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Poor metaboliser: what the result means for your treatment | CION Cancer Clinics
A poor metaboliser result means that, for one particular gene, your body processes certain medicines very slowly. It is not an illness, and it rarely rules treatment out. For some drugs it means a much smaller dose. For others it means a different drug, because a slow gene can cause too much of a medicine or too little of its active form. This page explains which, and what happens next. At CION Cancer Clinics, our team helps carriers and their families plan checks, next steps and support after a genetic result.
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Medically reviewed by Dr. Naresh GunduConsultant Medical Oncologist · MBBS, DNB (Internal Medicine), DM (Medical Oncology, AIIMS) · last reviewed September 2026, next review due September 2027
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On this page
- What does a poor metaboliser result actually mean?
- What does it mean for the genes tested in cancer care?
- What happens after a poor metaboliser result?
- The words around a metaboliser result, in plain language
- Too much drug or too little: how to tell which
- What this page cannot tell you
- Four things people assume about a poor metaboliser result
- Common questions about poor metaboliser results
The short answer
What does a poor metaboliser result actually mean?
It means that, for one specific gene, both copies work poorly or not at all, so your body processes certain medicines very slowly. It does not mean you are ill, and it rarely means you cannot be treated. It means the usual dose, or the usual choice of drug, may need to change for the medicines linked to that gene.
Why slow can mean too much or too little
Most drugs are active the moment you take them. The body's job is to break them down and clear them. If that clearing is slow, the drug builds up and side effects become more severe. A few drugs work the other way. They are given in an inactive form and the body has to switch them on. For those, a slow gene means too little of the active drug, and less benefit.
It applies to one gene at a time
A report may list several genes, each with its own label. You can be a poor metaboliser for one gene and entirely normal for the rest. The label only matters for the drugs that depend on that particular gene, so always read which gene it sits beside.
A poor metaboliser result is a note about medicines. It is not a diagnosis and it is not a cancer risk.Gene by gene
What does it mean for the genes tested in cancer care?
These four are the ones most likely to appear on a report before cancer treatment. The consequence differs sharply between them.
DPYD
Clears fluorouracil and capecitabine, used widely in bowel, breast, stomach and head and neck cancers. A poor metaboliser result usually means these drugs are avoided, because even a small dose can cause life-threatening side effects.
TPMT and NUDT15
Clear thiopurines such as mercaptopurine, used in leukaemia. In cancer care the drug is usually still given, but at a far smaller dose or on fewer days, with close blood count checks.
UGT1A1
Clears the active form of irinotecan. Two slow copies usually mean a reduced starting dose and closer watching for diarrhoea and low white cell counts.
CYP2D6
Switches on tamoxifen and some painkillers. For these, a poor result means less effect, not more side effects.
Medicines it touches
- Tamoxifen for breast cancer
- Codeine and tramadol for pain
- Several medicines outside cancer care
Not sure whether this applies to you?
Ask an oncologistStep by step
What happens after a poor metaboliser result?
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The report reaches your oncologist
The result is read against the treatment plan that was already taking shape. Nothing changes until someone qualified has matched the gene to your medicines.
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The team checks which planned drugs are affected
Often only one drug in the plan depends on that gene. The rest of the treatment goes ahead as it would have anyway.
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They choose one of three responses
A much lower dose, a different drug that does not depend on that gene, or the usual drug with closer monitoring. Published guidelines suggest which response fits each gene and drug pair.
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The result goes on your records
It should be written in your hospital file and, ideally, on your discharge summaries and prescriptions, so it is not lost when you change hospitals or doctors.
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You carry it for life
Your genes do not change. The result will matter again for any future prescription that uses the same pathway, including medicines that have nothing to do with cancer.
On your report
The words around a metaboliser result, in plain language
- Metaboliser status
- How fast your version of a gene processes certain drugs: normal, intermediate, poor, and for some genes rapid or ultrarapid.
- Poor metaboliser
- Both copies of the gene work poorly or not at all. Processing through that pathway is very slow.
- Diplotype
- The pair of gene versions you carry, one from each parent, written like *4/*4. The label is worked out from this pair.
- Activity score
- A number some reports give for genes such as DPYD and CYP2D6. The lower the score, the slower the gene.
- Prodrug
- A medicine given in an inactive form that the body must switch on. Tamoxifen and codeine work this way.
- Phenoconversion
- When another medicine slows a gene's enzyme, so a person with normal genes behaves like a poor metaboliser while taking it.
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Side by side
Too much drug or too little: how to tell which
The drug is active as given
The drug must be switched on
Fluorouracil, capecitabine, mercaptopurine, irinotecan
Tamoxifen, codeine, tramadol
A slow gene lets the drug build up
A slow gene leaves too little active drug
The risk is severe side effects
The risk is weaker benefit or poor pain relief
Usual answer: a smaller dose, or avoid
Usual answer: consider a different drug
Being straight with you
What this page cannot tell you
It cannot tell you what your own result means for your own treatment. That depends on the exact gene versions found, the drugs in your plan and your other medicines. What your specific result means is a question for the oncologist or genetic counsellor who ordered the test.
