Why Does the Same Medication Work Differently for Different People?
Two people with the same diagnosis receive the same prescription. One improves quickly. The other experiences side effects, sees no benefit, or both. The difference often comes down to genetics.
Pharmacogenomics — the study of how your genes affect your response to medicines — is helping to end the era of trial-and-error prescribing.
How Your Genes Affect Medication
- Drug absorption: How quickly your body takes up a medication from your digestive system
- Drug breakdown (metabolism): How efficiently your liver processes and clears a drug. Some people are 'rapid metabolisers' — the drug leaves their system so fast it has little effect. Others are 'poor metabolisers' — the drug builds up and causes toxicity
- Side effects: Certain genetic variants make some people far more likely to experience adverse reactions to specific medications
Real-World Examples
Painkillers: Codeine is converted to morphine in the body by an enzyme called CYP2D6. People with variants that make this enzyme over-active can experience dangerous morphine build-up from a standard dose.
Antidepressants: The effectiveness of SSRIs and other antidepressants varies significantly based on genetic variants in drug-metabolising enzymes. Pharmacogenomic testing can help identify which medications are more likely to work — and which to avoid.
Blood thinners: Warfarin dosing is notoriously difficult to get right. Variants in the VKORC1 and CYP2C9 genes affect how individuals respond, and genetic testing is increasingly used to guide safe starting doses.
What This Means for You
Pharmacogenomic testing doesn't replace your doctor — it gives them better information. Rather than adjusting doses based on how you react, testing can help predict the right medication from the start.
At Jeen, we're helping patients and clinicians move away from trial-and-error prescribing towards personalised, genetics-informed care.
