What you carry at this position in your DNA and what research says about it.
HIV Medication Sensitivity: 8/10 evidence, 17 publications.
You're viewing every possible genotype for this variant.
None of these is your result. Use the genotype toggle to compare them, then sign up and upload your DNA to see which one is yours.
Sample genotype (not yours): T/T
Typical HIV Medication Metabolism
Normal CYP2B6 enzyme function allows for typical clearance rates of medications like efavirenz and nevirapine.
What this genotype means: Normal drug metabolism; Typical risk of side effects.
Sample genotype (not yours): C/C
Severely Slower HIV Medication Metabolism
A complete loss of CYP2B6 enzyme activity causes extremely slow clearance of certain medications, significantly raising toxicity risks.
What this genotype means: Severely reduced drug clearance; High risk of severe skin reactions; Increased risk of drug toxicity.
Sample genotype (not yours): T/C
Slower HIV Medication Metabolism
Reduced CYP2B6 enzyme activity leads to slower clearance of certain medications, increasing toxicity risks.
What this genotype means: Slower drug clearance; Increased risk of severe skin reactions; Higher risk of liver enzyme elevation.
Gene function and biological role
Gene Function: CYP2B6 encodes a member of the cytochrome P450 superfamily of enzymes, which are primarily responsible for metabolizing clinical drugs in the liver [1]. This enzyme plays a critical role in the clearance of several widely used medications, including the antiretroviral drugs efavirenz and nevirapine [2]. It is also expressed in the brain, where it may influence the central nervous system side effects of these medications [1]. Variations in this gene can lead to dramatic differences in how quickly these drugs are cleared from the body [3].
Variant Impact: A single-nucleotide change at position 328 of the CYP2B6 protein replaces the amino acid isoleucine with threonine. This specific nucleotide change, also known as c.983T to C, defines the CYP2B6*18 allele, which is classified as a no function allele by pharmacogenomic databases [2]. The alteration disrupts the normal structure of the enzyme, leading to a complete loss of its metabolic activity [4]. Consequently, individuals carrying this variant experience significantly reduced clearance of drugs metabolized by CYP2B6 [5]. This variant is most commonly found in populations of African descent, while being extremely rare in other ancestries [3].
Key Insight: The CYP2B6*18 variant is virtually absent in European and East Asian populations but is carried by up to seven percent of people of African descent.
Location: CYP2B6. Chromosome 19 · 19q13.2 · 41.0 Mb
Estimated prevalence by population
Global 96.1%; European > 99.9%; African 86.4%; East Asian 100.0%; South Asian 99.9%; Americas 98.3%; Ashkenazi 100.0%; Finnish 100.0%; Middle Eastern 98.6%; Amish 100.0%.
Estimated TT genotype frequency.
Global 3.8%; European < 0.1%; African 13.1%; East Asian 0.0%; South Asian 0.1%; Americas 1.7%; Ashkenazi 0.0%; Finnish 0.0%; Middle Eastern 1.4%; Amish 0.0%.
Estimated TC genotype frequency.
Global 0.1%; European < 0.1%; African 0.5%; East Asian 0.0%; South Asian < 0.1%; Americas < 0.1%; Ashkenazi 0.0%; Finnish 0.0%; Middle Eastern < 0.1%; Amish 0.0%.
Estimated CC genotype frequency.
Showing effects for T/T genotype
Pharmacogenomics
Antimicrobial & Antiviral Response
Efavirenz Metabolism. Evidence: 8/10. Source: PMID 22354160. Normal Metabolism.
Nevirapine Toxicity. Evidence: 8/10. Source: PMID 23774940. Typical Risk.
Adverse Drug Reactions
Severe Skin Drug Reaction. Evidence: 8/10. Source: PMID 23774940. Typical Risk.
Drug-Induced Liver Injury. Evidence: 6/10. Source: PMID 20017669. Typical Risk.
Showing effects for C/C genotype
Efavirenz Metabolism. Evidence: 8/10. Source: PMID 22354160. Poor Metabolism.
Nevirapine Toxicity. Evidence: 8/10. Source: PMID 23774940. Increased Risk.
