Medical Oncologist
CION Cancer Clinics
Pharmacogenomics: why your genes change your dose | CION Cancer Clinics
Your genes build the enzymes that break down and clear medicines. If you inherited a slow version, a normal dose of certain cancer drugs can build up and cause severe side effects. This page explains how that happens, the four ways a gene can change a drug's behaviour, what else sets your dose, and who does not need to think about this at all. At CION Cancer Clinics, our team helps carriers and their families plan checks, next steps and support after a genetic result.
NG
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
17+specialists on panel
15,000+patients treated
35+centres across Telangana & AP
4.8★ / 800+Google rating
On this page
- Why can the same dose affect two people so differently?
- Four ways a gene can change how a drug behaves
- What happens to a drug after it is given?
- The words you will meet, in plain language
- Genes are one reason your dose is what it is
- What this page cannot tell you
- Four things families tell us, and what is actually true
- Common questions about genes and drug doses
The short answer
Why can the same dose affect two people so differently?
Because the enzymes that handle a drug are built from gene instructions, and those instructions differ slightly from person to person. If yours make a slow enzyme, a drug can build up and a normal dose can act like a large one. If they make a fast enzyme, the drug may clear before it has done its work.
This is inherited, and it is fixed
These differences are present from birth, in every cell. They came from your parents and they do not change with age, diet or treatment. That is why a single test can answer the question for life, and why brothers and sisters may share the same result.
Why it matters more in cancer care
Many cancer drugs work within a narrow margin. The dose that treats the cancer is not far from the dose that harms healthy cells. For most drugs and most people, that margin is managed with blood tests and careful watching. For a few drugs, one inherited difference can push a person straight past it, which is where a gene test earns its place.
This page is about inherited genes and medicines. It is not about inherited cancer risk, which is a separate question.How genes get involved
Four ways a gene can change how a drug behaves
Doctors group these effects in a few simple ways. Knowing which one applies tells you what the result is likely to change.
It clears the drug too slowly
The drug stays in the body longer than planned and its side effects become severe. This is the pattern with DPYD and fluorouracil, and with TPMT or NUDT15 and the thiopurine tablets used in leukaemia.
Usually leads to
- A lower starting dose
- Closer blood tests early on
It fails to switch the drug on
Some medicines must be changed by an enzyme before they work. If the enzyme is weak, less of the active form is made. Tamoxifen and CYP2D6 are the example people ask about, though the evidence here is mixed.
It sets off an immune reaction
Certain HLA genes, which help the immune system recognise the body, can make a person prone to a rare but serious skin reaction to a specific drug. The fix is to avoid that drug, since dose changes do not help.
It leaves a cell unprotected
In G6PD deficiency, red blood cells lack a protective enzyme. Some drugs, including rasburicase, can then break those cells apart. Here too the answer is a different drug.
Not sure whether this applies to you?
Ask an oncologistInside the body
What happens to a drug after it is given?
-
It enters the blood
Whether it is swallowed as a tablet or given through a drip, the drug ends up in the bloodstream and travels round the body, reaching the cancer and healthy tissue alike.
-
Enzymes start to break it down
Much of this happens in the liver. Each enzyme is built from a gene. This is the step where an inherited difference usually shows itself.
-
Some drugs are switched on along the way
A few medicines only become active after an enzyme changes them. For these, a weak enzyme means less active drug rather than more.
-
What is left is cleared
The broken-down drug leaves through the urine or the gut. Kidney and liver health affect this step, which is why those blood tests are checked before every cycle of chemotherapy.
-
The level in the blood decides the effect
Too little and the drug may not work well. Too much and side effects climb. The planned dose assumes average enzymes, so a slow or missing enzyme is what turns a standard dose into the wrong one.
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.
- Genotype
- The exact letters the laboratory read in your gene. This is the raw result.
- Phenotype
- What that genotype means for the enzyme, written as normal, intermediate or poor metaboliser. This is the line your doctor acts on.
- Star allele
- A shorthand name for a known gene version, written with an asterisk and a number after the gene name. It looks technical but is just a label.
- Prodrug
- A medicine that is given in an inactive form and has to be switched on by an enzyme before it works.
- Germline
- Present in every cell from birth, and therefore inherited. Drug-gene tests read germline genes. A somatic change is one found only inside the tumour.
Leave a number, we will call you
One field. No form to fill in, and no charge for the call.
Side by side
Genes are one reason your dose is what it is
Set by your genes
Set by everything else
How fast a particular enzyme works
How well your kidneys and liver work today
Whether a prodrug is switched on properly
Your height and weight, used for most doses
Whether you react to a specific drug
Other medicines and herbal remedies you take
The same for life, in any hospital
Changes during treatment and is rechecked
Being straight with you
What this page cannot tell you
It cannot tell you whether your dose is right. Doses are worked out from your size, your blood tests, your kidney and liver function, the other medicines you take and, for a few drugs, a gene result. Only your oncologist has all of those in front of them.
