CYP2D6*4 - The Most Important Drug Metabolism Gene
CYP2D6 is one of the most clinically significant drug-metabolizing enzymes in
the human body. Despite making up only about 2% of liver CYP450 content, it
metabolizes approximately 25% of all clinically used medications. The *4 allele| rs3892097
is the most common non-functional variant in European populations,
carried by about 25% of people.
The Mechanism
The CYP2D6*4 variant is a splice site mutation| A splice site mutation disrupts the boundary between coding and non-coding DNA, preventing correct protein assembly| C>T on the plus strand (historically called G1846A on the coding strand) at the intron 3/exon 4 boundary
that causes aberrant mRNA splicing, producing a completely non-functional enzyme.
Unlike variants that merely reduce activity, *4 abolishes CYP2D6 function entirely
from that allele. Individuals homozygous for *4 (TT) are classified as CYP2D6
poor metabolizers.
Prodrugs vs. Active Drugs
The clinical impact of CYP2D6 status depends on whether a medication is a prodrug| A prodrug is inactive until the body converts it to its active form
or an active drug (needs CYP2D6 to be eliminated).
For prodrugs like codeine and tramadol, poor metabolizers get NO pain relief
because these drugs cannot be converted to their active forms| Codeine is converted to morphine; tramadol to O-desmethyltramadol. This is not a matter of dose adjustment -
these drugs simply will not work.
For active drugs like many antidepressants| e.g. fluoxetine, paroxetine, venlafaxine,
beta-blockers, and tamoxifen, poor metabolizers accumulate higher drug
levels, increasing the risk of side effects and toxicity.
The Evidence
CYP2D6 pharmacogenomics has the strongest evidence base of any pharmacogene. The
Clinical Pharmacogenetics Implementation Consortium (CPIC) and the Dutch
Pharmacogenetics Working Group (DPWG) | Dutch Pharmacogenetics Working Group at PharmGKB have published dosing guidelines for over
30 CYP2D6 substrate medications. Major medical centers now routinely test CYP2D6
before prescribing certain medications. The Gaedigk activity score system | Gaedigk A et al. The CYP2D6 activity score. Clin Pharmacol Ther, 2008
translates complex CYP2D6 genotypes into a quantitative measure of predicted
enzyme activity, enabling standardized phenotype assignment.
What You Should Do
If you carry even one *4 allele, this is clinically actionable information. Share
your CYP2D6 status with all prescribing physicians and pharmacists. Consider
requesting your full CYP2D6 genotype through clinical pharmacogenomic testing,
as 23andMe only captures some of the known variants.
All Genotypes
Normal CYP2D6 activity
You have normal CYP2D6 enzyme activity. This enzyme metabolizes about 25% of all medications, including many antidepressants, opioids like codeine and tramadol, and beta-blockers like metoprolol. About 72% of people of European descent share this genotype. With normal activity, standard doses of these medications should work as expected for you.
Normal CYP2D6 activity
You have normal CYP2D6 enzyme activity. This enzyme metabolizes about 25% of all medications, including many antidepressants, opioids like codeine and tramadol, and beta-blockers like metoprolol. About 72% of people of European descent share this genotype. With normal activity, standard doses of these medications should work as expected for you.
Intermediate CYP2D6 metabolizer
You carry one copy of the CYP2D6*4 variant, which gives you intermediate enzyme activity. About 25% of Europeans share this genotype. This affects how you process about 25% of medications. For codeine and tramadol, you may get somewhat reduced pain relief since these prodrugs need CYP2D6 to convert them to their active forms. For drugs that are inactivated by CYP2D6 (like some antidepressants), they may build up slightly more in your system.
Intermediate CYP2D6 metabolizer
You carry one copy of the CYP2D6*4 variant, which gives you intermediate enzyme activity. About 25% of Europeans share this genotype. This affects how you process about 25% of medications. For codeine and tramadol, you may get somewhat reduced pain relief since these prodrugs need CYP2D6 to convert them to their active forms. For drugs that are inactivated by CYP2D6 (like some antidepressants), they may build up slightly more in your system.
Poor CYP2D6 metabolizer - medication adjustments needed
You are a CYP2D6 poor metabolizer. About 3% of Europeans share this genotype. This significantly affects how you process about 25% of medications. Codeine and tramadol will be INEFFECTIVE for pain relief because you cannot convert them to their active forms. Use alternative pain medications. Drugs inactivated by CYP2D6 (many antidepressants, metoprolol, tamoxifen) will accumulate in your system, increasing side effects.
Poor CYP2D6 metabolizer - medication adjustments needed
You are a CYP2D6 poor metabolizer. About 3% of Europeans share this genotype. This significantly affects how you process about 25% of medications. Codeine and tramadol will be INEFFECTIVE for pain relief because you cannot convert them to their active forms. Use alternative pain medications. Drugs inactivated by CYP2D6 (many antidepressants, metoprolol, tamoxifen) will accumulate in your system, increasing side effects.