CYP2D6

Overview

CYP2D6 belongs to the cytochrome P450 superfamily, a collection of enzymes responsible for oxidising foreign compounds and facilitating their removal from the body. Located on chromosome 22, CYP2D6 is expressed primarily in hepatocytes, where it processes an estimated 20 to 25 per cent of clinically prescribed drugs. The gene exhibits extraordinary genetic diversity, with more than 100 recognised variant alleles identified across global populations. This variation translates into substantial differences in enzyme activity between individuals, affecting how rapidly medications are converted to active or inactive forms. Because drug response depends partly on the rate of metabolism, CYP2D6 genotype has become a key consideration in prescribing decisions for certain therapeutic classes. Understanding an individual's CYP2D6 metaboliser status enables clinicians to adjust dosages or select alternative medications, reducing the risk of adverse effects or treatment failure.

What the gene does

The CYP2D6 enzyme catalyses phase I oxidation reactions, typically introducing a hydroxyl group into lipophilic substrates to render them more water-soluble. This modification facilitates conjugation by phase II enzymes and subsequent renal or biliary excretion. Substrates processed by CYP2D6 include tricyclic antidepressants, selective serotonin reuptake inhibitors, opioid analgesics such as codeine and tramadol, beta-blockers, antiarrhythmics, and certain antipsychotics. For some drugs, CYP2D6 activates a prodrug into its pharmacologically active metabolite; for others, it inactivates the parent compound. The enzyme operates within the endoplasmic reticulum membrane of hepatocytes, where it accepts electrons from NADPH via cytochrome P450 reductase to drive the oxidation cycle. CYP2D6 exhibits narrow substrate specificity compared with other P450 isoforms, with a relatively constrained active site that accommodates molecules containing a basic nitrogen atom at a defined distance from the site of oxidation. Unlike many cytochrome P450 genes, CYP2D6 is not substantially induced by xenobiotics, meaning enzyme levels remain relatively constant regardless of environmental exposures. This lack of inducibility underscores the importance of inherited genetic variation in determining individual metabolic capacity.

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Chromosome location

CYP2D6 resides on the long arm of chromosome 22 at band 22q13.2, within a cluster of related cytochrome P450 genes. The genomic locus spans approximately 4.4 kilobases and comprises nine exons. Flanking CYP2D6 are two pseudogenes, CYP2D7 and CYP2D8P, which share high sequence similarity but do not encode functional enzymes. This tandem arrangement creates a region prone to recombination events, contributing to the gene's structural complexity. Copy number variation is common at this locus: some individuals carry gene duplications or multiplications resulting in increased enzyme production, whilst others harbour complete gene deletions leading to absent activity. Hybrid genes formed by recombination between CYP2D6 and the adjacent pseudogenes can produce non-functional or partially functional enzymes. The genomic architecture of the CYP2D6 region complicates molecular genetic testing, as standard sequencing approaches may misalign reads from the pseudogenes, necessitating specialised assays to accurately determine copy number and hybrid structures.

Protein structure

Domain architecture has not been experimentally characterised in detail for this protein.

Key variants

CYP2D6 is one of the most polymorphic genes in the human genome, with allelic variants classified according to their impact on enzyme activity. Poor metaboliser alleles carry mutations that abolish or severely reduce catalytic function, often through frameshifts, premature stop codons, or splicing defects. Intermediate metaboliser alleles encode partially functional enzymes with diminished activity. Normal or extensive metaboliser alleles produce fully functional enzyme at typical expression levels. Ultrarapid metaboliser alleles typically arise from gene duplications or multiplications, leading to increased enzyme production. The frequency of these alleles varies markedly across populations: for example, poor metaboliser phenotypes are more common in individuals of European ancestry, whilst ultrarapid metaboliser genotypes occur at higher frequencies in certain Middle Eastern and North African groups. Because metaboliser status is determined by the combination of alleles inherited from both parents, individuals may fall anywhere along a continuum of activity. Pharmacogenomic testing identifies an individual's diplotype and predicts their metaboliser classification, guiding drug selection and dosing.

No pathogenic or likely-pathogenic ClinVar variants recorded yet for this gene.

Associated conditions

CYP2D6 variation does not cause a specific inherited disease in the traditional sense. Instead, genetic differences in this gene influence individual responses to a wide range of medications. Poor metabolisers may experience elevated drug concentrations and a greater risk of adverse effects when prescribed standard doses of CYP2D6 substrates. Conversely, ultrarapid metabolisers may achieve subtherapeutic drug levels, resulting in reduced efficacy or treatment failure. For prodrugs such as codeine, which require CYP2D6 to convert them into active metabolites, poor metabolisers derive little analgesic benefit, whilst ultrarapid metabolisers may experience toxicity from excessive active metabolite formation. These gene-drug interactions are clinically significant across multiple therapeutic areas, including psychiatry, pain management, cardiology, and oncology. Regulatory agencies and professional societies have issued guidance on CYP2D6 genotype-directed prescribing for selected medications, reflecting the growing role of pharmacogenomics in personalised medicine.

No disease links recorded for this gene in our reference set.

UK clinical status

Diet & lifestyle considerations

Lifestyle factors have limited direct influence on CYP2D6 enzyme activity, as the gene is not inducible by diet or environmental exposures in the manner of some other cytochrome P450 isoforms. However, concurrent use of certain medications can inhibit CYP2D6 function, effectively converting an extensive metaboliser into a phenotypic poor metaboliser. Potent CYP2D6 inhibitors include some selective serotonin reuptake inhibitors, antiarrhythmics, and antihistamines. Individuals taking these inhibitors alongside CYP2D6 substrate drugs may experience altered drug levels regardless of their underlying genotype. Grapefruit juice, which inhibits other P450 enzymes such as CYP3A4, does not substantially affect CYP2D6 activity. Smoking status and alcohol consumption also appear to have minimal impact on CYP2D6 metabolism. From a practical standpoint, individuals aware of their CYP2D6 metaboliser status should inform all prescribing clinicians, as this information remains relevant throughout life and across multiple therapeutic contexts. Maintaining an up-to-date list of all medications, including over-the-counter products, helps clinicians anticipate potential drug-drug interactions mediated by CYP2D6 inhibition.

Supplement considerations

There is no evidence that dietary supplements modify CYP2D6 enzyme activity or that specific nutritional interventions can compensate for genetically determined differences in metabolic capacity. Some herbal products have been investigated for potential effects on cytochrome P450 enzymes, but robust clinical data supporting supplement-based modulation of CYP2D6 are lacking. Individuals considering any supplement should discuss its use with a healthcare provider, particularly if they are taking medications known to be CYP2D6 substrates, as unexpected interactions could theoretically alter drug metabolism. The most effective approach to managing CYP2D6-related drug response variability is genotype-guided prescribing rather than attempting to influence enzyme activity through supplementation. Pharmacogenomic testing provides actionable information that clinicians can use to select appropriate medications and dosages, reducing reliance on trial-and-error prescribing and minimising the risk of adverse drug reactions.

Related genes

CYP2D6 functions within the broader cytochrome P450 system, a family of enzymes collectively responsible for metabolising the majority of pharmaceutical compounds. Other P450 genes, including CYP2C9, CYP2C19, and CYP3A4, also exhibit pharmacogenomic relevance and are often tested alongside CYP2D6 in comprehensive medication management panels. These genes can share overlapping substrate profiles, meaning that an individual's overall drug metabolism capacity depends on the combined activity of multiple enzymes. Understanding the interplay between different cytochrome P450 variants enables more refined predictions of drug response and supports the goal of truly personalised prescribing.

No related genes found in our reference set.

Frequently asked questions