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CION Cancer Clinics
Which cancer drugs have a gene test before them? | CION Cancer Clinics
A handful of cancer drugs have a gene test before the first dose. The main ones are fluorouracil, capecitabine, irinotecan, the thiopurine tablets used in childhood leukaemia, and rasburicase. The test reads how well your body clears the drug, so the dose can be made safe for you. This page lists the pairs, explains what the test does and does not tell you, and who does not need it. 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
- Which cancer drugs have a gene test before them?
- Which drug goes with which gene?
- Where does the test fit into treatment planning?
- The words you will meet, in plain language
- A drug-gene test and a tumour test are not the same thing
- What this page cannot tell you
- Four things families tell us, and what is actually true
- Common questions about drug-gene tests
The short answer
Which cancer drugs have a gene test before them?
Only a small group. The best known are fluorouracil and capecitabine, the thiopurine tablets used in childhood leukaemia, irinotecan, and a medicine called rasburicase. For each one, an inherited difference in a single gene can make an ordinary dose far too strong for that person.
What the test is looking for
Your body uses enzymes to break down and clear a drug. Each enzyme is built from a gene instruction. If you inherited a weak version of that instruction, the drug stays in your body longer and builds up. A normal dose then behaves like an overdose. A simple blood or saliva test can read that gene before the first dose is given.
What it does not do
This test does not decide whether a drug will work on your cancer. It decides how safely your body can handle the drug. Choosing the drug itself is often based on a separate test on the tumour, which is covered on our targeted therapy pages. The two are easily confused, and families often think one test was done when it was the other.
Most cancer drugs have no gene test before them. The list is short and changes slowly.The main pairs
Which drug goes with which gene?
Each of these pairs has published guidance behind it. How strictly the test is required differs between countries.
Fluorouracil and capecitabine: DPYD
These are used widely in bowel, stomach, breast and head and neck cancer. The DPYD gene makes the enzyme that clears them. People with a weak or missing enzyme can have severe diarrhoea, mouth sores and dangerously low blood counts. European regulators recommend testing before the first dose.
Mercaptopurine and thioguanine: TPMT and NUDT15
These tablets are the backbone of maintenance treatment in childhood acute lymphoblastic leukaemia. A weak TPMT or NUDT15 gene can drop the blood counts sharply. NUDT15 differences are more common in people of Asian ancestry, which makes this pair especially relevant in India.
Irinotecan: UGT1A1
Used mainly in bowel and pancreatic cancer. A common weak version of UGT1A1, the same one behind the harmless condition called Gilbert syndrome, can raise the chance of severe diarrhoea and a low white cell count.
Rasburicase: G6PD
Given to protect the kidneys when a fast-growing cancer breaks down quickly. In people with G6PD deficiency it can destroy red blood cells, so it is not safe to use. G6PD deficiency is found in several Indian communities.
Not sure whether this applies to you?
Ask an oncologistBefore the first dose
Where does the test fit into treatment planning?
The drug is chosen first
Your oncologist decides on a treatment plan based on the cancer, its stage and your overall health. Only once a drug on the short list is chosen does the gene test become relevant.
A sample is taken
For most of these tests a routine blood sample is enough. It can often be taken on the same visit as your other pre-treatment blood tests. No fasting is needed.
The laboratory reads one or two genes
This is a narrow test. It looks only at the genes linked to that drug and says nothing about your risk of cancer or about your relatives' health.
The dose is set
If the result is normal, the standard dose is used. If it shows a weak enzyme, the doctor may lower the starting dose and raise it carefully, or choose a different drug. Only a doctor can make that call.
The result stays with you
Your genes do not change, so this result holds for life. Keep a copy and show it to any doctor who prescribes you a new medicine.
On your report
The words you will meet, in plain language
- Pharmacogenomics
- The study of how your inherited genes change the way your body handles a medicine. It is sometimes shortened to drug-gene testing.
- Enzyme
- A protein that does a job in the body, such as breaking a drug down so it can be cleared.
- Normal metaboliser
- Your enzyme works as expected. The standard dose is usually used.
- Intermediate metaboliser
- Your enzyme works partly. The drug may need a lower starting dose.
- Poor metaboliser
- Your enzyme barely works or does not work. The drug may need a much lower dose, or a different drug may be chosen.
- Germline
- Present in every cell from birth. Drug-gene tests read your germline genes, which is why a blood sample works. The opposite word is somatic, meaning a change found only inside the tumour.
