CYP2B6 rs55705857: Understanding Your Genetic Variant
The rs55705857 variant is a genetic marker located within the CYP2B6 gene, which provides instructions for making an enzyme responsible for breaking down various medications. This variant is a key component of the CYP2B6*6 allele, which is associated with reduced enzyme function and altered clearance of drugs such as efavirenz, methadone, and bupropion.
What each genotype means
Normal enzyme activity
This genotype is typically associated with normal CYP2B6 enzyme function, often referred to as the *1/*1 diplotype in clinical literature. Individuals with this profile generally metabolize medications like efavirenz, methadone, and bupropion at standard rates. Please discuss your medication dosing and any concerns regarding drug response with your clinician or pharmacist.
This is the most common genotype globally, though frequencies vary significantly by ancestral background.
Intermediate enzyme activity
This genotype indicates the presence of one reduced-function allele, often corresponding to the *1/*6 diplotype. Research suggests this may lead to intermediate metabolism of certain drugs, potentially requiring dose adjustments for medications like efavirenz or methadone. Please consult your healthcare provider or pharmacist to interpret how this may influence your specific treatment plan.
Carried by a significant portion of the population, with frequencies ranging from approximately 15% to 40% depending on ancestry.
Reduced enzyme activity
This genotype is associated with reduced CYP2B6 enzyme activity, frequently identified as the *6/*6 diplotype in pharmacogenetic studies. Individuals with this profile may experience slower clearance of drugs such as efavirenz, which can increase the risk of side effects or altered therapeutic outcomes. Always discuss your genotype results and medication management with your prescribing physician or pharmacist.
This genotype is less common than the heterozygous state but is observed across various global populations at varying frequencies.
What is rs55705857 and Where is it Located?
The rs55705857 variant is a specific change in the DNA sequence of the CYP2B6 gene. In the field of genomics, an rsID (reference SNP cluster ID) like rs55705857 acts as a unique identifier for a single nucleotide polymorphism, or SNP. This variant is not an isolated mutation but is instead a functional component of the well-studied CYP2B6*6 haplotype. The CYP2B6 gene is located on chromosome 19 and encodes the cytochrome P450 2B6 enzyme. This enzyme is primarily expressed in the liver, where it plays a critical role in the metabolism of a wide range of endogenous compounds and exogenous substances, including therapeutic drugs. Because rs55705857 is linked to the *6 allele, it is frequently studied in the context of pharmacogenomics—the study of how an individual's genetic makeup influences their response to medications. By identifying this variant, researchers can better understand why some individuals process certain drugs more slowly than others.
The Role of the CYP2B6 Gene
The CYP2B6 gene belongs to the cytochrome P450 superfamily, a group of enzymes that are essential for the detoxification and metabolism of many drugs. The enzyme produced by this gene is responsible for the oxidative metabolism of several clinically significant medications. When the CYP2B6 gene carries variants that reduce the enzyme's activity, the body may struggle to clear these drugs from the bloodstream at a normal rate. This can lead to higher-than-expected plasma concentrations of the medication, which may increase the risk of side effects or toxicity. Conversely, in some cases, the enzyme is required to activate a drug; if the enzyme is less active, the medication may not be as effective. Because of its central role in drug clearance, variations in CYP2B6 are a major focus for clinicians aiming to optimize drug therapy, particularly for patients taking antiretroviral treatments for HIV or maintenance therapies for opioid use disorder.
Research Associations and Evidence Strength
Research has consistently linked the CYP2B6*6 allele, which includes rs55705857, to reduced enzyme function. Studies have shown that individuals carrying this variant often exhibit slower clearance of drugs like efavirenz, a common HIV medication, and methadone, used in opioid treatment. For efavirenz, reduced clearance can lead to higher plasma levels, which has been associated with an increased risk of neuropsychological side effects. Similarly, in patients taking methadone, this variant can influence how quickly the drug is eliminated, potentially impacting the stability of the treatment. While the association between this genetic profile and altered drug metabolism is well-documented in scientific literature, the clinical evidence strength is often categorized as limited or evolving. This is because drug response is complex and influenced by many factors, including other genetic variants, drug-drug interactions, and individual health status. Consequently, while these findings are scientifically significant, they are used as one piece of a larger puzzle in clinical decision-making.
What You Can and Cannot Do With This Information
Understanding your genetic status regarding rs55705857 can provide insight into how your body might process specific medications, but it is not a diagnostic tool. You cannot use this information to predict your exact response to a drug, nor should you ever change your medication dosage or stop taking a prescribed treatment based on genetic test results. Pharmacogenomic data is intended to be used by healthcare professionals to help guide personalized treatment plans. If you have concerns about how your genetics might affect your medication, the most important step is to have a conversation with your doctor or a clinical pharmacist. They can interpret these results within the context of your overall health, current medications, and clinical history. Always prioritize professional medical advice over self-interpretation of genetic data, as clinical decisions require a comprehensive evaluation that goes beyond a single genetic variant.
How common is this variant?
The minor allele frequency for variants associated with the CYP2B6*6 haplotype typically ranges from 0.15 to 0.40 across global populations, though it varies significantly by ancestry.
Frequently asked questions
Does having the rs55705857 variant mean I will have side effects?
Not necessarily. While this variant is associated with slower drug metabolism, many factors influence whether you experience side effects, including your overall health, other medications you take, and your specific dosage. Genetic markers are only one piece of the puzzle.
Should I get tested for CYP2B6 variants?
Genetic testing for pharmacogenomics is usually recommended by a healthcare provider if you are starting a medication known to be affected by these variants. You should discuss the necessity and potential benefits of such testing with your doctor.
Can I change my medication based on my genotype?
No. You should never change, stop, or start any medication based on genetic test results without consulting your physician. Only a qualified healthcare professional can determine if a dosage adjustment or alternative medication is appropriate for you.
Is rs55705857 the only variant that affects CYP2B6?
No, the CYP2B6 gene is highly polymorphic, meaning there are many different variants that can affect its function. The rs55705857 variant is just one of several markers used to identify specific star alleles like *6.
Sources & further reading
Educational information only, last refreshed 9/27/2026. Not medical advice — these associations describe population statistics, not individual predictions.
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Get my report — $29Related variants in CYP2B6
Defines the non-functional CYP2B6*18 allele (c.983T>C; p.Ile328Thr) leading to impaired metabolism of efavirenz.
Core coding variant c.1459C>T (p.Arg487Cys) defining CYP2B6*5, associated with reduced clearance of substrates.
Missense alteration defining the CYP2B6*18 allele, causing profound enzymatic deficiency and impaired clearance of efavirenz and methadone.
A coding variant that defines the decreased-function CYP2B6*4 allele, influencing plasma concentrations of efavirenz, methadone, and bupropion.
Missense alteration defining the nonfunctional CYP2B6*18 allele common in African populations that slows clearance of efavirenz.
Encodes CYP2B6 c.1459C>T (p.Arg487Cys) defining the CYP2B6*5 allele, associated with reduced clearance of efavirenz and other substrates.