Intermediate Metabolizer: What It Means for Your Medications

An intermediate metabolizer (IM) is someone with reduced — but not absent — enzyme activity in a drug-metabolizing gene. If you have received this result for a gene like the CYP2D6 gene or the CYP2C19 gene, it means your body processes certain medications more slowly than someone with normal enzyme function, but faster than a poor metabolizer.

What Causes Intermediate Metabolizer Status?

Your metabolizer phenotype is determined by your diplotype — the combination of two alleles you carry, one from each parent. Each allele is assigned a function value: normal-function (1.0), decreased-function (0.5), or no-function (0). Your total activity score is the sum of both alleles.

For CYP2D6, intermediate metabolizer corresponds to an activity score of 0.25–1.0. Common diplotype examples:

• *1/*4 — one normal-function allele + one no-function allele (activity score 1.0)
• *1/*5 — one normal-function allele + one gene deletion (activity score 1.0)
• *41/*4 — one decreased-function allele + one no-function allele (activity score 0.5)

For CYP2C19, intermediate metabolizer typically means carrying one normal-function allele and one no-function allele (e.g., *1/*2). See the CYP2C19 gene page for the full allele table.

How Intermediate Metabolizer Affects Medications

The clinical impact depends on whether a drug is an active medication or a prodrug:

• Active drugs (work as-is): Cleared more slowly than in normal metabolizers, which can result in moderately higher drug levels. Side effects may be more likely at standard doses, though the effect is less pronounced than in poor metabolizers.
• Prodrugs (must be converted to active form): Activation is reduced but not eliminated. The drug may still work, but with potentially lower efficacy.

Examples by Gene

CPIC recommendations are gene- and drug-specific. See individual medication pages for full guideline details.

Intermediate vs. Poor vs. Normal Metabolizer

Intermediate metabolizer sits between normal and poor on the activity spectrum:

• Poor metabolizer — Very low or absent enzyme activity. CPIC often recommends dose reductions or alternative drugs.
• Intermediate metabolizer — Reduced but not absent activity. Recommendations are typically more moderate — dose monitoring or modest adjustments rather than switching drugs.
• Normal metabolizer — Expected enzyme function. Standard dosing applies.
• Ultrarapid metabolizer — Increased activity, often from gene duplications. Can cause drugs to be cleared too quickly or prodrugs to be activated too rapidly.

How Common Is Intermediate Metabolizer?

Intermediate metabolizer is the second most common phenotype after normal metabolizer for most pharmacogenes:

• CYP2D6 IM: A CPIC-compiled analysis of more than 60,000 subjects across major world populations found genotypic intermediate metabolizer prevalence ranging from about 0.4% to 11%, varying considerably by ancestry.
• CYP2C19 IM: Generally more common than CYP2D6 IM — roughly 20–25% in European-ancestry populations, and approaching 50% in some East Asian populations, though estimates vary by study and cohort.

Because intermediate metabolizer status is common, many people carry a result that is clinically relevant but not as urgent as poor or ultrarapid metabolizer. The key is knowing which specific medications are affected for your genotype.

How to Interpret This Result

Intermediate metabolizer status is context, not a directive. Here is what it means in practice and what to discuss with your clinician:

• Not all medications are affected equally — Only drugs primarily metabolized by the flagged gene have CPIC recommendations for intermediate metabolizers. Your results identify only those interactions with clinical guideline support.
• Questions to ask your prescriber — "Does my current medication list include drugs metabolized by this gene?" and "Does my IM status change how you would dose or monitor these medications?" A provider-ready summary is included in your results to facilitate this conversation.
• IM status does not mean a drug will not work — It means the drug may behave differently at standard doses. Many people on medications metabolized by their IM gene tolerate them without issue — the phenotype provides information for a prescriber to consider alongside other clinical factors.