Maarg Manthan · Topic 5.16

Role of Genetics/Pharmacogenetics in Pharmacology

Science & Technology › Human Health and Diseases · Topic 5.16

Pharmacogenetics is the study of how a person's genes affect the way they respond to drugs, and pharmacogenomics is the wider study across the whole genome. Differences in genes for drug-metabolising enzymes, drug transporters and drug targets explain why the same dose can work well in one patient, fail in another and cause serious side effects in a third. The goal is personalised or precision medicine, in which the choice and dose of a drug is matched to the genetic make-up of the patient. Well-known examples include CYP2C19 variants that change the effect of the heart drug clopidogrel, HLA-B*1502 that predicts severe skin reactions to the epilepsy drug carbamazepine, and TPMT variants that affect thiopurine drugs. Genetic testing before treatment can improve safety.

5 min read · UPSC Prelims · Science & Technology · Human Health and Diseases

At a Glance

Aim Right drug and dose for each genotype
Key genes CYP2D6, CYP2C19, TPMT, HLA-B
Wider field Pharmacogenomics
Result Personalised medicine
Indian projects IndiGen, GenomeIndia
Exam link UPSC Prelims, Science & Technology: Human Health and Diseases

Where Does It Fit?

This is the sixteenth topic of the macro on human health and diseases. It joins genetics with pharmacology and prepares for the closing topic on medical imaging. Previous topic: Drug Cost Structure and Pricing in India. Next topic: Imaging Methods: CT Scan, MRI, PET Scan.

How Do Genes Change Drug Response?

Gene or marker	Effect on drug response
CYP2D6	Enzyme that breaks down many drugs, such as some antidepressants and codeine. Slow or ultra-fast metabolisers need different doses
CYP2C19	Activates the antiplatelet drug clopidogrel. Some people have reduced function and get less benefit
CYP2C9 and VKORC1	Affect the dose of the blood thinner warfarin
TPMT	Breaks down thiopurine drugs used in leukaemia and autoimmune disease. Low activity causes severe toxicity at normal doses
HLA-B*1502	Predicts severe skin reactions such as Stevens-Johnson syndrome to carbamazepine, common in some Asian populations
HLA-B*5701	Predicts a hypersensitivity reaction to the HIV drug abacavir
G6PD deficiency	An X-linked trait in which some drugs such as primaquine cause red cell destruction, as in Non-Chromosomal (Mendelian) Inheritance Disorders

How Is Pharmacogenetics Used?

Testing before treatment: a blood or saliva sample is tested for gene variants before certain drugs are prescribed.
Dose adjustment: doses are raised, lowered or changed to another drug according to the result.
Cancer therapy: tumours are tested for mutations to choose targeted drugs, such as trastuzumab for HER2-positive breast cancer, as in Monoclonal Antibody Therapy.
Drug development: companies test drugs in people with specific genotypes to increase the chance of success, and drugs are approved with companion diagnostic tests.
Safer prescribing: fewer adverse reactions and fewer hospital stays, which saves cost.
Population studies: allele frequencies differ between populations, so Indian data are needed for Indian patients.

What Else Should You Remember and Why Does It Matter?

Indian projects: the CSIR’s IndiGen programme sequenced about a thousand Indian genomes and made an open database, and the GenomeIndia project, reported in 2025, has sequenced thousands more genomes from many groups. These give the base for pharmacogenomic research.
Genetic diversity: India has many endogamous populations with distinct variants, so tests built on European data may not fit.
Cost and access: sequencing costs have fallen sharply but testing is not yet widely available in public hospitals.
Ethics and privacy: genetic data are sensitive, so consent, storage and protection against discrimination are important, as in Molecular Biology and Human Disease.
Not only genes: age, weight, diet, other drugs, liver and kidney function and the gut microbiome also affect drug response.
Nutrigenomics: the study of how genes affect response to nutrients, a related field.
Links to AMR: genomic tests of bacteria show their resistance genes so that the right antibiotic can be chosen, as in Antibiotic Medications and Antimicrobial Resistance (AMR).
Exam pattern: definitions, gene-drug examples and the role of genome programmes.
CYP enzymes: CYP enzymes, short for cytochrome P450 enzymes, process around three quarters of the drugs in common use, so variants in their genes matter widely.
Genetic variation: genetic variation between people is the source of different drug responses, and some variants are common in one region but rare in another.
Adverse drug reactions: adverse drug reactions cause many hospital admissions, and part of them could be avoided by genetic testing and by dose advice.
Single nucleotide polymorphism: most gene variants in pharmacogenetics are single base changes called SNPs, which tests read on a chip or by sequencing.
Regulators and labels: drug labels in some countries now list genetic markers, and regulators approve certain cancer drugs only with a matching test.
Cost-effectiveness: testing makes economic sense when the reaction is severe, the drug is widely used, and a test is cheap, as with HLA tests before some drugs.

