What you carry at this position in your DNA and what research says about it.
Medication Metabolism: 8/10 evidence, 45 publications.
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Sample genotype (not yours): G/G
Typical Drug Metabolism Speed
You carry two copies of the common, fully functional version of the CYP2B6 gene. This means your liver processes medications like efavirenz, nevirapine, and methadone at a normal, expected rate.
What this genotype means: Standard clearance of efavirenz and nevirapine; Typical response to methadone; Standard sensitivity to propofol anesthesia.
Sample genotype (not yours): G/T
Slower Drug Metabolism Speed
You carry one copy of the decreased-function T variant, which moderately reduces your liver's ability to break down certain medications. This can lead to higher drug levels in your system and an increased risk of side effects.
What this genotype means: Moderately elevated blood levels of efavirenz; Increased risk of neuropsychiatric side effects from efavirenz; Potential sensitivity to standard doses of propofol.
Sample genotype (not yours): T/T
Slowest Drug Metabolism Speed
You carry two copies of the decreased-function T variant, which significantly slows your liver's ability to clear certain medications. This can lead to highly elevated drug levels and a much higher risk of side effects.
What this genotype means: Significantly elevated blood levels of efavirenz and nevirapine; High risk of severe neuropsychiatric side effects from efavirenz; Likely greater sensitivity to methadone and propofol.
Gene function and biological role
Gene Function: The CYP2B6 gene encodes a key member of the cytochrome P450 family of enzymes in the liver. This enzyme is responsible for breaking down and clearing several clinically important medications from the body. It plays a critical role in processing antiretroviral drugs used to treat HIV, such as efavirenz and nevirapine. Additionally, it is involved in the metabolism of methadone, certain anesthetics like propofol, and chemotherapy agents like cyclophosphamide. Variations in this gene can significantly alter how quickly these substances are cleared, leading to differences in drug efficacy and side effect risks.
Variant Impact: The rs3745274 variant, also known as c.516G>T or p.Gln172His, is a missense mutation in the CYP2B6 gene. This genetic change replaces the amino acid glutamine with histidine at position 172 of the enzyme. The resulting structural alteration significantly reduces the enzyme's ability to metabolize its target drugs. This specific variant is one of the most widely studied and clinically relevant pharmacogenomic markers. It is a defining marker for the CYP2B6*9 and *6 star alleles, which are associated with decreased drug clearance.
Key Insight: This single genetic letter change can cause up to a four-fold increase in blood levels of certain HIV medications, significantly raising the risk of neurological side effects.
Location: CYP2B6. Chromosome 19 · 19q13.2 · 41.0 Mb
Estimated prevalence by population
Global 51.7%; European 58.2%; African 39.7%; East Asian 63.2%; South Asian 38.8%; Americas 43.5%; Ashkenazi 54.3%; Finnish 66.6%; Middle Eastern 50.5%; Amish 79.7%.
Estimated GG genotype frequency.
Global 39.7%; European 36.2%; African 46.6%; East Asian 32.6%; South Asian 47.0%; Americas 44.9%; Ashkenazi 38.8%; Finnish 30.1%; Middle Eastern 41.1%; Amish 19.2%.
Estimated GT genotype frequency.
Global 8.6%; European 5.6%; African 13.7%; East Asian 4.2%; South Asian 14.2%; Americas 11.6%; Ashkenazi 6.9%; Finnish 3.4%; Middle Eastern 8.4%; Amish 1.2%.
Estimated TT genotype frequency.
Showing effects for G/G genotype
Pharmacogenomics
Antimicrobial & Antiviral Response
Efavirenz Response. Evidence: 8/10. Source: PMID 15622315. Typical Metabolism.
Nevirapine Response. Evidence: 8/10. Source: PMID 19228205. Typical Metabolism.
Pain Medication Response
Methadone Metabolism. Evidence: 8/10. Source: PMID 21790905. Typical Metabolism.
