CYP2D6 rs1135840: What Your Genotype Means
The rs1135840 variant is a well-studied genetic marker located within the CYP2D6 gene, which plays a critical role in how the body processes many medications. It is frequently used by researchers to help identify specific genetic patterns that influence the rate at which an individual metabolizes drugs.
What each genotype means
Typical enzyme activity
This genotype is often associated with normal CYP2D6 enzyme function, which is responsible for metabolizing many common medications. Because this variant is just one of many factors that influence how your body processes drugs, it does not provide a complete picture of your metabolic profile. Please discuss your medication history and any concerns about drug response with your clinician or pharmacist.
This is a common genotype found in many populations, though exact frequencies vary significantly by ancestry.
Potential altered enzyme activity
Carrying one copy of the G allele (often referred to as part of specific star alleles like *2, *10, *17, or *41) may be associated with reduced or altered CYP2D6 enzyme activity. The clinical significance of this genotype depends on your other CYP2D6 genetic markers, as this variant is typically analyzed in combination with others to determine your overall metabolizer status. Please consult with your healthcare provider or a pharmacist to understand how this might influence your response to specific medications.
This genotype is frequently observed in European and Asian populations, where the G allele is a common marker for various CYP2D6 star alleles.
Likely reduced enzyme activity
Having two copies of the G allele is often associated with decreased CYP2D6 enzyme activity compared to the reference sequence. This may affect how your body clears certain drugs, potentially requiring dosage adjustments for medications metabolized by this enzyme. Because this variant is part of a complex gene, please discuss these results with your doctor or pharmacist to determine if this information is relevant to your current or future treatments.
This genotype is found in various populations, with higher prevalence in specific groups where the associated star alleles are more common.
Understanding the rs1135840 Variant
The rs1135840 variant, also known as 4180G>C or S486T, is a single nucleotide polymorphism (SNP) situated within the CYP2D6 gene on chromosome 22. In genetics, a SNP represents a variation at a single position in the DNA sequence. Because the CYP2D6 gene is highly polymorphic—meaning it contains many different variations—rs1135840 serves as a useful marker for scientists. It is often analyzed in conjunction with other SNPs to help categorize the gene into specific 'star alleles,' which are standardized labels used to describe the functional status of the enzyme. While this SNP is a key piece of the puzzle, it does not act alone; its clinical significance is usually interpreted based on the broader genetic context of the entire CYP2D6 gene.
The Role of the CYP2D6 Gene
The CYP2D6 gene provides instructions for making an enzyme called cytochrome P450 2D6. This enzyme is primarily found in the liver and is responsible for breaking down, or metabolizing, approximately 20% of commonly prescribed medications. These include a wide range of drugs, such as antidepressants, antipsychotics, and certain pain relievers. When the CYP2D6 enzyme functions normally, it helps clear these substances from the body at a predictable rate. However, genetic variations can lead to enzymes that are either more active, less active, or completely non-functional. Because rs1135840 is associated with these variations, it is a subject of significant interest in the field of pharmacogenomics, which studies how an individual's genetic makeup influences their response to drug therapy.
Research and Clinical Evidence
Research has established that rs1135840 is a marker for reduced enzyme activity, which can impact how quickly a person clears specific drugs from their system. The evidence for its role in pharmacogenomics is considered moderate, as it is often found in combination with other variants that define an individual's metabolic phenotype. Studies have shown that individuals with certain genotypes at this position may experience different drug-response profiles compared to those with the wild-type (normal) sequence. It is important to note that while this SNP is a valuable research tool, it is not a diagnostic test on its own. Clinical decisions regarding medication dosing should always be made by a qualified healthcare provider or pharmacist who can interpret genetic information in the context of a patient's full medical history and current treatment plan.
Population Frequency and Diversity
The frequency of the rs1135840 variant varies significantly across different global populations. It is considered a common variant, particularly in European populations, where it has been extensively studied. Because the CYP2D6 gene is highly diverse, the prevalence of specific alleles—and the SNPs that define them—can differ between ethnic groups. For example, research comparing Han and Tibetan populations has noted significant differences in the distribution of this and other CYP2D6-related polymorphisms. These variations highlight the importance of considering ancestry when conducting pharmacogenetic research and designing clinical trials. Understanding these frequency differences helps scientists better predict how drug metabolism might vary across diverse groups of people, ultimately contributing to more personalized approaches in medicine.
What You Can Do With This Information
If you have received information about your rs1135840 genotype, it is best viewed as a piece of educational data rather than a medical instruction. You cannot use this information to diagnose a condition or determine if a medication is 'right' or 'wrong' for you. Pharmacogenomics is a complex field where many factors—including age, liver function, other medications, and overall health—interact with your genetics to determine how you respond to a drug. If you are concerned about how a specific medication affects you, or if you are interested in how your genetics might influence your treatment, the most effective step is to have a conversation with your doctor or pharmacist. They can provide professional guidance, review your current prescriptions, and determine if further testing or dosage adjustments are appropriate for your specific clinical situation.
How common is this variant?
The rs1135840 variant is common in European populations, though its frequency varies significantly across different global ancestries.
Frequently asked questions
Is rs1135840 a diagnostic test for drug side effects?
No, rs1135840 is not a diagnostic test. It is a genetic marker that researchers use to study how the CYP2D6 enzyme functions, but it cannot predict with certainty whether you will experience side effects from a specific medication.
Should I change my medication based on my rs1135840 result?
You should never change your medication or dosage based on genetic results without consulting your doctor. Only a healthcare professional can evaluate your genetic data alongside your medical history to determine if a change in your treatment plan is necessary.
What does it mean to be a 'poor metabolizer' of a drug?
A poor metabolizer has a reduced ability to break down certain drugs, which can cause the medication to build up in the body. This may increase the risk of side effects, which is why doctors may adjust dosages for these individuals.
Does everyone have the same CYP2D6 gene?
No, the CYP2D6 gene is highly polymorphic, meaning there is a great deal of variation between people. These variations are what lead to the different ways individuals process medications.
Sources & further reading
Educational information only, last refreshed 10/7/2026. Not medical advice — these associations describe population statistics, not individual predictions.
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This variant is a key marker for the CYP2D6*10 allele, which results in reduced enzymatic activity and altered metabolism of various psychiatric and analgesic medications.
This variant defines the CYP2D6*4 non-functional allele, which causes impaired metabolism of numerous clinically prescribed medications including antidepressants, tamoxifen, and codeine.
Defines the CYP2D6*17 allele (p.Thr107Ile), which exhibits reduced enzyme activity towards substrates like codeine and certain antidepressants.
Defines the CYP2D6*29 variant (p.Val338Met) conferring decreased metabolic capacity, primarily observed in populations of African descent.
Promoter variant defining the CYP2D6*41 allele, associated with reduced enzyme expression and decreased metabolic capacity for various drugs.
Defines the CYP2D6*6 deletion variant (1707delT) leading to a premature stop codon and loss of enzyme function.