Pharmacogenetics !

How inherited differences in genes change the way a person handles a drug or reacts to it, and when a genetic test can guide treatment.

Updated 2026-11-01

Pharmacogenetics studies gene variants that change drug response. Some variants change pharmacokinetics (enzymes that activate or clear a drug), others change the target or the immune system's reaction. Regulators list these gene–drug pairs in labels, and expert groups such as CPIC publish guidance on how to use test results.

Enzyme variants (pharmacokinetics)

CYP2C9 (and VKORC1, the warfarin target): warfarin sensitivity
CYP2C19: clopidogrel activation
CYP2D6: codeine and tramadol activation; many antidepressants
TPMT and NUDT15: azathioprine and mercaptopurine; low activity causes severe myelosuppression
DPYD: fluorouracil and capecitabine; deficiency causes severe toxicity
BCHE (plasma cholinesterase): suxamethonium; atypical enzyme prolongs paralysis
NAT2 (acetylation): slow acetylators keep sulfapyridine (from sulfasalazine) longer

Immune and target variants

HLA-B*57:01: abacavir hypersensitivity (screening before starting)
HLA-B*15:02: carbamazepine and severe skin reactions, mainly in people of Asian ancestry
G6PD deficiency: haemolysis with primaquine, dapsone or rasburicase
RYR1 variants: malignant hyperthermia with volatile anaesthetics and suxamethonium

Using the results

The FDA Table of Pharmacogenomic Biomarkers in Drug Labeling lists which labels mention a gene and in which section (boxed warning, dosing, contraindications). CPIC guidelines translate a genotype into a recommendation (use as usual, adjust, or choose another drug). A result is lifelong, so it is worth recording in the patient's chart.

Limits

Genes explain only part of the variation: age, kidney and liver function, interactions, adherence and diet also matter. Allele frequencies differ between populations, including within Latin America, so data from one population may not fit another.

Test yourself

Think of the answer, then tap the question to see it.

Why does a CYP2C19 poor metaboliser get less benefit from clopidogrel?

Clopidogrel is a prodrug activated mainly by CYP2C19; with two loss-of-function alleles less active metabolite forms, so platelet inhibition is weaker.

Why can codeine be dangerous in a CYP2D6 ultrarapid metaboliser?

CYP2D6 converts codeine to morphine; ultrarapid metabolisers make more morphine faster, with a risk of opioid toxicity.

Which test is done before starting abacavir and why?

HLA-B*57:01, because carriers have a high risk of a serious hypersensitivity reaction.

What do TPMT and NUDT15 variants change?

They reduce the breakdown of thiopurines such as azathioprine and mercaptopurine, raising the risk of severe bone marrow suppression.

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Example drugs

← All of pharmacology

Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: Table of Pharmacogenomic Biomarkers in Drug Labeling (U.S. Food and Drug Administration, 2026); What is pharmacogenomics? (MedlinePlus Genetics (U.S. National Library of Medicine), 2026); DailyMed - WARFARIN SODIUM tablet (DailyMed (U.S. National Library of Medicine), 2026); DailyMed - CLOPIDOGREL BISULFATE tablet (DailyMed (U.S. National Library of Medicine), 2026).

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Representative model, for educational purposes only.