Poor, intermediate and rapid metabolisers: What do they mean for your medication?

Two people taking the same medication at the same dose may experience different effects. One person might respond as expected, while another experiences side effects or finds the treatment less effective.

There are several reasons for this, including age, other medicines, health conditions and genetic differences.

Some genes influence the enzymes responsible for processing certain medications. Depending on their inherited genetic variants, people may be classified into different drug metaboliser types, including poor, intermediate, normal, rapid and ultra-rapid metabolisers.

Understanding these classifications can help explain why medication response varies, although genetics is only one part of the picture.

Key takeaways

• Drug metaboliser types describe predicted activity of particular enzymes involved in processing certain medicines.

• Poor and intermediate metabolisers generally have reduced activity of a specific enzyme, while rapid and ultra-rapid metabolisers have increased activity where those classifications apply.

• Faster or slower metabolism does not automatically mean a medication will be more or less effective.

• Your metaboliser status can differ between enzymes, meaning you may process different medications in different ways.

• Pharmacogenetic testing can provide information relevant to some prescribing decisions, but results must be interpreted by an appropriate healthcare professional.

What are drug metaboliser types?

Drug metaboliser types are classifications used in pharmacogenomics to describe how genetic differences may affect the activity of enzymes involved in medication metabolism.

Medication metabolism is the process through which the body chemically changes medicines, often to help remove them or convert them into another substance.

Many medications are processed by enzymes in the liver, although metabolism can also occur elsewhere in the body.

Some of these enzymes are influenced by inherited genetic variations.

Depending on the enzyme and genetic variants involved, a person may be described as a poor, intermediate, normal, rapid or ultra-rapid metaboliser.

These classifications are sometimes called metaboliser phenotypes.

They describe predicted enzyme activity, not a person's overall ability to tolerate medication.

What are the five main drug metaboliser types?

Five commonly discussed classifications are:

This is a general overview. Not every gene uses all five categories, and classification criteria differ between enzymes.

For example, CYP2C19 uses poor, intermediate, normal, rapid and ultra-rapid metaboliser categories.

CYP2D6, however, generally uses poor, intermediate, normal and ultra-rapid classifications without a separate rapid metaboliser category.

The clinical importance of any classification depends on the medication involved and the evidence supporting that particular gene-drug relationship.

What does being a poor metaboliser mean?

A poor metaboliser has genetic variants associated with little or no functional activity of a particular drug-metabolising enzyme.

For medications that rely on that enzyme for breakdown, reduced activity may lead to slower clearance and greater drug exposure.

In some circumstances, this can increase the likelihood of concentration-related side effects.

However, this is not true for every medication.

Some medicines need to be converted into an active form before they can produce their intended effect. If the relevant enzyme has little activity, that conversion may be reduced.

For example, codeine requires conversion into morphine, partly through the CYP2D6 enzyme, to provide much of its pain-relieving effect.

People classified as CYP2D6 poor metabolisers may produce substantially less morphine from codeine, potentially reducing its effectiveness.

This illustrates why poor metabolism does not always mean stronger medication effects.

What is an intermediate metaboliser?

An intermediate metaboliser has reduced activity of a particular enzyme compared with someone classified as a normal metaboliser.

The enzyme still functions, but its activity is predicted to be lower.

Depending on the medication, this may influence how much of the drug remains in the body or how much active substance is produced.

For some medicines, intermediate metabolism may have clinical implications. For others, the difference may be less significant or may not require a prescribing change.

Intermediate metaboliser status should therefore not be interpreted as evidence that a medication is unsuitable.

A clinician needs to consider the specific gene, medicine, dose, treatment response and available prescribing guidance.

What is a normal metaboliser?

A normal metaboliser has genetic variants associated with enzyme activity within the expected range.

This generally suggests that the enzyme processes relevant medicines at a rate considered typical for that classification.

However, normal metaboliser status does not guarantee that a medication will work or that side effects will not occur.

Medication response is influenced by many other factors, including:

• Kidney and liver function

• Age and general health

• Other prescribed medicines

• Drug interactions

• The condition being treated

• Individual sensitivity to medication

Some medicines are also processed through several different pathways.

A normal result for one enzyme therefore provides only part of the information needed to assess medication response.

What does being a rapid metaboliser mean?

A rapid metaboliser has genetic variants associated with increased activity of a particular enzyme.

