Pharmacogenomic (PGx) Testing: What to Know

What Is Pharmacogenomic (PGx) Testing?

Pharmacogenomic (PGx) testing is a genetic test that evaluates how your DNA affects your response to medications.

It focuses on genes involved in how drugs are:

• absorbed
• metabolized (broken down)
• transported in the body
• recognized by receptors

These genetic differences can explain why the same medication may work well for one person, cause side effects in another, or be ineffective in someone else.

PGx testing supports a more personalized approach to treatment. It helps clinicians use genetic information alongside clinical data to guide safer and more effective prescribing.

When Is PGx Testing Recommended?

PGx testing can be used both before starting treatment and after problems occur.

It may be recommended if:

• A medication causes unexpected side effects
• A treatment does not work as expected
• Multiple medication changes have already been tried
• A drug has known gene-related prescribing guidance
• Long-term therapy is planned
• The patient is taking several medications at once

In some cases, testing is done proactively. This means results are available before medications are prescribed, which can help avoid trial-and-error approaches.

How Is PGx Testing Conducted?

PGx testing is simple for the patient.

A sample is collected using a cheek swab or blood draw. No special preparation is usually required.

In the laboratory, DNA is analyzed to identify variants in specific pharmacogenes, which are genes that influence how your body processes and responds to medications. Different technologies may be used, including targeted genotyping or next-generation sequencing (NGS).

NGS allows testing of multiple genes at the same time, making it a cost-effective option for broader panels. However, more complex genes may require additional methods for accurate analysis.

Results are typically available within a few days to a few weeks.

Which Genes Are Commonly Tested?

PGx panels include genes with known clinical impact on drug response.

Commonly tested genes include:

• CYP2D6, CYP2C19, CYP2C9, CYP3A5: These control enzymes (proteins in the liver that break down medications) that break down many medications
• SLCO1B1: Affects how drugs are transported into cells
• TPMT, DPYD: Influence how certain drugs are metabolized and can affect toxicity risk
• VKORC1: Affects sensitivity to some blood-thinning medications
• HLA genes: Associated with immune-related drug reactions

Some genes, such as CYP2D6, are complex and may have many variants, duplications, or structural changes. This can make testing and interpretation more challenging.

What Do PGx Results Show?

PGx results describe how your body is likely to process certain medications.

You may be classified into different metabolizer phenotypes, which describe how your body processes certain medications:

• Poor metabolizer - your body breaks down the drug very slowly, which may increase the risk of side effects or drug buildup
• Intermediate metabolizer - your body processes the drug more slowly than normal, which may require dose adjustments
• Normal (extensive) metabolizer - your body processes the drug at the expected rate, and standard dosing is usually appropriate
• Rapid metabolizer - your body breaks down the drug faster than normal, which may reduce the drug’s effectiveness
• Ultrarapid metabolizer - your body processes the drug very quickly, which can make the medication less effective or require higher doses

These phenotypes are based on your genetic variants and help guide medication choice and dosing.

In addition to metabolizer status, PGx reports often include drug-gene interactions (DGI). These describe how specific genetic variants may affect your response to a particular medication, such as increasing the risk of side effects or reducing effectiveness.

In some cases, results may also show:

• Increased risk of serious side effects
• Reduced likelihood of treatment effectiveness
• A need to avoid a specific drug

Clinical guidelines from organizations such as CPIC and DPWG are often used to interpret these results and provide recommendations.

Which Medications Are Affected by PGx?

PGx testing is relevant across many medical fields.

It is commonly used for medications in:

• Mental health (antidepressants, antipsychotics)
• Cardiology (blood thinners, heart medications)
• Pain management
• Oncology (cancer treatments)
• Neurology (seizure medications)
• Gastroenterology and infectious diseases

More than 300 medications include pharmacogenomic information in their labeling, and over 100 have dosing recommendations linked to genetic results.

What Do PGx Results Mean for Treatment?

PGx results do not make decisions on their own. They provide guidance that must be combined with clinical judgment.

They may help:

• Choose a more suitable medication
• Adjust the starting dose
• Avoid drugs with a higher risk
• Plan closer monitoring

Final decisions depend on the full clinical context, including the patient’s condition, history, and other medications.

Some PGx reports also include drug-drug interactions (DDI). These show how different medications may affect each other when taken together. This is especially important for patients taking multiple drugs, where interactions can change how medications work or increase the risk of side effects.

When both drug-gene interactions (DGI) and drug-drug interactions (DDI) are considered together, treatment decisions can be more precise and safer.

Benefits of PGx Testing

PGx testing can improve medication management in several ways.

It may:

• Reduce trial-and-error prescribing
• Lower the risk of adverse drug reactions
• Improve treatment effectiveness
• Support safer use of multiple medications
• Provide lifelong genetic information that can be reused

Its impact is strongest when there is clear evidence for a specific gene-drug interaction.

Limitations of PGx Testing

PGx testing has important limitations.

• Not all medications have strong genetic guidance
• Evidence varies between gene-drug pairs
• Results may differ depending on the testing panel
• Interpretation can be complex, especially with multiple medications
• Non-genetic factors still influence drug response

In some cases, results may include variants with unclear clinical meaning, which can make interpretation more difficult.

Challenges in Clinical Use

Despite growing evidence, PGx testing is not yet widely used in all healthcare settings.

Key challenges include:

• Limited familiarity among clinicians
• Differences between clinical guidelines
• Lack of standardized reporting formats
• Difficulty integrating results into clinical workflows
• Complexity of interpreting certain genes (such as CYP2D6)

There is also a need for better tools to combine results from multiple genes, especially when patients take several medications.

A multidisciplinary approach is often required. Physicians, pharmacists, and genetic counselors may work together to interpret results and apply them in practice.

Which Specialist Should You Consult?

PGx testing may involve several healthcare professionals.

These may include:

• Primary care physicians
• Psychiatrists
• Cardiologists
• Oncologists
• Clinical pharmacologists

Pharmacists and genetic counselors play a key role in interpreting results and helping guide treatment decisions, especially in complex cases.