Your genes can influence how medicines work in your body, and this genetic information could help doctors prescribe medications more safely. Two people taking the same dose of the same drug might experience very different effects—one could feel better, while the other might not benefit or could suffer serious side effects. These differences are often partly due to genetic variations that affect how the body processes or responds to medications. Pharmacogenomics is the study of how genes influence a person’s response to drugs. Some genes produce proteins that help the body break down medicines, while others affect how the drugs act in the body. Variations in these genes can change how effective a treatment is or increase the risk of side effects. However, genes are just one factor. Age, organ function, other medications, and the condition being treated can all influence how a person reacts to a drug. Allergic reactions also have multiple causes unrelated to genetics. A study of nearly 500,000 people from the UK Biobank found that 99.5% of participants had genetic variants that could cause them to react unusually to at least one drug. Almost a quarter of these people had already been prescribed a medicine affected by one of these genes. However, the practical use of this information is limited to cases where a person is taking or could be prescribed a relevant drug, and where there is strong evidence to guide doctors in adjusting treatment. In such situations, genetic testing can help avoid harmful reactions or ineffective treatment. While genetic differences account for only a portion of adverse drug reactions—ranging from mild to life-threatening—these reactions can be serious. A 2022 study at a Liverpool NHS trust found that adverse drug reactions contributed to 16.5% of adult medical admissions in one month. Researchers estimated this could cost NHS England £2.21 billion annually, though this is an extrapolated estimate based on a single trust. Testing is already used for certain medicines. For example, the DPYD gene is tested before some chemotherapy drugs, as certain variants can reduce the body’s ability to break them down. Similarly, the HLA-B gene is tested before prescribing the HIV drug abacavir, as a specific variant increases the risk of a severe allergic reaction. A national study in England is now exploring how to expand pharmacogenomic testing across the healthcare system, including integrating test results into clinical computer systems. In the Netherlands, pharmacogenomic guidance is already part of the national medicines database, allowing doctors and pharmacists to access this information at the point of care. However, access to testing remains a separate challenge. For example, a person recovering from a heart attack may be prescribed clopidogrel, a statin, and omeprazole. Genetic variants in the CYP2C19 gene can affect how well clopidogrel works, and in some cases, alternative treatments may be recommended. Variants can also influence the risk of muscle pain from statins or how effective omeprazole is in protecting the stomach. Genetic testing can provide doctors with additional information when making prescribing decisions, but it is not the only factor. A sample for testing can be obtained through a blood sample or a cheek swab, and the results can be stored in a patient’s medical record for future use. As new evidence and guidelines emerge, the interpretation of these results may also evolve. While genetic testing can reduce the trial-and-error aspect of prescribing, especially when there is strong evidence supporting a drug-gene relationship, wider use requires reliable testing, updated clinical guidelines, and proper training for healthcare professionals. Ensuring fair access to testing is also crucial, so that those who need it most can benefit from this emerging science.