What you carry at this position in your DNA and what research says about it.
Drug Metabolism: 5/10 evidence, 22 publications.
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Sample genotype (not yours): T/T
Slower Tamoxifen Clearance and Poorer Opioid Pain Relief
Processing tamoxifen more slowly may enhance its effectiveness in breast cancer treatment by keeping active metabolites in the body longer. However, this same slow clearance pattern is linked to a weaker response to several common opioid painkillers.
What this genotype means: Slower clearance of active breast cancer treatment metabolites; Reduced pain relief from morphine, fentanyl, and oxycodone; Typical liver safety profile with tuberculosis medications.
Sample genotype (not yours): C/C
Faster Tamoxifen Clearance and Better Opioid Pain Relief
Active tamoxifen metabolites are cleared from the body more rapidly, which could potentially reduce the drug's long-term efficacy. However, there is a stronger response to morphine pain relief and a reduced risk of cardiovascular events on clopidogrel.
What this genotype means: Rapid clearance and inactivation of active tamoxifen metabolites; Enhanced pain relief and greater sensitivity to morphine; Increased risk of severe withdrawal symptoms during methadone therapy.
Sample genotype (not yours): T/C
Altered Painkiller Response and Higher Medication Toxicity Risk
A single copy of each allele results in an intermediate metabolism profile for several common medications. There is an increased risk of liver irritation from certain tuberculosis therapies and a potentially weaker response to some opioid painkillers.
What this genotype means: Increased susceptibility to liver injury from tuberculosis therapies; Potentially weaker pain relief from standard morphine doses; Intermediate clearance rates for tamoxifen metabolites.
Gene function and biological role
Gene Function: The UGT2B7 gene encodes a key phase II drug-metabolizing enzyme called UDP-glucuronosyltransferase 2B7 [1]. Active primarily in the liver, this enzyme performs glucuronidation, a process that attaches a sugar molecule to lipophilic compounds to make them water-soluble for excretion [1]. It plays a vital role in detoxifying a wide range of foreign substances, including environmental carcinogens and commonly prescribed medications [2]. Among its major substrates are breast cancer therapies, anti-epileptic drugs, and various opioid painkillers [2]. Variations in this gene can significantly alter enzyme activity, leading to substantial differences in how people process these substances [1].
Variant Impact: Known also as Y268H, this missense variant results in an amino acid substitution from tyrosine to histidine at position 268 of the UGT2B7 enzyme. The change directly alters the catalytic region of the enzyme, modifying its ability to bind and clear specific drugs [1]. Presence of the T allele is associated with significantly lower glucuronidation activity for certain compounds, such as active tamoxifen metabolites [3]. Conversely, the C allele can lead to faster clearance or altered sensitivity for other substances, including several opioid medications [4]. These functional differences explain why individuals experience varying levels of drug efficacy and side effects [3].
Key Insight: This variant alters how the liver processes both breast cancer therapies and common pain medications, meaning a genotype that is highly beneficial for one treatment can be disadvantageous for another.
Location: UGT2B7. Chromosome 4 · 4q13.2 · 69.1 Mb
Estimated prevalence by population
Global 19.2%; European 28.4%; African 8.3%; East Asian 9.2%; South Asian 15.7%; Americas 11.6%; Ashkenazi 23.8%; Finnish 18.3%; Middle Eastern 17.9%; Amish 26.6%.
Estimated TT genotype frequency.
Global 46.5%; European 49.8%; African 41.0%; East Asian 42.2%; South Asian 47.8%; Americas 44.9%; Ashkenazi 50.0%; Finnish 48.9%; Middle Eastern 48.8%; Amish 50.0%.
Estimated TC genotype frequency.
Global 34.3%; European 21.9%; African 50.6%; East Asian 48.6%; South Asian 36.5%; Americas 43.5%; Ashkenazi 26.2%; Finnish 32.8%; Middle Eastern 33.3%; Amish 23.5%.
Estimated CC genotype frequency.
Showing effects for T/T genotype
Pharmacogenomics
Chemotherapy Response
Tamoxifen Response. Evidence: 3/10. Source: PMID 17664247. Slower Clearance.
Pain Medication Response
Opioid Pain Response. Evidence: 5/10. Source: PMID 31607718. Poorer Response.
Methadone Withdrawal. Evidence: 5/10. Source: PMID 22676193. Typical Risk.