Anesthesia Response
Propofol Sensitivity. Evidence: 8/10. Source: PMID 26141406. Typical Sensitivity.
Chemotherapy Response
Cyclophosphamide Toxicity. Evidence: 5/10. Source: PMID 25428516. Typical Risk.
Showing effects for G/T genotype
Efavirenz Response. Evidence: 8/10. Source: PMID 15622315. Slower Metabolism.
Nevirapine Response. Evidence: 8/10. Source: PMID 19228205. Slower Metabolism.
Methadone Metabolism. Evidence: 8/10. Source: PMID 21790905. Slower Metabolism.
Propofol Sensitivity. Evidence: 8/10. Source: PMID 26141406. Higher Sensitivity.
Cyclophosphamide Toxicity. Evidence: 5/10. Source: PMID 25428516. Altered Risk.
Showing effects for T/T genotype
Efavirenz Response. Evidence: 8/10. Source: PMID 15622315. Slowest Metabolism.
Nevirapine Response. Evidence: 8/10. Source: PMID 19228205. Slowest Metabolism.
Methadone Metabolism. Evidence: 8/10. Source: PMID 21790905. Slowest Metabolism.
Propofol Sensitivity. Evidence: 8/10. Source: PMID 26141406. Highest Sensitivity.
What this genotype means for the people who carry it
Sample genotype: GG (Homozygous)
Clinical guidelines indicate that having two copies of the functional G allele results in normal CYP2B6 enzyme activity. When taking medications processed by this enzyme, such as the HIV drug efavirenz, your body is expected to clear the drug at a standard rate, keeping blood levels within the target therapeutic range. This typical clearance rate is associated with a standard risk of side effects. For individuals undergoing methadone therapy, a typical response is expected, compared to those with slower-metabolizing variants. Similarly, standard dosing guidelines apply if you ever require anesthesia with propofol or treatment with cyclophosphamide.
Standard clearance of efavirenz and nevirapine; Typical response to methadone; Standard sensitivity to propofol anesthesia.
Sample genotype: GT (Heterozygous)
Having one copy of the T allele means you are classified as having decreased CYP2B6 enzyme function. If you are prescribed the HIV medication efavirenz, this slower metabolism can lead to moderately elevated drug concentrations in your blood. Higher drug levels are linked to an increased likelihood of experiencing early central nervous system side effects, such as sleep disturbances or vivid dreams. Slower clearance is also observed with nevirapine, which may moderately increase the risk of drug-related toxicities. For those on methadone therapy, this slower breakdown can sometimes mean a lower dose is sufficient to achieve the desired effect. Additionally, you may exhibit increased sensitivity to the anesthetic propofol during medical procedures.
Moderately elevated blood levels of efavirenz; Increased risk of neuropsychiatric side effects from efavirenz; Potential sensitivity to standard doses of propofol.
Sample genotype: TT (Homozygous)
Carrying two copies of the T allele results in a profound reduction in CYP2B6 enzyme activity. Clinical studies show that individuals with this genotype experience significantly elevated blood concentrations of the HIV drug efavirenz. These high levels are strongly associated with severe, early neuropsychiatric side effects, often leading to early discontinuation of the medication. Slower metabolism of nevirapine also increases the risk of severe skin reactions and liver toxicity. For methadone therapy, individuals with this genotype are typically much more sensitive to it, with a greater risk of toxicity and side effects. Furthermore, standard doses of the anesthetic propofol may have a more pronounced or prolonged effect, meaning lower doses are often advisable.
Significantly elevated blood levels of efavirenz and nevirapine; High risk of severe neuropsychiatric side effects from efavirenz; Likely greater sensitivity to methadone and propofol.
Variants with shared genetic architecture
rs28399499 · CYP2B6 Gene · Chr 19 · HIV Medication Sensitivity
rs2279343 · CYP2B6 Gene · Chr 19 · Drug Metabolism and Mastocytosis
rs3211371 · CYP2B6 Gene · Chr 19 · Medication Response
Content updated August 2026.
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