Severe Skin Drug Reaction. Evidence: 8/10. Source: PMID 23774940. Increased Risk.
Showing effects for T/C genotype
Efavirenz Metabolism. Evidence: 8/10. Source: PMID 22354160. Slower Metabolism.
Drug-Induced Liver Injury. Evidence: 6/10. Source: PMID 20017669. Increased Risk.
What this genotype means for the people who carry it
Sample genotype: TT (Homozygous)
Two copies of the T allele ensure that the CYP2B6 enzyme functions normally, allowing the body to break down and clear medications at a typical rate [1]. When taking antiretroviral therapies such as efavirenz, individuals with this genotype maintain standard drug concentrations in their blood [2]. This helps avoid the excessively high drug levels that can trigger central nervous system side effects or liver toxicity [3]. It also means the risk of developing rare but severe hypersensitivity reactions, such as Stevens-Johnson Syndrome, remains at the typical baseline [4]. No special dosing adjustments are generally recommended based on this genotype alone.
Normal drug metabolism; Typical risk of side effects.
Sample genotype: CC (Homozygous)
Two copies of the C allele result in a complete loss of CYP2B6 enzyme activity, leading to a poor metabolizer phenotype [1]. Without active CYP2B6, the body struggles to clear medications like efavirenz and nevirapine, leading to exceptionally high drug concentrations in the blood [2]. These elevated levels significantly increase the risk of severe adverse drug reactions [3]. Specifically, individuals with this genotype have a heightened risk of developing life-threatening skin conditions, such as Stevens-Johnson Syndrome and Toxic Epidermal Necrolysis, when exposed to nevirapine [4]. Clinical guidelines often suggest alternative medications or careful monitoring for individuals with this metabolic profile [5].
Severely reduced drug clearance; High risk of severe skin reactions; Increased risk of drug toxicity.
Sample genotype: TC (Heterozygous)
One copy of the C allele reduces overall CYP2B6 enzyme activity because this variant produces a non-functional enzyme [1]. This slower clearance rate can cause medications like efavirenz to build up to higher-than-normal levels in the bloodstream [2]. Elevated drug levels are associated with an increased risk of side effects, including central nervous system issues and depression [3]. Furthermore, when taking nevirapine, this genotype is linked to a higher likelihood of severe hypersensitivity reactions like Stevens-Johnson Syndrome and Toxic Epidermal Necrolysis [4]. Slower metabolism can also elevate the risk of drug-induced liver injury [5].
Slower drug clearance; Increased risk of severe skin reactions; Higher risk of liver enzyme elevation.
Variants with shared genetic architecture
rs3745274 · CYP2B6 Gene · Chr 19 · Medication Metabolism
rs2279343 · CYP2B6 Gene · Chr 19 · Drug Metabolism and Mastocytosis
rs3211371 · CYP2B6 Gene · Chr 19 · Medication Response
Pharmacogenomics knowledge for personalized medicine.
Association between CYP2B6 polymorphisms and Nevirapine-induced SJS/TEN: a pharmacogenetics study.
Content updated August 2026.
ReadYourDNA provides educational interpretation of DNA data you already have. It is not diagnostic, not a substitute for professional medical advice, diagnosis, or treatment, and is not intended to diagnose, treat, cure, or prevent any disease. Talk to a clinician before acting on any result.
Raw DNA Data Reports for 23andMe & AncestryDNA · About ReadYourDNA · FAQ · Methodology & Science · Pricing · Support and complaints · Legal & Contact · Health Traits · Pharmacogenomics · Carrier Status · Variant Explorer · The Human Karyotype · Privacy Policy · Terms of Service · Accessibility · Consumer Health Data Privacy · Your Privacy Choices · Editorial Standards · Your Data, Checkable · Sample Report · ReadYourDNA and Promethease compared · What is inside a 23andMe raw data file · What is inside an AncestryDNA raw data file · What a chatbot can and cannot do with your raw DNA file · Where a raw DNA file can still be uploaded in 2026
Written, maintained and operated by Martin Pechhacker, sole proprietor, Vienna, Austria. Trade register GISA 40064420. support@readyourdna.com