Antimicrobial & Antiviral Response
Tuberculosis Drug Liver Injury. Evidence: 5/10. Source: PMID 37868740. Typical Risk.
Anticoagulant & Blood Thinner Response
Clopidogrel Response. Evidence: 5/10. Source: PMID 41097034. Typical Benefit.
Showing effects for C/C genotype
Tamoxifen Response. Evidence: 3/10. Source: PMID 17664247. Faster Clearance.
Opioid Pain Response. Evidence: 5/10. Source: PMID 31607718. Typical Response.
Methadone Withdrawal. Evidence: 5/10. Source: PMID 22676193. Higher Severity.
Tuberculosis Drug Liver Injury. Evidence: 5/10. Source: PMID 37868740. Increased Risk.
Clopidogrel Response. Evidence: 5/10. Source: PMID 41097034. Higher Benefit.
Showing effects for T/C genotype
Tamoxifen Response. Evidence: 3/10. Source: PMID 17664247. Typical Clearance.
What this genotype means for the people who carry it
Sample genotype: TT (Homozygous)
Glucuronidation of active tamoxifen metabolites, specifically 4-OH-tamoxifen and endoxifen, is approximately 28% lower in those with this genotype compared to C/C carriers [1]. This slower inactivation keeps the highly active forms of the drug circulating longer, which may improve its ability to prevent breast cancer recurrence [1]. On the other hand, processing of several opioid medications is altered, often leading to a faster clearance or poorer overall pain relief [2]. Clinical studies have noted a weaker response to morphine, fentanyl, and oxycodone in individuals carrying these alleles [2] [3] [4]. Conversely, codeine may be processed more efficiently, meaning it produces a stronger effect for pain control [5]. There is also a typical, non-elevated risk of liver damage when taking certain tuberculosis medications compared to C allele carriers [6].
Slower clearance of active breast cancer treatment metabolites; Reduced pain relief from morphine, fentanyl, and oxycodone; Typical liver safety profile with tuberculosis medications.
Sample genotype: CC (Homozygous)
Enzymatic activity is highly efficient with this genotype, resulting in the rapid clearance of active tamoxifen metabolites [1]. This fast inactivation can lower the circulating levels of the drug's most potent forms, potentially affecting its protective benefits against breast cancer recurrence [1]. In contrast, pain management with morphine is highly effective, often meaning a lower dose is sufficient to achieve adequate relief [2]. However, if undergoing methadone maintenance therapy, there is a higher risk of experiencing severe opioid withdrawal symptoms [3]. There is also an increased susceptibility to liver damage when taking certain tuberculosis medications [4]. On a positive note, individuals taking clopidogrel show a decreased likelihood of experiencing major cardiovascular events [5].
Rapid clearance and inactivation of active tamoxifen metabolites; Enhanced pain relief and greater sensitivity to morphine; Increased risk of severe withdrawal symptoms during methadone therapy; Decreased risk of cardiovascular events when taking clopidogrel.
Sample genotype: TC (Heterozygous)
Possessing one copy of the variant allele leads to a mixed drug-processing profile that falls between the two homozygous genotypes. Pain relief from standard doses of morphine or oxcarbazepine may be less effective due to altered metabolic rates [1] [2]. When undergoing treatment for tuberculosis, there is a significantly higher likelihood of experiencing drug-induced liver injury [3]. Methadone metabolism is also slightly slower, though this does not typically translate to the severe withdrawal symptoms seen in C/C homozygotes [4]. Tamoxifen metabolism remains relatively typical, providing a standard balance of active metabolite clearance [5]. Understanding these intermediate clearance rates helps explain why standard drug dosages sometimes yield unexpected side effects or reduced efficacy [1].
Increased susceptibility to liver injury from tuberculosis therapies; Potentially weaker pain relief from standard morphine doses; Intermediate clearance rates for tamoxifen metabolites.
Variants with shared genetic architecture
rs12233719 · UGT2B7 Gene · Chr 4 · Lung Cancer Risk
rs28365062 · UGT2B7 Gene · Chr 4 · Zidovudine Metabolism
rs7438135 · UGT2B7 Gene · Chr 4 · Medication Response
Genotyping panel for assessing response to cancer chemotherapy.
Glucuronidation of active tamoxifen metabolites by the human UDP glucuronosyltransferases.
The pharmacogenetics of codeine pain relief in the postpartum period.
Content updated August 2026.
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