For medicines that are inactivated through that enzyme, increased activity may result in faster breakdown and lower exposure.

However, the effect depends on the medication and its metabolic pathway.

For medicines requiring enzyme activity to become active, increased metabolism may instead increase formation of the active substance.

The rapid metaboliser classification is used for CYP2C19, but it is not a standard separate classification for every drug-metabolising enzyme.

It should not be assumed that someone classified as a rapid metaboliser for CYP2C19 processes every medicine more quickly.

What is an ultra-rapid metaboliser?

An ultra-rapid metaboliser has genetic variants associated with substantially increased activity of a particular enzyme.

In some cases, this can cause a medication to be broken down more quickly than expected.

For certain medicines, lower drug exposure could affect treatment response.

However, increased enzyme activity can also increase the conversion of some medications into active substances.

Codeine provides an important example.

People classified as CYP2D6 ultra-rapid metabolisers may convert codeine into morphine more rapidly, potentially increasing the risk of serious adverse effects.

This is why ultra-rapid metabolism does not simply mean that someone needs a higher medication dose.

The implications must be assessed for the specific medicine involved.

How do CYP450 enzymes affect medication metabolism?

Cytochrome P450 enzymes, often called CYP450 enzymes, are a group of proteins involved in the metabolism of many medicines.

Different CYP enzymes process different substances.

Two enzymes commonly discussed in pharmacogenomics are CYP2D6 and CYP2C19.

CYP2D6

CYP2D6 contributes to the metabolism of several medications, including certain antidepressants, antipsychotics and pain medicines.

Genetic variation can influence CYP2D6 activity, potentially affecting medication exposure or activation.

For example, CYP2D6 activity is relevant to codeine because the enzyme helps convert it into morphine.

The standard CYP2D6 phenotype system generally distinguishes poor, intermediate, normal and ultra-rapid metabolisers.

The effect of CYP2D6 variation differs between medications.

CYP2C19

CYP2C19 contributes to the metabolism of certain antidepressants, proton pump inhibitors and other medicines.

It is also involved in activating clopidogrel, a medicine used to reduce the risk of harmful blood clots.

People with reduced CYP2C19 activity may produce less of clopidogrel's active metabolite, which can affect its antiplatelet action.

Unlike CYP2D6, CYP2C19 uses a separate rapid metaboliser classification alongside poor, intermediate, normal and ultra-rapid categories.

Not all medicines processed by CYP2C19 are affected in the same way.

Why doesn't slower metabolism always mean more side effects?

It is easy to assume that slower metabolism means a medicine stays in the body longer and therefore causes more side effects.

That can happen with certain medications, but it is not a universal rule.

The important distinction is whether the enzyme is primarily involved in breaking down an active medicine or converting an inactive substance into an active one.

Medicines that are broken down into less active substances

For some medications, reduced enzyme activity can slow the breakdown of the active drug.

This may increase exposure and, depending on the medicine, contribute to concentration-related side effects.

Medicines that need to be activated

Some medications are known as prodrugs.

A prodrug requires conversion within the body to produce some or all of its intended therapeutic effect.

If the relevant enzyme has reduced activity, less active substance may be formed.

As a result, slower metabolism may reduce effectiveness rather than increase it.

This is why metaboliser status must always be interpreted in relation to a particular medication.

Can you be a poor metaboliser for one medication and a rapid metaboliser for another?

Yes.

A person does not have one universal drug metaboliser type.

Instead, their genetic profile may indicate different activity levels for different enzymes.

For example, someone could be classified as a CYP2D6 poor metaboliser and a CYP2C19 rapid metaboliser.

These classifications describe different enzyme pathways.

Even medications processed by the same enzyme may be affected differently, depending on how that enzyme contributes to their activity and clearance.

This is one reason pharmacogenetic reports often contain information about several genes rather than assigning a single overall metabolism score.

Can medication or health conditions change your metaboliser status?

Your inherited genetic variants generally remain stable throughout your life.

However, actual enzyme activity can be affected by factors beyond genetics.

Some medications inhibit or induce particular metabolic enzymes, changing how other medicines are processed.

For example, a person whose genetics predict normal CYP2D6 activity may experience reduced CYP2D6 function while taking a strong CYP2D6 inhibitor.