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Side by side
A drug-gene test and a tumour test are not the same thing
Drug-gene test
Tumour test
Reads your own inherited genes
Reads changes found only in the cancer
Uses a blood or saliva sample
Usually uses a biopsy or surgery sample
Asks how safely you can take a drug
Asks whether a drug is likely to work
Holds for life and for any future drug
Can change as the cancer changes
Being straight with you
What this page cannot tell you
It cannot tell you whether your treatment plan needs one of these tests. That depends on the exact drugs chosen, the doses planned and what else you are taking. Your oncologist is the person who decides, and it is fair to ask them directly before treatment starts.
It cannot read a result you are holding
Laboratories use different names and codes for the same gene differences. A result that says one thing to a pharmacist may say something else to a family member reading it online. What your specific result means for your dose is a question for the doctor who ordered the test.
Who this does not apply to
Most people having cancer treatment will not need a drug-gene test, because most cancer drugs do not have one. Surgery and radiotherapy have none at all. If none of the drugs named here is in your plan, this test is unlikely to change anything for you. Evidence for some other pairs, such as tamoxifen and CYP2D6, is still mixed, and guidelines disagree on whether testing helps.
A normal result does not mean a drug will have no side effects. It means one known cause of severe reactions has been checked.Commonly believed
Four things families tell us, and what is actually true
"The biopsy already tested his genes, so this is done."
A biopsy test looks at the tumour. It does not usually report the inherited genes that decide how the body clears a drug. The two are separate tests on separate samples.
"If the test is abnormal, she cannot have chemotherapy."
Usually she still can. Most abnormal results lead to a lower starting dose that is raised if she copes well. A different drug is chosen only when the enzyme is barely working.
"Nobody in our family reacted badly to medicine, so we are fine."
Most relatives have never taken these particular drugs, so there was nothing to react to. A weak enzyme stays hidden until the drug is given.
"This test will tell us if cancer runs in the family."
It will not. A drug-gene test looks only at how medicines are handled. Inherited cancer risk needs a different test and a conversation with a genetic counsellor.
Questions we are asked
Common questions about drug-gene tests
Is DPYD testing done before capecitabine in India?
It is available at several laboratories, but it is not yet done for every patient everywhere. Practice varies between hospitals and between doctors. If capecitabine or fluorouracil is in your plan, ask your oncologist whether a DPYD test is advised and how soon the result can come back.
Does the test delay the start of treatment?
Sometimes by a few days, depending on the laboratory. Doctors weigh that short wait against the risk of a severe reaction. In some cases treatment starts at a cautious dose while the result is awaited. Your oncologist will explain what they plan to do in your case.
Is this a genetic test for cancer risk?
No. It reads only the genes that handle a few drugs. It does not look at the genes linked to inherited breast, ovarian or bowel cancer. If you are worried about inherited cancer risk, that is a separate referral to a genetic counsellor.
What if my child is starting leukaemia maintenance tablets?
Ask the treating team whether TPMT and NUDT15 have been checked, or whether blood counts are being used to guide the dose instead. Both approaches are used. NUDT15 matters more in Asian children, so it is a reasonable question to raise early.
Do I need to repeat the test for a new drug?
Not for the same gene. Your inherited genes do not change, so a valid result can be reused for life. A new drug linked to a different gene would need that other gene read, which is why some laboratories offer a wider panel.
Should everyone on tamoxifen be tested for CYP2D6?
Guidelines disagree and most do not require it. Some studies suggested people with a weak CYP2D6 gene gain less from tamoxifen, while others did not. What matters more in daily practice is avoiding certain antidepressants that block the same enzyme. Ask your oncologist before starting any new medicine.
Are the medicines given alongside chemotherapy tested too?
Rarely, though some are on published gene lists. One example is carbamazepine, sometimes used for nerve pain, where an HLA test is advised for some people of Asian ancestry. Your doctor or pharmacist can tell you whether any of your supportive medicines has a gene test attached.
Can I ask for the test myself?
You can raise it, and a good oncologist will welcome the question. The result is only useful to the doctor who is choosing your doses, so the test is best ordered through them. A test ordered alone, without a planned drug, rarely changes anything.
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)
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Sources
- Cancer Research UK — Capecitabine (Xeloda)
- US Food and Drug Administration — Table of Pharmacogenomic Biomarkers in Drug Labeling
- Clinical Pharmacogenetics Implementation Consortium (CPIC) — CPIC Guidelines
- MedlinePlus Genetics — What is pharmacogenomics?
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
Pharmacogenomics: why your genes change your doseHow one gene can turn a normal dose into an overdoseTPMT and NUDT15 before thiopurinesThe pair that matters most in childhood leukaemiaG6PD deficiency and cancer drugsWhy rasburicase is checked against G6PD firstAsking for a drug-gene test before treatment startsHow to raise it with your oncologist
Talk to us
Is a drug-gene test part of your treatment plan?
Ask us which drugs are planned and whether any of them has a gene test attached. We will explain the answer in plain language. One helpline serves every CION centre.