Why CYP2D6 Matters More Than Almost Any Other Gene
CYP2D6 is a single gene, but it has an outsized impact on medicine. It encodes a liver enzyme responsible for metabolizing roughly one in four prescribed drugs, including opioids, antidepressants, antipsychotics, beta-blockers, and anti-cancer agents. Variants in this gene are among the most actionable findings in pharmacogenomics.
What makes CYP2D6 uniquely complex is its extreme polymorphism. Over 100 star alleles have been cataloged, including gene deletions, duplications, and hybrid arrangements with the neighboring pseudogene CYP2D7. This complexity means that CYP2D6 phenotype prediction requires more than looking at a single SNP.
The Star Allele System
CYP2D6 variants are named using the star allele nomenclature. Each star allele represents a specific combination of sequence variants (haplotype) that produces a defined level of enzyme function:
- *1 — Normal function (reference allele). Produces a fully active enzyme.
- *2 — Normal function. Common variant with full enzyme activity.
- *4 — Non-functional. A splicing defect (rs3892097, G>A) prevents production of active enzyme. Most common loss-of-function allele in European populations (~20% frequency).
- *6 — Non-functional. A single-base deletion causes a frameshift. Less common than *4.
- *10 — Reduced function. A missense variant (rs1065852, C>T, P34S) produces an unstable enzyme with reduced activity. Most common reduced-function allele in East Asian populations (~40% frequency).
- *5 — Gene deletion. The entire CYP2D6 gene is missing. No enzyme produced from this allele.
- *1xN / *2xN — Gene duplication. Multiple copies of a functional allele lead to ultra-rapid metabolism.
Metabolizer Categories
Your CYP2D6 diplotype (the combination of your two alleles) determines your metabolizer phenotype. CPIC assigns an Activity Score (AS) to each allele: 1 for normal function, 0.5 for reduced function, and 0 for non-functional. Your total AS predicts your phenotype:
- Poor Metabolizer (AS = 0) — e.g., *4/*4, *4/*5, *5/*6. Approximately 5–10% of Europeans.
- Intermediate Metabolizer (AS = 0.5–1) — e.g., *4/*10, *1/*5. About 10–15% of most populations.
- Normal Metabolizer (AS = 1.5–2) — e.g., *1/*1, *1/*2. The majority of the population.
- Ultra-rapid Metabolizer (AS > 2) — e.g., *1/*1xN, *2/*2xN. About 1–10% depending on ethnicity (higher in North African and Middle Eastern populations).
Drugs Affected by CYP2D6
Opioids: Codeine and Tramadol
Codeine and tramadol are prodrugs that require CYP2D6 to convert them into their active forms (morphine and O-desmethyltramadol, respectively). Poor metabolizers get minimal pain relief. Ultra-rapid metabolizers produce active metabolite too quickly, risking respiratory depression. CPIC recommends avoiding codeine and tramadol in both poor and ultra-rapid metabolizers.
Tamoxifen
Tamoxifen, used in breast cancer treatment, is converted by CYP2D6 into endoxifen, its primary active metabolite. Poor metabolizers produce less endoxifen and may have reduced therapeutic benefit. Some oncologists use CYP2D6 testing to guide the choice between tamoxifen and aromatase inhibitors.
SSRIs and Antidepressants
Several antidepressants including paroxetine, fluoxetine, venlafaxine, and nortriptyline are CYP2D6 substrates. Poor metabolizers may experience more side effects at standard doses. CPIC provides dosing guidelines for tricyclic antidepressants and some SSRIs based on CYP2D6 phenotype.
Ondansetron (Zofran)
The anti-nausea drug ondansetron is partially metabolized by CYP2D6. Ultra-rapid metabolizers clear the drug faster and may experience reduced antiemetic efficacy.
Checking CYP2D6 SNPs from 23andMe Data
If you have 23andMe raw data, you can look up key CYP2D6 SNPs to get a partial picture of your genotype. The most informative positions include:
- rs3892097 — Defines *4 (non-functional). G is normal, A indicates *4.
- rs1065852 — Defines *10 (reduced function). C is normal, T indicates *10.
- rs5030655 — Defines *6 (non-functional). Look for a deletion variant.
- rs16947 — Defines *2 (normal function). Useful for haplotype resolution.
Use the SNP Lookup tool to check these rsIDs, or try the Pharmacogenomics Checker for a gene-level summary of CYP2D6 metabolizer status and affected medications.
For developers building PGx applications, the SciRouter Genomics API provides programmatic access to pharmacogenomic annotations across all major PGx genes.