What Does Poor Metabolizer Mean? CYP2D6 & CYP2C19 Explained

A poor metabolizer is someone whose body breaks down certain medications much more slowly than normal — or not at all — due to genetic variants in drug-metabolizing enzymes. This can cause drugs to build up to unsafe levels, or prevent prodrugs from being activated. Poor metabolizer is one of five categories used in pharmacogenomics to describe how your genes affect your response to medications.

This guide explains each metabolizer status in plain language — what it means, how it affects specific medications, and what to discuss with your healthcare provider.

What Is Metabolizer Status?

Your body uses enzymes to break down (metabolize) medications. The genes that encode these enzymes can carry variants that increase or decrease enzyme activity. Your combination of genetic variants — called your diplotype — determines your metabolizer phenotype: how efficiently a specific enzyme works in your body.

Clinical guidelines from CPIC (Clinical Pharmacogenetics Implementation Consortium) and DPWG (Dutch Pharmacogenetics Working Group) use these metabolizer phenotypes to make dosing and prescribing recommendations. For a broader introduction, see what is pharmacogenomics and how pharmacogenomic testing works.

The Five Metabolizer Phenotypes Explained

Poor Metabolizer (PM)

A poor metabolizer has very low or absent enzyme activity. This has different consequences depending on whether a medication is an active drug or a prodrug:

• Active drugs (medications that work as-is): The drug is cleared more slowly, causing it to build up in your body. This can increase the risk of side effects at standard doses.
• Prodrugs (medications that must be converted to an active form): The drug is not converted effectively, resulting in reduced or no therapeutic effect.

Example: CYP2D6 poor metabolizer status means patients may not effectively convert codeine to morphine, its active metabolite — potentially resulting in inadequate pain relief. CPIC guidelines recommend considering an alternative analgesic for these patients.

Intermediate Metabolizer (IM)

An intermediate metabolizer has reduced but not absent enzyme activity. Medications processed by the affected enzyme may be cleared more slowly than expected, which can lead to higher drug levels or reduced prodrug activation.

Example: CYP2C19 metabolizer status matters here — intermediate metabolizers may have reduced activation of clopidogrel (Plavix), potentially affecting its antiplatelet efficacy. CPIC guidelines recommend considering an alternative antiplatelet agent in certain clinical scenarios.

Normal Metabolizer (NM)

A normal metabolizer has expected enzyme function. Standard medication doses are typically appropriate, and no genotype-based dosing adjustments are recommended. This is the most common phenotype for most pharmacogenes.

Note: The term "normal metabolizer" replaced the older term "extensive metabolizer" in current CPIC nomenclature. If you see either term in your results, they mean the same thing.

Rapid Metabolizer (RM)

A rapid metabolizer has increased enzyme activity compared to normal. Active drugs may be cleared faster than expected, potentially reducing their effectiveness at standard doses. Not all pharmacogenes have a recognized rapid metabolizer category — it depends on the specific gene and its known functional variants.

Ultrarapid Metabolizer (UM)

An ultrarapid metabolizer has significantly increased enzyme activity, often caused by gene duplication (carrying extra copies of a functional gene). This can have serious clinical consequences:

• Active drugs: Cleared too quickly for standard doses to maintain therapeutic levels.
• Prodrugs: Converted too rapidly, potentially producing dangerous levels of the active metabolite.

Example: CYP2D6 metabolism in ultrarapid metabolizers may convert codeine to morphine too quickly, which can cause respiratory depression — a risk serious enough to warrant an FDA boxed warning. CPIC guidelines recommend avoiding codeine entirely for these patients.

How Diplotypes and Activity Scores Determine Your Phenotype

Your metabolizer phenotype is not assigned directly from a single variant — it is calculated from your diplotype (the combination of two alleles you carry, one from each parent) using an activity score system standardized by CPIC.

Each allele is assigned a value based on its function: a normal-function allele scores 1, a decreased-function allele scores 0.5, and a no-function allele scores 0. Your total activity score is the sum of both alleles. For example, if you carry one normal-function allele and one no-function allele, your activity score is 1 + 0 = 1.0.