Where the evidence is debated
Some gene and drug pairs are firmly established, such as DPYD with fluorouracil. Others are argued over. For CYP2D6 and tamoxifen, studies disagree on how much the result changes outcomes, and not every oncologist tests for it. Studies in Indian patients are small for most pairs.
Tests do not look at every variant
Laboratories check a set list of known gene versions. A rare version may be missed, and a result is only as complete as the panel used. Ask which variants were covered if the report does not say.
Who this does not apply to
If you are not taking or planning a drug linked to the gene, the result changes nothing today. It is not a cancer risk test, and it is different from tumour tests that guide targeted treatment.
Commonly believed
Four things people assume about a poor metaboliser result
"Poor metaboliser means my body is weak or my liver is failing."
It describes one inherited enzyme, not your overall health. Your liver and kidneys can be entirely normal. Many people only discover the result when a test is done before treatment.
"It means I cannot have chemotherapy."
It usually affects one drug. Most treatment plans go ahead with a dose change or a swap for that one medicine, and the remaining drugs are unaffected.
"A smaller dose means the treatment will not work."
For drugs that build up, the smaller dose gives you roughly the same exposure a normal dose gives other people. The dose is cut to match your body, not to weaken the treatment.
"If I am poor for one gene, every medicine is dangerous for me."
Each gene handles its own set of drugs. A poor DPYD result says nothing about how you handle tamoxifen, antibiotics or most everyday medicines.
Questions we are asked
Common questions about poor metaboliser results
Is a poor metaboliser result the same as a drug allergy?
No. An allergy is the immune system reacting to a drug. A poor metaboliser result is about how fast the body processes it. Both are worth recording, but they are handled differently, and a metaboliser result often means a dose change rather than avoiding the drug completely.
Will I always be a poor metaboliser?
For that gene, yes. Your genes do not change, so the result stays true for life. Other medicines can make things worse by slowing the same enzyme further, which is why every doctor should see the full list of what you take.
I am a CYP2D6 poor metaboliser on tamoxifen. Should I stop?
Do not stop on your own. Guidelines suggest discussing whether a different hormone medicine would suit you, but the evidence is debated and the right answer depends on your age, menopause status and other treatment. Take the report to your oncologist and ask directly.
Can a poor metaboliser result be wrong?
Laboratory errors are uncommon but possible, and a result that does not fit how you have handled a drug before is worth questioning. Your oncologist may repeat the test or check it at a second laboratory before making a major change.
Should my children or siblings be tested?
Not routinely. Each child inherited one of your two copies, and siblings may share your result. It carries no cancer risk, so testing only helps when one of the linked drugs is being considered. Telling relatives about your result is usually enough.
Does it affect everyday painkillers?
It can. A CYP2D6 poor metaboliser gets little pain relief from codeine or tramadol, because the body cannot switch them on properly. Paracetamol and many other painkillers are not affected. Tell whoever manages your pain about the result.
What should I do with the report?
Keep the original safe and a photo on your phone. Show it to every doctor, dentist and pharmacist who prescribes for you, including after cancer treatment ends. A result that stays in one hospital file cannot protect you anywhere else.
Does this result affect insurance or marriage?
It is a note about how you handle certain drugs, not a disease and not a cancer risk. It is not the kind of finding that marks a family. If you are worried about disclosure, raise it with the counsellor who explains the result.
Your Specialists
Meet CION's oncologists. Bring your family history or genetic report to them.
Our medical oncologists see people with a strong family history of cancer, arrange genetic counselling and testing where it fits, and plan the checks that follow.
Medical Oncologist
Dr. C. Raghavendra Reddy
MBBS(Gold Medal), DNB(General Medicine), DM(Medical Oncology)(Gold Medal)
Medical Oncologist
Dr. Bharati Devi Gorantla
MBBS, MD(General Medicine), DM(Medical Oncology)(Adyar,Chennai), ECMO, MRCP SCE(UK)
Medical Oncologist
Dr. Owais Mohammed
MBBS, MD (General Medicine), DrNB (Medical Oncology), ECMO, MRCP SCE (Medical Oncology) (UK)
Medical Oncologist
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Sources
- Clinical Pharmacogenetics Implementation Consortium (CPIC) — CPIC Guidelines
- Clinical Pharmacogenetics Implementation Consortium (CPIC) — CPIC Guideline for Fluoropyrimidines and DPYD
- MedlinePlus Genetics — What is pharmacogenomics?
- US Food and Drug Administration — Table of Pharmacogenomic Biomarkers in Drug Labeling
This page is general information, not a prescription. Do not change or stop any treatment based on what you read here. If anything is worrying you, contact your own treating team — or call our helpline and we will help you reach the right specialist.
Keep reading
Related pages
Intermediate metaboliser results and dose adjustmentThe middle result, and how it differs from poorTPMT and NUDT15 before thiopurinesThe two genes behind thiopurine dosingCYP2D6 and tamoxifenThe debated pair, explained properlyCarrying your pharmacogenomic result for future careMaking sure every future prescriber sees it
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