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Pharmacogenetics

Poor, intermediate, normal and ultra-rapid metabolisers explained

Pharmacogenetic reports sort people into groups such as poor, intermediate, normal and ultra-rapid metabolisers. The words sound like a verdict on the body, but they describe only how quickly one enzyme works. What that means depends on whether the medicine needs the enzyme to switch it on or to clear it, which is why the same label can mean opposite things for different drugs.

Last checked: 2026-10-03

Key facts

At a glance.

  • Labels describe speed

    The labels grade how fast one enzyme works; they are not a judgement on overall health or liver function.

  • Gene copies

    MedlinePlus Genetics describes TPMT risk by copy number: two normal copies, one low-activity copy, or two low-activity copies.

  • Direction depends on the drug

    For clopidogrel a poor metaboliser gets too little active drug, whereas for a drug cleared by the enzyme a poor metaboliser gets too much.

  • Population differences

    The MHRA gives different prevalence figures for DPD deficiency and codeine ultra-rapid metabolism, showing that estimates depend on population.

  • A result informs, a clinician decides

    The FDA table lists many gene-drug pairs with varying strength of evidence, and only some change practice.

Where the labels come from

Each person inherits two copies of most genes, one from each parent. For an enzyme gene such as CYP2C19 or CYP2D6, those copies may be fully working, partly working or not working. MedlinePlus Genetics illustrates the idea for CYP2C19: normal metabolisers carry working copies, intermediate metabolisers have one functional and one non-functional copy, poor metabolisers have two non-functional copies, and ultra-rapid metabolisers carry a variant, CYP2C19*17, that increases activity.

A laboratory combines the variants it finds into an overall activity estimate for that enzyme and assigns the matching label. Different labs can use slightly different category boundaries, so it is worth reading the report's own definitions.

Why the same label means different things

Imagine two kinds of medicine. A prodrug is inactive until an enzyme converts it. Clopidogrel and codeine are examples. A drug that is already active and is cleared by an enzyme is the second kind. Warfarin, cleared by CYP2C9, is an example.

For a prodrug, a poor metaboliser makes too little of the active product, so the medicine may not work well, while an ultra-rapid metaboliser makes too much, risking toxicity. For an active drug cleared by the enzyme, the pattern reverses: poor metabolisers accumulate the drug and may need less, while ultra-rapid metabolisers may clear it too fast. That is the logic behind codeine caution in ultra-rapid metabolisers and clopidogrel concern in poor metabolisers.

  • Prodrug, poor metaboliser: reduced effect.

  • Prodrug, ultra-rapid metaboliser: increased effect and possible toxicity.

  • Active drug cleared by the enzyme, poor metaboliser: higher blood levels.

  • Active drug cleared by the enzyme, ultra-rapid metaboliser: lower blood levels.

Not every gene works through enzymes

Some pharmacogenetic results have nothing to do with speed. Transporter genes, such as SLCO1B1, affect how well a medicine moves into the liver. Immune genes, such as HLA-B*57:01 and HLA-B*15:02, predict hypersensitivity, and the answer there is to avoid the drug, not to adjust a dose. The metaboliser vocabulary does not apply cleanly to these.

A good report states which mechanism applies and what the gene-drug pair is linked to. If your report simply lists labels, ask the clinician or pharmacist what each means for your current medicines.

How common the extremes are

The extremes are uncommon but not negligible, and frequencies vary by ancestry. The MHRA reports complete DPD deficiency in roughly 0.01 to 0.5 per cent and partial deficiency in 3 to 9 per cent of Caucasian populations, and quotes about 1 to 2 per cent of the UK population as CYP2D6 ultra-rapid metabolisers of codeine in one update, while an earlier review quoted a higher figure for Caucasians. That is a reminder that numbers depend on how and where they were measured.

MedlinePlus Genetics similarly lists different common variants of UGT1A1 and CYP2C9 in different ancestries. A test designed around one group's variants may miss rarer variants in another, so interpretation takes ancestry into account.

What a result can and cannot do

The FDA's table of pharmacogenetic associations lists many gene-drug pairs, grouped by strength of evidence. Only some have regulatory advice to test before prescribing, such as HLA-B*57:01 with abacavir, and for others, such as CYP2D6 and tamoxifen, the UK regulator found the evidence insufficient to recommend routine genotyping.

So a result is rarely a command. It is one input beside kidney and liver function, age, interacting medicines, the condition being treated and how the medicine is actually working. Where a medicine can be monitored directly, such as warfarin with INR, monitoring tends to count for more than genotype.

Using a result well

Keep a copy of your report, because the genes do not change and the result may be relevant for decades. Share it with your GP and pharmacist, and ask them to record it in your notes with the specific gene and phenotype.

Do not change or stop any prescribed medicine on the basis of a result you have read yourself. Ask the prescriber what the result means for each of your medicines, and whether it changes the plan. A separate guide on this site explains how pharmacogenomic testing is arranged.

Frequently asked

Questions people ask.

Quick, plain-English answers to the questions we hear most.

  • What is a poor metaboliser?

    Someone whose enzyme for a particular gene works little or not at all, usually because both gene copies are non-functional.

  • What is an ultra-rapid metaboliser?

    Someone whose enzyme works faster than normal, often because of an increased-function variant such as CYP2C19*17 or extra gene copies.

  • Does poor metaboliser mean I am unhealthy?

    No. The label describes speed of one enzyme and says nothing about general health.

  • Why can poor metabolisers need less of one drug and more of another?

    If the drug is active and cleared by the enzyme, levels rise. If it is a prodrug needing activation, too little active drug forms.

  • Do metaboliser labels apply to all genes?

    No. Transporter and immune genes such as SLCO1B1 and HLA-B work differently, so the labels do not fit them neatly.

  • How common are extreme metabolisers?

    Fairly uncommon, and figures vary by ancestry. The MHRA gives 3 to 9 per cent for partial DPD deficiency in Caucasian populations.

  • Will my result change over time?

    The gene result does not, but how you respond can change with age, illness and other medicines.

  • Should I change my medicines based on a result?

    No. Discuss it with your prescriber or pharmacist first.

  • Where can I read about arranging testing?

    The pharmacogenomic testing guide on this site covers how tests work and what they can show.

Where this comes from

Sources and checking.

This guide is for information, not medical advice.

Your GP or consultant knows your history and can tell you which parts apply to you. If your symptoms are severe, rapidly worsening, or come with red-flag features, seek urgent medical care. Figures and availability change, so treat them as a guide and confirm with the provider.

Published 2026-10-03, next review 2027-10-03.

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