Pharmacogenetics – A Path Towards More Suitable Medication Selection

Why does a medication work well for one person but have an insufficient effect or cause unpleasant side effects in another? Although various factors play a role, including age, health status, lifestyle, and the use of other medications, our genetics may also provide part of the answer.

Pharmacogenetics (pharmacogenomics) is a field of science that studies how genetic variations influence an individual's response to medications. It helps explain why the same medication may affect people differently and how genetic information can be used to personalise treatment.

How Do Genes Influence the Effects of Medications?
After a medication is taken, the body must absorb it, transport it to its site of action, and sometimes convert or metabolise it into its active form. The medication must also be eliminated from the body to prevent it from accumulating to potentially dangerous concentrations. These processes involve various proteins encoded by our genes.

If a particular gene contains a specific genetic variation, it may alter the function of the corresponding protein. Some genetic variations reduce the body's ability to absorb certain medications, meaning they may not work as expected. Due to their genetic characteristics, some individuals break down certain medications more slowly, while others metabolise them more quickly than usual.

As a result, the concentration of a medication in the body may be higher or lower than expected. This can affect both treatment effectiveness (if the concentration is too low, the therapy may not achieve the desired effect) and the risk of side effects (if the concentration is too high, more severe side effects may be more likely to occur).

One of the most important genes in pharmacogenetics is CYP2D6, which encodes an enzyme involved in the metabolism of approximately 25% of commonly used medications, including opioids, antidepressants, and tamoxifen.

Meanwhile, variations in the CYP2C19 gene may affect the metabolism of certain antidepressants and proton pump inhibitors (medications that reduce stomach acid production).

Genetic differences do not have the same clinical impact on all medications. Therefore, the relationship between each gene and medication must be evaluated individually.

What Can Pharmacogenetic Testing Reveal?
A pharmacogenetic test analyses specific genetic variations associated with the effectiveness or safety of particular medications, using a saliva sample.

Pharmacogenetic testing can help a doctor assess whether a particular medication is suitable for a patient by identifying:

Whether the individual's body may metabolise certain medications more slowly or more rapidly.
Whether choosing an alternative medication or adjusting the dosage may be necessary in certain cases.
Whether there is an increased risk of specific side effects.
The role of pharmacogenetics has already been studied and applied in various medical fields, including psychiatry, cardiology, oncology, and neurology.

Clinical guidelines have been developed for certain gene–drug combinations to help healthcare professionals interpret genetic information and make appropriate treatment decisions.

However, pharmacogenetic testing is not a universal solution for all medications or all patients. The usefulness of testing depends on the specific medication, the genes being analysed, and the strength of the available scientific evidence.

Pharmacogenetics and Personalised Medicine
Traditionally, medication selection and dosage are often determined based on clinical guidelines, the patient's health status, and their observed response during treatment.

Pharmacogenetics provides additional information that, in certain situations, can help healthcare professionals make better-informed decisions before starting treatment or when the treatment outcome differs from what was expected.

Common situations in which pharmacogenetic testing may be useful include:

- High cholesterol levels – Certain variants of the SLCO1B1 gene increase the likelihood of muscle pain and weakness when taking cholesterol-lowering medications such as atorvastatin or fluvastatin.

- Depression – Specific variants of the CYP2D6 or CYP2C19 genes are associated with an increased likelihood of altered metabolism of antidepressants such as sertraline and venlafaxine.

- Prevention of blood clots – Some genetic variants are associated with a lower required dose of the anticoagulant medication warfarin.

- HIV – Variants of the HLA-B gene influence the likelihood of developing a severe skin reaction to abacavir, while variants of the CYP2B6 gene may affect the nervous system during treatment with efavirenz.

- Multiple sclerosis – Certain variants of the TPMT and UGT1A1 genes have been associated with an increased risk of immune system suppression during treatment with azathioprine.

The results of a pharmacogenetic test provide information about variations in the genes that have been analysed. These variations may indicate how certain medications are likely to affect you.

However, pharmacogenetic testing does not provide a complete picture of your health, as your genetic profile is not the only factor determining how medications work.

Other important factors include interactions with other medications, liver and kidney function, age, coexisting medical conditions, and individual circumstances.

Therefore, pharmacogenetic test results should always be interpreted alongside other relevant clinical information.

Pharmacogenetics represents an important step forward in the development of personalised medicine. A better understanding of genetic differences helps improve our knowledge of how medications work and creates opportunities for increasingly targeted, personalised treatment approaches.

GenEra offers pharmacogenetic tests covering various groups of medications, including:

Antidepressants
Proton pump inhibitors
Weight management medications
Antiepileptic and antipsychotic medications
Pharmacogenetic test results do not replace a medical consultation and should not, on their own, be used as a basis for independently discontinuing medication or changing its dosage.

References and Additional Information
CYP2D6 overview:

https://www.clinpgx.org/gene/PA128/overview

International clinical guidelines for healthcare professionals:

https://www.clinpgx.org/cpic/guidelines

Common situations in which pharmacogenetic testing may be useful:

Centers for Disease Control and Prevention . Atlanta: U.S. Department of Health and Human Services; Pharmacogenomics; [Accessed: 21 October 2026].

https://www.cdc.gov/genomics-and-health/pharmacogenomics/index.html

Pharmacogenetics and personalised medicine:

Hefti, E., & Blanco, J. G. (2016). Documenting Pharmacogenomic Testing with CPT Codes. Journal of AHIMA, 87(1), 56–59.