The Science of Genetic Variability

24 Aug 2025

8 Min Read

Choong Chiau Ling (Academic Contributor)

IN THIS ARTICLE

Contributed by Ms Choong Chiau Ling, whose research focuses on pharmacogenetics in kidney transplantation, particularly how genetic polymorphisms in CYP3A5 and ABCC2 affect outcomes such as acute graft rejection, costs, and quality of life in tacrolimus-treated recipients. She can be reached at cl.choong@taylors.edu.my.

Imagine this: Two patients walk into a clinic with the same condition. They’re both prescribed the same medication at the same dose. One recovers within days; the other develops rashes and ends up in the emergency room. What just happened?

For decades, modern medicine has relied on standardised treatments—dosages based on average outcomes from clinical trials. But the truth is, the ‘average’ patient doesn’t exist. Beneath the surface, each of us carries a unique genetic code that shapes how our body responds to medication. These subtle genetic differences can determine whether a drug heals or harms.

This is where the science of pharmacogenomics steps in, revealing why medicine is not a one-size-fits-all equation—and how the future of healthcare is becoming more personal than ever.

What Is Pharmacogenomics and Pharmacogenetics?

Pharmacogenomics is the study of how a person’s genes influence their response to drugs. It’s an area of science that is transforming the old ‘trial-and-error’ approach into something far more precise: medication tailored to fit an individual’s genetic profile.

Closely related is pharmacogenetics, which focuses on how a variation in a single gene affects drug response. Pharmacogenomics, on the other hand, considers the bigger picture—the combined impact of multiple genetic variations across the entire genome.

Lab wearing purple protective gloves tests for COVID Delta Variant

_{Unlike general pharmacology, which studies how drugs act in the body, or genetics, which studies inherited traits, pharmacogenomics bridges both—focusing on how your DNA can change the way a drug works for you}

This field began gaining momentum after the Human Genome Project mapped our DNA in the early 2000s. Since then, advances in genomic data have given researchers powerful tools to uncover patterns in drug responses that were previously invisible.

The Science Behind the Reaction

When you take a medicine, it doesn’t simply ‘work’ or ‘not work’—it goes on a complex journey through your body. This journey involves:

Absorption: how the drug enters your bloodstream.
Distribution: how it travels to the tissues and organs.
Metabolism: how your body breaks it down, often in the liver.
Elimination: how it is removed from your body, usually through the kidneys or bile.

A group of enzymes known as the CYP450 family—including CYP2D6, CYP3A4, and CYP3A5—plays a central role in drug metabolism. Variations in the genes that produce these enzymes can lead to big differences in how people process the same medication.

Some people are poor metabolisers, meaning their bodies process drugs slowly. As a result, the medicine stays in their system for longer, which can increase the risk of toxic side effects. Others are ultra-rapid metabolisers, whose bodies break down drugs so quickly that the medicine may leave the system before it has a chance to take effect.

Doctor And Nurse Suture Patient In Emergency Room

_{Even a tiny change in DNA known as a genetic polymorphism, can make a big difference in how a drug works. The CYP3A5 gene is a good example. It influences how the drug tacrolimus (used alongside other medicines to prevent organ rejection after a transplant) is metabolised. Patients with the CYP3A5 expresser genotypes break down tacrolimus more quickly than non-expressers, meaning they often require higher doses to achieve the same therapeutic effect.}

When the Same Drug Works Differently

Pharmacogenomics isn’t just theory—it’s already reshaping the way some drugs are prescribed.

Codeine and CYP2D6

Codeine needs CYP2D6 to convert it into morphine, which provides pain relief. Ultra-rapid metabolisers produce too much morphine too quickly, risking dangerous side effects such as breathing problems. Poor metabolisers produce very little morphine, so they get almost no pain relief. Because of this, codeine is now prescribed with caution in many populations.

Warfarin and VKORC1/CYP2C9

Warfarin is a common blood thinner, but the ‘right’ dose varies depending on genetic variants in VKORC1 and CYP2C9. A dose that’s too high can cause internal bleeding; too low can lead to dangerous blood clots.

**Abacavir and HLA-B*5701**

Abacavir, used to treat HIV, can cause severe allergic reactions in patients with the HLA-B*5701 variant. Testing for this genetic marker is now standard practice in many treatment guidelines to prevent life-threatening hypersensitivity.

A young Asian female doctor and a young male doctor are rounding in the hospital ward

_{In some Asian populations, the way people metabolise drugs can be quite different from Western averages. For example, up to 50% of East Asians have genetic variations in CYP2D6 that make them poor metabolisers of codeine, meaning the drug may provide little or no pain relief.}

Personalised Medicine Today and the Road Ahead

Pharmacogenomics is already making a significant impact across various areas of clinical practice. In oncology, testing for mutations such as Epidermal Growth Factor Receptor (EGFR) or Human Epidermal Growth Factor Receptor 2 (HER2) helps guide targeted cancer treatments, ensuring patients receive therapies most likely to be effective.

In psychiatry, variations in the CYP2C19 gene influence how individuals respond to certain antidepressants, affecting both effectiveness and side-effect profiles. In cardiology, differences in the same gene can determine how well clopidogrel works to prevent strokes, highlighting the importance of tailoring medication to each patient’s genetic makeup.

AI and big data are opening new possibilities. Predictive models that combine genetic information with patient health records can help forecast treatment outcomes. Machine learning may soon help doctors instantly identify the best drug for each patient.

What This Means for the Next Generation of Pharmacists

For future pharmacists, this evolution offers exciting opportunities. The role of the pharmacist will extend beyond dispensing medicines to include genomic consultations—helping patients and doctors interpret genetic results and adjust treatments accordingly.

Pharmacy programmes are beginning to incorporate pharmacogenomics into their curriculum, equipping graduates with skills in genetic interpretation, ethical considerations, and patient communication. Career paths may expand into clinical pharmacy, drug development, regulatory science, or even AI-driven healthcare systems that use genetic data to design treatment strategies.

Career paths may expand into clinical pharmacy, drug development, regulatory science, or even AI-driven healthcare systems that use genetic data to design treatment strategies.

Equitable access to personalised medicine is vital, yet challenges span regulation, cost, and acceptance by stakeholders. Strategies include integrating pharmacogenetic testing into public care, building local labs, and cost-effective assays. Proving cost-effectiveness and embedding guidelines with education ensures sustainable, inclusive adoption.

— Ms Choong Chiau Ling

Conclusion

Genetics explains why medicine isn’t one-size-fits-all. The move from trial and error to precision and prediction is transforming healthcare—and pharmacists will be at the centre of this shift.

As the science advances, the challenge isn’t the technology; it’s ensuring every patient benefits, no matter their background or location. For students entering pharmacy today, pharmacogenomics is not just an interesting subject—it’s the future of personalised healthcare.

As medicine shifts towards precision and personalised care, pharmacists are at the front line of this transformation. Discover how Bachelor of Pharmacy programme equips you with the scientific foundations, clinical knowledge, and ethical understanding to make a real impact in people’s lives.

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