance is established for warfarin dosing.

Consumer genotyping arrays from 23andMe and AncestryDNA typically include the variants defining *2 and *3. Coverage of *8 and *11 varies by platform and chip version. See our methodology for details on variant coverage.

CPIC Clinical Guidelines

CPIC has published Level A guidelines (strongest evidence) for CYP2C9 interactions with warfarin (in combination with VKORC1), phenytoin, and several NSAIDs. These guidelines provide specific dosing recommendations based on metabolizer phenotype.

For warfarin, the CPIC guideline uses a dosing algorithm that incorporates both CYP2C9 and VKORC1 genotypes alongside clinical factors (age, height, weight, interacting medications). The algorithm produces a genotype-guided dose estimate that can help clinicians reach the target therapeutic range more quickly and with fewer dose adjustments. DecodeMyBio reports include only CPIC Level A and Level B interactions, and gene pages like this one and the CYP2C19 gene provide background context for understanding your results. The current CPIC warfarin guideline update (Johnson et al., 2017; PMID: 28198005) incorporates CYP2C9 alleles *2, *3, *5, *6, *8, and *11 in its dosing algorithm.

Get Your CYP2C9 Results

If you have raw DNA data from 23andMe, AncestryDNA, MyHeritage, or FamilyTreeDNA, you can upload it to DecodeMyBio to learn your CYP2C9 metabolizer status. Your Medication Safety Report will include your CYP2C9 diplotype, metabolizer phenotype, and any CPIC-guideline drug-gene interactions relevant to your genotype. You can also view a sample report to see what your results will look like.

For information on the inherent limitations of consumer-grade pharmacogenomic analysis, review our limitations page before sharing results with your provider.

For a guide to extracting pharmacogenomic insights from consumer DNA services, see pharmacogenomics from raw DNA data, or browse our learning center for all educational guides.

Frequently Asked Questions

Beyond Medications: Nutrition and Methylation

If you're exploring how your DNA affects nutrition or methylation, your raw data also contains variants in genes like MTHFR, FUT2, VDR, and others that influence nutrient metabolism. See our Nutrition & Methylation Report for a nutrigenomic analysis from the same DNA data.