Pharmacogenomics is the study of how an individual’s genetic makeup influences their response to medicines. Rather than treating everyone the same, it asks a simple but powerful question: will this drug work well for this person, and will it cause harm? For UK laboratory science, biotechnology and clinical diagnostics, pharmacogenomics is moving from research curiosity to practical tool. It sits at the heart of modern personalised medicine, helping clinicians choose the right drug at the right dose while avoiding serious adverse reactions.

What pharmacogenomics actually means

Your genes encode enzymes, transporters and receptors that determine how a medicine is absorbed, distributed, metabolised and excreted. Small genetic variants — often single nucleotide polymorphisms — can change the activity of these proteins. Some people metabolise a drug so quickly that it is cleared before it can work. Others metabolise it so slowly that standard doses build up to toxic levels. Pharmacogenomics identifies these differences before a prescription is written.

This is not about rare inherited diseases. The variants involved are common. For example, around 8% of people of European ancestry are poor metabolisers of CYP2D6, while up to 20% of East Asian individuals carry a CYP2C19 variant that reduces clopidogrel activation. In a UK population as diverse as ours, these frequencies matter every day in hospitals and general practice.

The key enzymes and genes that shape drug response

Most pharmacogenomic testing in the UK focuses on a defined set of genes with strong evidence for clinical action. The cytochrome P450 family is central. CYP2D6 handles codeine, tramadol, tamoxifen and many antidepressants. CYP2C19 activates clopidogrel and metabolises proton pump inhibitors and some antifungals. CYP2C9 is crucial for warfarin and phenytoin. Other important examples include TPMT and NUDT15 for thiopurines such as azathioprine, DPYD for fluorouracil and capecitabine, and HLA-B*57:01 for abacavir hypersensitivity.

• CYP2D6: codeine, tramadol, tamoxifen, many antidepressants and antipsychotics.
• CYP2C19: clopidogrel, citalopram, omeprazole, voriconazole.
• CYP2C9: warfarin, phenytoin, NSAIDs.
• TPMT/NUDT15: azathioprine, mercaptopurine, thioguanine.
• DPYD: fluorouracil, capecitabine, tegafur.
• HLA-B*57:01: abacavir; HLA-B*15:02: carbamazepine.

Each gene has star alleles — numbered variants that predict enzyme activity. A patient might be a poor, intermediate, normal or ultra-rapid metaboliser. That classification, not the raw genotype, guides the prescribing decision.

How UK laboratories test for pharmacogenomic variants

Testing typically starts with a blood sample or a buccal swab. In NHS laboratories, genotyping is usually performed by polymerase chain reaction (PCR) followed by allele-specific probes, or by microarray and next-generation sequencing panels. Some specialist centres use long-read sequencing to resolve complex regions such as the CYP2D6 gene, which is prone to structural variation. Results are reported as a diplotype, then translated into a predicted metaboliser phenotype.

Turnaround times vary. Reactive testing — when a result is needed before a specific drug — can be delivered in a few days. Pre-emptive testing, where a panel is stored in the electronic record for future use, is increasingly favoured. The NHS Genomic Medicine Service and Genomic Laboratory Hubs are working to make pharmacogenomic panels more accessible, but coverage is still uneven across the UK. Point-of-care tests exist for a few variants, yet most testing remains laboratory-based for reasons of quality, scale and cost-effectiveness.

Where it changes prescribing today

Several clinical areas already benefit. In cardiology, CYP2C19 genotyping before clopidogrel after percutaneous coronary intervention helps identify poor metabolisers who may need an alternative antiplatelet. In oncology, DPYD testing before fluoropyrimidine chemotherapy is now recommended to prevent severe or fatal toxicity, and TPMT/NUDT15 testing before thiopurines is standard in many centres. In psychiatry, CYP2D6 and CYP2C19 results can guide antidepressant and antipsychotic selection, reducing trial-and-error prescribing. In pain management, codeine and tramadol are ineffective in some CYP2D6 poor metabolisers and dangerously potent in ultra-rapid metabolisers, particularly in breastfeeding women.

HIV medicine uses HLA-B*57:01 screening to avoid abacavir hypersensitivity, and epilepsy services use HLA-B*15:02 to reduce carbamazepine-induced Stevens-Johnson syndrome in at-risk populations. These are not theoretical benefits. They are measurable reductions in hospital admissions and patient harm.

Practical steps for safer prescribing and patient conversations

Pharmacogenomics does not replace clinical judgement. It adds a layer of evidence. When a result is available, prescribers should consider the drug’s therapeutic window, the availability of alternatives, and the patient’s other medicines, age, renal and liver function. A poor metaboliser of CYP2D6 may still receive a standard drug if the medicine is a prodrug and the clinical effect is monitored closely — but often a dose change or switch is safer.

• Check whether a pharmacogenomic result already exists in the patient’s record before prescribing a high-risk drug.
• Use validated clinical decision support tools that translate genotype into actionable advice.
• Document the result clearly, including the phenotype and the date of testing.
• Explain to patients that a genetic result is lifelong and can inform future prescriptions, not just the current one.
• Respect consent and data governance — genomic data is sensitive and must be handled within UK GDPR and NHS information standards.

Laboratory scientists and clinical scientists play a vital role in ensuring results are accurate, interpretable and communicated in plain language. Warm, clear conversations reduce anxiety and build trust. A patient who understands why a drug was changed is more likely to adhere to treatment and report side effects early.

The road to routine use in the NHS

Pharmacogenomics is already improving care in selected specialties, but wider adoption requires investment in laboratory capacity, standardised reporting, clinician education and equitable access. Biodiversity in the UK population means that no single test panel will fit everyone; local demographics should guide service design. The future lies in pre-emptive panels, integrated into electronic prescribing systems, so that a patient’s genetic profile is available at the moment of decision. That is personalised medicine in practice — not a distant ambition, but a practical extension of the diagnostic laboratory into everyday prescribing.