The activity score thresholds differ slightly between genes. Here are the standard ranges for the two most common pharmacogenes:

Activity score ranges based on CPIC guideline updates. CYP2C19 phenotype assignment also considers specific allele combinations; see CYP2C19 gene page for details.

What Happens in Practice: Phenotype and Drug Examples

The same phenotype can have opposite clinical effects depending on whether a drug is metabolized directly or is a prodrug that must be activated. The table below shows concrete examples across multiple genes:

These examples illustrate why understanding your metabolizer status for each gene — not just one — provides a more complete picture. See the CYP2D6 gene and CYP2C19 gene pages for full variant and guideline details.

Why the Same Status Means Different Things for Different Drugs

The clinical impact of your metabolizer status depends on the specific drug-gene pair. Being a poor metabolizer for an active drug means the drug builds up (risk of toxicity), while being a poor metabolizer for a prodrug means it is not activated effectively (risk of treatment failure).

Additionally, the same person can have different metabolizer phenotypes for different enzymes. You could be a CYP2D6 poor metabolizer (affecting opioids and antidepressants) and a CYP2C19 normal metabolizer (standard clopidogrel activation) — or any other combination. This is why a comprehensive report covering multiple pharmacogenes provides more complete information than testing a single gene. For more on how to read these results, see understanding your Medication Safety Report.

Metabolizer Phenotypes by Gene

The table below summarizes key pharmacogenes, the medications they affect, and the approximate prevalence of poor metabolizer status. Frequencies vary by ancestry and gene — the figures below are general ranges drawn from published clinical literature.

Prevalence descriptions are qualitative summaries based on published CPIC guidelines and PharmVar data. Exact frequencies depend on specific populations studied. See our data sources page for references.

What Does Intermediate Metabolizer Mean?

An intermediate metabolizer (IM) has reduced — but not absent — enzyme activity. If you see "intermediate metabolizer" on a pharmacogenomic report, it means you carry at least one decreased-function or loss-of-function allele for the gene in question, while retaining some residual enzyme activity. Intermediate metabolizers process certain medications more slowly than normal metabolizers but faster than poor metabolizers.

Clinically, intermediate metabolizer status can matter in several ways depending on the drug-gene pair:

• Standard medications — drugs cleared by the affected enzyme may reach higher-than-expected blood levels, which can increase the risk of side effects. CPIC guidelines for some genes recommend starting at a lower dose for intermediate metabolizers.
• Prodrugs — medications that must be converted to their active form (e.g., clopidogrel activated by the CYP2C19 enzyme) may have reduced efficacy in intermediate metabolizers because less of the drug is converted to the active compound.
• Psychiatric medications — many SSRIs and tricyclic antidepressants are metabolized by the CYP2D6 enzyme or the CYP2C19 enzyme. An intermediate metabolizer phenotype for either gene can influence drug exposure and tolerability. See the CYP2D6 antidepressants list for specific examples.

Intermediate metabolizer is the most common non-normal phenotype across most pharmacogenes. For example, roughly 25–35 % of people of European ancestry are CYP2D6 intermediate metabolizers, and a similar proportion are CYP2C19 intermediate metabolizers. Because IM status is so prevalent, understanding its implications can be valuable even when the clinical impact is less dramatic than for poor metabolizers.

It is important to note that intermediate metabolizer status does not automatically require a medication change. Whether a dose adjustment is appropriate depends on the specific drug, the gene involved, and your overall clinical picture. Always discuss your results with your prescriber before making any changes.

What To Do With Your Metabolizer Status

If you have metabolizer status results — from any testing service — the most important step is to share them with your healthcare provider. These results are informational: they provide genetic context that your prescriber can consider alongside your full medical history, current medications, and other clinical factors.

• Do not change, stop, or adjust any medication based solely on your metabolizer status.
• Keep your results on file for future prescribing decisions — pharmacogenomic information remains relevant throughout your lifetime.
• Understand that metabolizer status is one factor among many. Drug response is also influenced by age, weight, organ function, other medications, and environmental factors.

