Pharmacogenomics is the study of how a person’s genes affect their response to medicines. Inherited differences — often in genes that activate or break down a drug, or in immune-type (HLA) markers — can change how well a medicine works and the risk of side effects. This is especially relevant in Singapore and Asia, where some markers are more common. The aim is safer, more personalised prescribing, but a key rule holds throughout: never start, stop or change any medicine based on a genetic or direct-to-consumer test without your doctor or pharmacist.
Worried about your child’s health or development?
Message our care team on WhatsApp. Tell us what’s going on and we’ll help you find the right paediatrician — free, with no obligation.
Chat with our care team on WhatsAppNot sure who to see? Find the right specialist →
In a medical emergency, or for severe or sudden symptoms such as breathing difficulty, chest pain, fainting or stroke signs, call 995 immediately.
A severe skin rash with blistering or peeling, or sores in the mouth, eyes or genitals — especially with fever — while on a new medicine can be life-threatening: stop the medicine and seek urgent care, calling 995 if severe. Facial or throat swelling with trouble breathing is a 995 emergency.
What it is
Pharmacogenomics studies how inherited differences in genes affect the way people respond to medicines. Those differences — often in genes controlling how the body activates, breaks down or reacts to a drug — can change both how well a medicine works (efficacy) and the risk of side effects or serious harm (safety). The goal is more personalised prescribing: choosing a medicine, or flagging who needs extra caution, based partly on genetic makeup. It does not replace clinical judgement — the doctor and pharmacist interpret any result alongside the whole clinical picture.
Who it affects and testing
Anyone can carry these differences, and some variants are more common in particular ancestry groups, which makes pharmacogenomic testing especially relevant in Singapore’s and Asia’s populations. Testing — usually a blood or saliva sample analysing specific gene variants or HLA markers — is typically considered before starting certain higher-risk medicines, or when someone has had an unusual response. When it is appropriate is a clinical decision, and results are only meaningful when interpreted by a clinician, because the same result can mean different things for different drugs.
Skin-reaction examples (HLA markers)
Some of the clearest examples involve immune-type HLA markers. Carrying HLA-B*15:02 is linked to a much higher risk of severe skin reactions (Stevens-Johnson syndrome and toxic epidermal necrolysis) with the anti-seizure and nerve-pain medicine carbamazepine, and this marker is more common in some Asian populations; Singapore’s Health Sciences Authority (HSA) has issued guidance recommending HLA-B*15:02 testing before starting carbamazepine in certain at-risk new patients so an alternative can be chosen if the test is positive. Similarly, HLA-B*58:01 is associated with severe skin reactions with the gout medicine allopurinol and is more frequent in some Asian groups.
Drug-metabolism examples
Other examples involve how the body processes a drug. Inherited low activity of the TPMT enzyme, or certain NUDT15 variants (the latter notably relevant in East and Southeast Asian populations), raises the risk of severe bone-marrow suppression with thiopurine medicines such as azathioprine, so testing before starting helps identify who needs extra caution. The antiplatelet clopidogrel, used after heart stents and some strokes, is activated by the CYP2C19 enzyme, and “poor metabolisers” convert little of it, reducing its effect. And the blood thinner warfarin is dosed carefully and monitored with blood tests partly because inherited variants affect how people respond — this guide gives no doses.
What results mean
A result can help the care team choose a safer drug, prefer an alternative, or flag who needs closer monitoring. It is a risk indicator, not a diagnosis of harm: many people without a variant can still react, and a negative test does not guarantee safety, so context always matters. Interpretation and any change to treatment are always doctor- or pharmacist-led.
The most important safety rule
Do not start, stop, switch or change the dose of any medicine based on a genetic result or a direct-to-consumer test on your own — always go through your doctor or pharmacist, who interpret the result in your clinical context. Be cautious about commercial direct-to-consumer genetic tests, which vary in quality and clinical validity and are no substitute for professional advice, and which this guide does not endorse. If you have such a report, bring it to a clinician rather than acting on it.
Emergencies and family implications
A severe skin rash, blistering, peeling, or sores in the mouth, eyes or genitals — especially with fever — while on a new medicine can signal a life-threatening reaction: stop the medicine and seek urgent care immediately, calling 995 if it is severe or rapidly worsening. Swelling of the face, lips, tongue or throat with difficulty breathing is a 995 emergency. Because these variants are inherited, a high-risk marker may run in the family and in certain ethnic groups, so it can be worth telling blood relatives to mention it to their own doctors — whether they should be tested is a decision for their clinicians.
How to access it in Singapore
Singapore’s Health Sciences Authority issues pharmacogenomic and drug-safety guidance, including on HLA-B*15:02 testing before carbamazepine in at-risk patients, and testing is available through restructured hospitals and specialist services, arranged and interpreted by your doctor or pharmacist. Cardiac genetics for clopidogrel and warfarin questions is available at the national heart centres. Our medication safety guides and HealthHub offer further information.
Related genetics & genomic-health topics
- Genetic testing
- Genetic counselling
- Carrier screening
- Hereditary breast & ovarian cancer
- Lynch syndrome
- Down syndrome
- Prenatal genetic testing
- Familial hypercholesterolaemia
- Rare & inherited diseases
- Thalassaemia
- Paediatricians in Singapore
Sources
- MedlinePlus — Pharmacogenomics: medlineplus.gov
- MedlinePlus — CYP2C19 gene: medlineplus.gov
- Health Sciences Authority (Singapore): hsa.gov.sg