By Next Biosciences

19 October 2026

Why does the same medicine work differently for different people?

Your genes affect how your body absorbs, breaks down, and responds to medicine. Some people clear a medicine so fast it barely works. Others clear it so slowly it builds up and causes side effects. The study of this is called pharmacogenomics, and a pharmacogenomic test helps your doctor choose a medicine and dose with your genes in mind.

You take a painkiller and feel nothing, while it works perfectly for your friend. Or a medicine everyone says is gentle leaves you feeling awful. It's easy to assume it's in your head. Often, it's in your genes.

What is pharmacogenomics?

Pharmacogenomics combines pharmacology, the study of medicines, with genomics, the study of genes. It looks at how small differences in your genes change the way your body handles medicine.

These differences are very common. In a study of nearly 500,000 people in the UK Biobank, 99.5% carried at least one gene variant that could change how they respond to a medicine.

How do genes change the way medicine works?

Most medicines are broken down by enzymes in your liver. Your genes decide how active those enzymes are, which puts you into one of these groups for each enzyme:

Type What happens What it can mean
Poor metaboliser Medicine is broken down slowly More side effects at a normal dose
Intermediate metaboliser Medicine is broken down a bit slower than usual Sometimes a lower dose is needed
Normal metaboliser Medicine is processed as expected Standard dosing usually works
Ultrarapid metaboliser Medicine is broken down very fast A normal dose may not work, or some medicines become too strong

You can be a normal metaboliser for one enzyme and a poor metaboliser for another, so your response depends on which medicine you take.

Which common medicines are affected by genes?

The US Food and Drug Administration keeps a list of gene and medicine links that are strong enough to affect prescribing. A few examples:

  • Codeine (CYP2D6): your body turns codeine into morphine to relieve pain. Poor metabolisers get little pain relief. Ultrarapid metabolisers make morphine too quickly, which can be dangerous. Codeine is in many pain and cough medicines sold at South African pharmacies.

  • Clopidogrel (CYP2C19): a blood thinner used after heart attacks and stents. People with reduced CYP2C19 activity may get less protection from it.

  • Simvastatin (SLCO1B1): a cholesterol medicine. Some gene variants raise the risk of muscle pain and damage, especially at higher doses.

  • Warfarin (CYP2C9 and VKORC1): a blood thinner where genes affect the dose a person needs.

  • Antidepressants and antipsychotics (CYP2D6 and CYP2C19): genes affect how quickly many of these are processed. Read more in our blog on pharmacogenomics in psychiatric care.

Why does pharmacogenomics matter in South Africa?

Southern Africa has the greatest genetic diversity on the continent, and gene variants that change how medicines work differ a lot between groups. For example, reduced-function versions of CYP2C19 have been found in roughly one in five people in some South African groups.

Studies have also reported adverse drug reactions in about 14% of hospitalised patients in South Africa, around twice the international rate. Knowing how your body handles medicine is one way to lower that risk.

Who should consider pharmacogenomic testing?

It may be useful if you:

  • Have had side effects from a medicine at a normal dose

  • Have tried a medicine that didn't work for you

  • Take several long-term medicines

  • Are about to start a long-term treatment, such as for depression, heart disease or high cholesterol

  • Want information on file for future prescriptions

What is PharmaGene?

PharmaGene is an in-house pharmacogenomic test from Next Biosciences. It looks at how genetic variation may affect how you process medicine, how well it works and your risk of side effects.

  1. Order online at nextbio.co.za, then complete a test requisition form and give a saliva sample. No blood is needed.

  2. Results are usually ready within four weeks of the lab receiving your sample.

  3. You review them with your healthcare provider, who uses them to guide prescribing.

Your genes don't change, so your results can be kept on file and used for future prescriptions. PharmaGene doesn't diagnose any condition.

Important: never stop or change a medicine without speaking to your doctor first.

Find out more about PharmaGene from Next Biosciences

 

FAQ

  1. What is pharmacogenomics in simple terms? It's the study of how your genes affect the way your body responds to medicine.

  2. Why doesn't a medicine work for me when it works for others? Genes affect how fast your body breaks down many medicines. If you process a medicine very quickly, a normal dose may not be enough.

  3. Do I only need a pharmacogenomic test once? Yes. Your genes don't change, so one test can guide prescriptions for years.

  4. Can pharmacogenomic testing tell me exactly which medicine to take? No. It gives your doctor extra information to help choose a medicine and dose, alongside your health history and other factors.

 

Sources

•      Table of pharmacogenetic associations, US Food and Drug Administration, updated September 2026

•      Pharmacogenetics at scale: an analysis of the UK Biobank, McInnes et al., preprint, 2020

•      Pharmacogenomic research in South Africa: lessons learned and future opportunities in the rainbow nation, Warnich et al., Current Pharmacogenomics and Personalized Medicine, 2011

•      PharmaGene, Next Biosciences

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