If you don't have pharmacogenomic results yet, your existing DNA data from 23andMe or AncestryDNA may already contain the relevant genetic variants. See what to do with your 23andMe raw data to learn how, or compare pharmacogenomics testing options to understand your choices.

Frequently Asked Questions

Can my metabolizer status change over time?

Your genetic metabolizer status does not change — it is determined by your DNA. However, your real-world drug metabolism can be affected by other factors, a concept called phenoconversion. Other medications, liver function, age, and environmental exposures can alter effective enzyme activity even if your genetic status remains the same. This is why pharmacogenomic information should always be interpreted alongside your current clinical context.

What's the difference between "extensive metabolizer" and "normal metabolizer"?

They mean the same thing. "Extensive metabolizer" was the original term used in pharmacogenomics literature. CPIC updated the nomenclature to "normal metabolizer" to improve clarity. If your results use either term, the clinical interpretation is the same: expected enzyme activity with standard dosing recommendations.

I'm a poor metabolizer — should I stop my medication?

No. A poor metabolizer result does not automatically mean your current medication is wrong for you. The clinical significance depends on the specific drug-gene pair, your dose, and your overall treatment context. Discuss your results with your prescriber, who can evaluate whether any adjustments are appropriate.

Can 23andMe tell me my metabolizer status?

23andMe tests some pharmacogenomically relevant variants but does not provide metabolizer phenotype interpretation for most genes in their standard reports. However, your 23andMe raw data file contains many of these variants, which can be analyzed by a pharmacogenomic service like DecodeMyBio. See what to do with your 23andMe raw data.

How accurate is metabolizer status from consumer DNA data?

Consumer genotyping arrays are highly accurate for the specific variants they test. However, they do not cover all possible functional variants, and structural changes like gene deletions cannot be detected from array data. A "normal metabolizer" result reflects the tested variants only. For a full discussion of these caveats, see our limitations page.

What is the difference between poor metabolizer and intermediate metabolizer?

A poor metabolizer has very low or absent enzyme activity, while an intermediate metabolizer has reduced but not absent activity. Both may require dosing adjustments depending on the specific drug-gene pair, but the clinical impact is typically more pronounced for poor metabolizers. CPIC guidelines provide separate recommendations for each phenotype.

What does intermediate metabolizer mean?

Intermediate metabolizer means you have reduced — but not absent — enzyme activity for a specific pharmacogene. Your body still processes the medication, but more slowly than a normal metabolizer. Depending on the drug-gene pair, this may lead to higher drug levels (for standard medications) or reduced activation (for prodrugs like clopidogrel). CPIC guidelines provide gene-specific recommendations for intermediate metabolizers, which may include dose reductions or closer monitoring. Intermediate metabolizer is the most common non-normal phenotype, so understanding what it means is relevant for a large portion of the population.

Does being a poor metabolizer mean I cannot take a medication?

Not necessarily. A poor metabolizer result means your body processes certain drugs differently, not that all medications are unsafe. The clinical significance depends on the specific drug-gene pair — for some medications, a dose adjustment may be recommended; for others, an alternative drug may be preferred. Always discuss results with your healthcare provider before making any changes.

What is an activity score in pharmacogenomics?

An activity score is a numerical value assigned to your combination of gene alleles (diplotype) to determine your metabolizer phenotype. Each allele receives a value — typically 1 for normal function, 0.5 for decreased function, and 0 for no function — and the two values are summed. For example, a CYP2D6 activity score of 0 means poor metabolizer (two no-function alleles), while a score of 1.25–2.0 means normal metabolizer. CPIC uses activity scores to standardize phenotype assignment across clinical guidelines.

References

1. Clinical Pharmacogenetics Implementation Consortium (CPIC) Guidelines. cpicpgx.org
2. DPWG Guidelines — PharmGKB. pharmgkb.org
3. PharmVar — Pharmacogene Variation Consortium. pharmvar.org
4. FDA Table of Pharmacogenomic Biomarkers in Drug Labeling. fda.gov

For details on the clinical sources used in DecodeMyBio's analysis, see our data sources page.