Pharmacogenomics: matching medicines to your genes

Pharmacogenomics is the study of how an individual’s genetic makeup influences their response to medicines. The term combines pharmacology, the science of drugs, with genomics, the study of genes and their functions. For decades, medicine prescribing has largely followed a "one-size-fits-all" approach, relying on standard doses designed for the average person. However, healthcare providers have long observed that the same medicine can work well for one person, cause severe side effects in another, and have no effect at all in a third. Pharmacogenomics seeks to explain these differences at the molecular level, moving healthcare toward a model of personalised prescribing where treatments are tailored to a person’s unique genetic profile.

How genes influence medicine response

Genes provide the instructions for making proteins, including the enzymes responsible for metabolising, or breaking down, medicines in the body. The most well-known family of these enzymes is the cytochrome P450 (CYP) system. Natural variations in the genes that produce these enzymes—often referred to as polymorphisms—can significantly alter enzyme function.

Because of these variations, individuals are generally classified into different metaboliser types. For example, a person might be a "poor metaboliser" for a specific enzyme, meaning their body breaks down a medicine very slowly, potentially leading to a build-up of the drug and an increased risk of toxicity. Conversely, an "ultra-rapid metaboliser" might process a medicine too quickly for it to have any therapeutic effect. In addition to metabolism, genetic differences can also affect how a medicine interacts with its target in the body, a concept known as pharmacodynamics. For instance, variations in cell receptors can determine whether a specific cancer treatment will be able to bind to and destroy a tumour.

Examples of gene-based labelling

Regulatory authorities worldwide have increasingly incorporated pharmacogenomic information into official medicine labels to guide safer prescribing. For example, the antiretroviral medicine abacavir carries a strong warning advising that patients be screened for the HLA-B*5701 gene variant before starting treatment. Individuals with this variant are at a high risk of experiencing a severe, potentially fatal hypersensitivity reaction. Similarly, the antiepileptic drug carbamazepine recommends genetic testing for the HLA-B*1502 allele in certain populations, as its presence is linked to serious skin reactions.

Another common example involves clopidogrel, a blood thinner. Clopidogrel is a prodrug, meaning it must be activated by the CYP2C19 enzyme in the liver to work effectively. Patients with certain CYP2C19 variants process the drug poorly, which can lead to an increased risk of cardiovascular events. In South Africa, the South African Health Products Regulatory Authority (SAHPRA) evaluates pharmacogenomic data during the medicine registration process, ensuring that local package inserts reflect these critical safety warnings. Patients and providers can use tools like HealthSA to verify a medicine’s registration details, which links back to these approved labels.

The promise of personalised prescribing

The primary goal of pharmacogenomics is to optimise therapy by predicting an individual’s response before the first dose is taken. By matching the right medicine to the right patient, healthcare providers can avoid the traditional trial-and-error approach to prescribing. This has the potential to improve clinical outcomes, as patients are more likely to receive a medicine that will be effective for them from the outset.

Furthermore, identifying genetic risks before treatment can significantly reduce the incidence of adverse drug reactions, which are a major cause of hospitalisation worldwide. The World Health Organization (WHO) also recognises the value of this field, particularly as it relates to the responsible use of essential medicines listed on the WHO Essential Medicines List (EML), where maximising safety and efficacy in diverse global populations is paramount.

Current limits and challenges

Despite its considerable promise, the routine use of pharmacogenomics in everyday clinical practice faces several hurdles. One major barrier is cost and accessibility. Genetic testing can be expensive, and it is not universally covered by public health systems or medical aids, making it less accessible in lower-income settings. There are also practical challenges related to implementing pharmacogenomic data in busy clinical environments.

Healthcare providers need clear, evidence-based guidelines on how to interpret genetic test results and translate them into prescribing decisions, and this information must be integrated seamlessly into electronic health records. Finally, it is important to recognise that genetics is only one piece of the puzzle. A person’s response to a medicine is influenced by a complex interplay of factors, including age, organ function, diet, and interactions with other medicines. Pharmacogenomics is therefore a powerful tool to guide prescribing, but it is not an absolute guarantee of how an individual will react.