It cannot interpret your result
Reports use star allele names and phenotype labels that differ between laboratories. The same gene version can carry different advice for different drugs. What your specific result means for your treatment is a question for the doctor who ordered the test.
Who this does not apply to
Most people having cancer treatment will never need a drug-gene test. Most cancer drugs have no gene test at all, and for them the dose is guided by size, blood counts and how you cope. Evidence is also thinner for Indian patients than for European ones. Much of the research was done abroad, and studies in Indian families so far are small.
A slow enzyme is not a disease. It only matters when one of a few specific drugs is given.Commonly believed
Four things families tell us, and what is actually true
"A strong body can take a strong dose."
Strength and fitness do help with recovery, but they do not change how fast an enzyme clears a drug. A fit young person with a slow enzyme can react more severely than a frail older one with a normal enzyme.
"He reacted badly, so the hospital gave him the wrong dose."
Not necessarily. Severe side effects happen for many reasons, and most are not caused by genes at all. A gene test can rule one cause in or out. It cannot explain every reaction.
"A lower dose means weaker treatment."
For someone with a slow enzyme, a lower dose can produce the same level of drug in the blood as a full dose does in others. The aim is the right amount for that body.
"If my result is normal, my children's will be too."
Not always. Children inherit one copy of each gene from each parent, so their result can differ from yours. It only matters if they ever need one of the drugs involved.
Questions we are asked
Common questions about genes and drug doses
Is pharmacogenomics the same as genetic testing for cancer?
No. Genetic testing for cancer risk looks for inherited faults that raise the chance of getting cancer. Pharmacogenomics looks only at genes that affect how medicines are handled. The two use similar laboratory methods, but they answer completely different questions and one does not replace the other.
Can my dose change after the test?
It can. If the result shows a slow enzyme, your doctor may lower the starting dose and raise it once they see how you cope. If it shows a missing enzyme, they may choose another drug. A normal result usually means the standard plan goes ahead unchanged.
Do Indians process cancer drugs differently?
Some gene versions are more or less common depending on ancestry. NUDT15 differences, for example, are more common in Asian populations, and G6PD deficiency is found in several Indian communities. That said, ancestry cannot replace a test for any single person.
Does diet or lifestyle change the result?
The gene result itself never changes. What can change is how an enzyme behaves on a given day. Some medicines, herbal products and even grapefruit juice can slow certain enzymes down. Tell your oncologist about everything you take, including traditional remedies.
Why were my relatives never tested?
Because the test only matters when one of a few drugs is planned. Most people go through life without ever needing it. A result in one family member does not mean the others should be tested now. It becomes relevant only if they are prescribed one of those drugs.
What does poor metaboliser mean?
It means the enzyme made from that gene barely works or does not work. For a drug cleared by that enzyme, a standard dose could build up to harmful levels. It does not mean you are unwell. Ask the doctor who ordered the test what it means for your specific treatment.
Should I keep the report?
Yes. The result holds for life. Keep a paper copy and a photo on your phone, and show it to any doctor or pharmacist who starts you on a new medicine. It can save you from repeating the test and from a reaction that was preventable.
Does a normal result mean I will have no problems?
No. It means one known cause of severe reactions has been checked and ruled out. You can still get the usual side effects of treatment, and your team will still watch your blood counts closely. Report any worrying symptom as you normally would.
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
Medical Oncologist
Want a specific doctor for your case? Mention them when booking.
Book Free ConsultationBook an appointment with our specialist
Share your name and number — we'll call you back within 30 minutes to schedule your consultation.
Sources
- MedlinePlus Genetics — What is pharmacogenomics?
- Clinical Pharmacogenetics Implementation Consortium (CPIC) — CPIC Guidelines
- US Food and Drug Administration — Table of Pharmacogenomic Biomarkers in Drug Labeling
- MedlinePlus Genetics — Dihydropyrimidine dehydrogenase deficiency
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
Which cancer drugs have a gene test before themThe short list of drug and gene pairsWhat a poor metaboliser result meansIf your report uses that phraseIntermediate metaboliser results and dose adjustmentThe in-between result, explainedCYP2D6 and tamoxifenThe pair people ask about most, and why evidence is mixed
Talk to us
Wondering whether your genes affect your treatment?
Tell us which drugs are planned and we will explain whether any of them has a gene test attached. We will answer honestly, including when a test would change nothing. One helpline serves every CION centre.