This is sometimes described as phenoconversion, where observed or expected enzyme activity differs from the genetic prediction because of another influence.

Health conditions, particularly those affecting liver function, may also influence medication metabolism.

This means a genetic result should not be considered separately from current medications and clinical circumstances.

How are drug metaboliser types identified?

Drug metaboliser classifications can be predicted using pharmacogenetic testing.

These tests examine selected genetic variants associated with the function of particular enzymes.

A laboratory interprets the detected variants and may assign a predicted metaboliser phenotype for each relevant gene.

For example, a report might identify someone as a CYP2C19 intermediate metaboliser or a CYP2D6 poor metaboliser.

However, different tests may examine different variants, and not every possible genetic change is necessarily included.

Some genetic patterns can be difficult to interpret, particularly when a test cannot determine exactly which variants are present or how they affect enzyme function.

In these circumstances, a result may be uncertain or indeterminate rather than producing a clear metaboliser classification.

A healthcare professional can help determine whether the findings are relevant to a particular medication.

Can medication response DNA testing help?

Pharmacogenetic testing can provide information about inherited differences that may influence how certain medicines are processed or how someone responds to them.

Ready Health offers medication response DNA testing using a cheek-swab sample collected at home.

Its published service information describes two options:

• Mental Health Panel: Focused on medications used for mental health conditions.

• Comprehensive Package: Covers a broader range of medication categories.

Ready Health's testing information includes genes such as CYP2D6 and CYP2C19, which are relevant to established pharmacogenetic relationships for certain medicines.

The results are provided in a report intended to support discussions with a healthcare professional.

However, genetic testing cannot guarantee that a particular medication will work, predict every side effect or remove the need for clinical monitoring.

Its usefulness depends on whether there is reliable evidence connecting the tested genetic variants with the medicine being considered.

How do doctors use metaboliser information when prescribing?

Where relevant pharmacogenetic guidance exists, clinicians may consider metaboliser status when choosing a medicine or deciding whether a different dose or alternative treatment is appropriate.

The information is considered alongside:

• The medical condition being treated

• Current medications and possible interactions

• Previous treatment response

• Side effects experienced

• Liver and kidney function

• Other relevant clinical factors

Not every medicine has an established gene-based prescribing recommendation.

Even where guidance exists, genetic information does not replace clinical judgement.

Importantly, you should never stop, start or change the dose of prescribed medication based solely on a pharmacogenetic result without discussing it with your prescriber.

Understanding what your metaboliser type means for treatment

Poor, intermediate, normal, rapid and ultra-rapid metaboliser classifications describe predicted activity of specific enzymes involved in processing medicines.

These differences can influence medication exposure, activation and, in some cases, treatment response.

However, the meaning of a metaboliser classification depends on the enzyme, the medicine and the individual's wider health circumstances.

A poor metaboliser result is not automatically bad, and a rapid metaboliser result is not necessarily better.

If you have questions about how genetic differences might relate to your medication, an appropriately qualified healthcare professional can explain whether pharmacogenetic information is relevant.

Ready Health's range of health tests and checks includes medication response DNA testing for people interested in exploring available testing options.

The most useful interpretation is always one that considers genetic findings alongside your medical history and current treatment.

FAQs

What are the main drug metaboliser types?

The commonly discussed categories are poor, intermediate, normal, rapid and ultra-rapid metabolisers. However, not every enzyme uses all five classifications.

Is being a poor metaboliser dangerous?

Not necessarily. Poor metaboliser status describes reduced activity of a particular enzyme. Its clinical significance depends on the medicine involved, including whether that enzyme breaks down or activates the drug.

Does being a rapid metaboliser mean medication won't work?

No. Rapid metabolism can affect the exposure or activation of certain medicines, but it does not automatically mean treatment will be ineffective.

Can you have different metaboliser types for different genes?

Yes. You may be a poor metaboliser for one enzyme and a normal or rapid metaboliser for another. There is no single classification that applies to every medicine.

Can genetic testing tell me which medication is best?

Pharmacogenetic testing can provide useful information for certain medicines where established gene-drug relationships exist. It cannot reliably identify the best treatment for every person or guarantee a particular outcome.

Should I change my medication if I'm a poor metaboliser?

No. Do not change or stop prescribed medication based on a genetic result without speaking to your prescriber. They can assess whether the result is clinically relevant and whether any action is needed.