As an analytical point, one-size-fits-all dosing ignores human diversity. Pharmacogenetics promises safer and more effective care, but needs local data, trained doctors and affordable tests.

Why Is This Topic Important?

Facts Gene-drug examples asked
Concept Personalised medicine
Policy Genome projects

What Came Next?

The next topic covers imaging methods: CT scan, MRI and PET scan. Next topic: Imaging Methods: CT Scan, MRI, PET Scan.

Key People and Terms

Pharmacogenetics Study of genes and drug response
Genotype Genetic make-up
Metaboliser Person who breaks down a drug
Companion diagnostic Test used with a drug
Allele frequency Share of a gene variant in a population
Precision medicine Care matched to the patient

Exam Corner

Points to Remember

CYP2C19 and clopidogrel.
HLA-B*1502 and carbamazepine.
TPMT and thiopurines.
G6PD and primaquine.
IndiGen: CSIR genome project.

Do Not Confuse With

Pharmacogenetics and pharmacogenomics One gene versus whole genome
Pharmacokinetics and pharmacodynamics What the body does to the drug versus drug action
Genotype and phenotype Gene make-up versus observed response

Memory Hook

Genes decide the drug, CYP breaks it down; HLA warns of reactions, TPMT dose to crown; IndiGen maps Indian codes.

Mains Angle

Questions ask about precision medicine and ethics. These points add depth.

Concept Gene-drug links
Use Testing and dosing
Issues Cost, ethics, data

A Question You May Face

An original practice question, not a past paper question.

“Explain the concept of pharmacogenetics and discuss its significance and challenges for health care in India.”

How to Answer

Concept Gene-drug.
Examples CYP, HLA.
India Genome data.
Issues Ethics, cost.

GS Relevance

Prelims: definitions, gene-drug examples, Indian genome projects. GS Paper 3: science and technology, biotechnology, health.

Frequently Asked Questions

What is pharmacogenetics?

Pharmacogenetics is the study of how differences in a person's genes affect their response to drugs, including how fast the drug is broken down, how well it works and the risk of side effects. It aims to choose the right drug and dose for each patient, which is called personalised medicine.

How do CYP enzymes affect drug action?

CYP enzymes, such as CYP2D6 and CYP2C19, are made in the liver and break down or activate many medicines. Variants of their genes make some people slow metabolisers, who may have too much drug and side effects, or fast metabolisers, who may have too little, so doses or drug choice may need to change.

Why is HLA-B*1502 testing important?

The HLA-B*1502 variant is found more often in some Asian populations and predicts a high risk of severe skin reactions, such as Stevens-Johnson syndrome, to the epilepsy and nerve pain drug carbamazepine. Testing before treatment lets doctors choose a safer drug, which shows how genetic tests can prevent serious harm.

What is IndiGen?

IndiGen is a genome sequencing programme of the Council of Scientific and Industrial Research, which sequenced about a thousand Indian genomes and released data as a reference for research. It helps to study genetic variants in the Indian population, including those that affect disease and drug response.

What are the limits of pharmacogenetic testing?

Genes explain only part of the variation in drug response, since age, weight, diet, other drugs and organ function also matter. Tests can be costly and are not widely available, reference data from Indian populations are still limited, and genetic information raises questions of privacy and consent.

PYQ Practice — Statement Analysis

1 Pharmacogenetics studies how genes affect response to drugs.

True

It supports personalised medicine.

2 CYP enzymes are mainly found in the kidney.

False

They are mainly in the liver.

3 HLA-B*1502 is linked with reactions to carbamazepine.