CYP2D6 Poor Metabolizer: Symptoms, Medications, and What It Means

This article is for educational purposes only. It is not medical advice, diagnosis, or treatment. Do not change or stop medications based on genetic information without consulting your prescriber.

You just learned you are a CYP2D6 poor metabolizer — or you suspect you might be because medications have not worked as expected. This page explains what that means, which drugs are affected, what symptoms people report, and what your prescriber can do with this information.

What Poor Metabolizer Means

The CYP2D6 enzyme is one of the most important drug-metabolizing enzymes in the body, responsible for processing approximately 25% of all prescribed medications. Your CYP2D6 metabolizer phenotype is determined by the combination of gene variants (star alleles) you inherited — one from each parent.

A poor metabolizer (PM) has little to no functional CYP2D6 enzyme activity. This typically results from carrying two non-functional alleles — for example, CYP2D6 *4/*4, *4/*5, or *3/*4. Approximately 5–10% of European-ancestry populations are CYP2D6 poor metabolizers. Frequency varies by ethnicity.

With reduced or absent enzyme activity, drugs that depend on CYP2D6 for metabolism behave differently in your body compared to someone with normal enzyme function.

Which Medications Are Affected

CYP2D6 metabolizes drugs across multiple therapeutic areas. The following table lists major drug classes with CPIC guideline evidence:

For a detailed list of CYP2D6-affected antidepressants specifically, see our pharmacogenomics for depression guide.

What Symptoms People Report

The "symptoms" of being a CYP2D6 poor metabolizer are not caused by the genotype itself — they are caused by the altered drug levels that result from reduced metabolism. What people experience depends on whether the affected medication is an active drug or a prodrug:

• For active drugs (SSRIs, TCAs, antipsychotics): Poor metabolizers clear the drug more slowly, leading to higher blood levels at standard doses. People may report increased side effects — nausea, dizziness, sedation, dry mouth, tremor, or other drug-specific adverse effects.
• For prodrugs (codeine, tramadol): Poor metabolizers cannot efficiently convert the prodrug to its active form. People may report that the medication "does not work" — inadequate pain relief despite taking the prescribed dose.

In both cases, the person often does not know that the issue is pharmacogenomic. They may describe the experience as "side effects," "sensitivity," or "the medication not working" — without connecting it to their genetics.

Side Effects vs Non-Response: The Difference

This distinction is critical for understanding CYP2D6 poor metabolizer outcomes:

• Codeine → no effect (prodrug failure): Codeine must be converted by CYP2D6 to morphine. In a poor metabolizer, this conversion is severely reduced. The person takes codeine but gets little to no pain relief. The drug is not "toxic" — it simply does not work.
• SSRIs → excess side effects (drug accumulation): Paroxetine (Paxil) is primarily metabolized by CYP2D6. In a poor metabolizer, the drug builds up in the bloodstream because it is cleared more slowly. The person may experience pronounced nausea, sedation, or other side effects at standard doses.

Both scenarios — non-response and excessive side effects — can be explained by the same underlying issue: altered CYP2D6 enzyme activity. Understanding which drugs are prodrugs and which are active drugs is the key to predicting the direction of the effect.

For more on metabolizer phenotypes including intermediate and ultrarapid, see our metabolizer phenotype guide.

Clinical Context

Knowing that you are a CYP2D6 poor metabolizer gives your prescriber actionable information. CPIC guidelines provide specific recommendations for many CYP2D6 drug-gene pairs:

• Dose adjustment: For some drugs (e.g., tricyclic antidepressants), CPIC recommends a 50% dose reduction for poor metabolizers as a starting point.
• Alternative drug selection: For prodrugs like codeine, CPIC recommends avoiding the drug entirely and selecting a non-CYP2D6-dependent alternative for pain management.
• Enhanced monitoring: For some drugs, standard dosing may still be used with closer monitoring of plasma levels and side effects.
• Drug interaction awareness: Even normal metabolizers can become "phenotypic poor metabolizers" when taking strong CYP2D6 inhibitors (e.g., fluoxetine, paroxetine, bupropion). Your prescriber should consider both genetic and drug-induced effects on CYP2D6 activity.

For an overview of how DNA tests for medication work and what options exist, see our comparison guide.

For background on pharmacogenomics generally, see our introductory article.

Medical disclaimer: This article is for informational purposes only and does not constitute medical advice, diagnosis, or treatment. DecodeMyBio provides informational pharmacogenomic reports — not clinical diagnostic testing. Always consult a qualified healthcare provider before making any medication changes.