Rows of pipette tips on an automated laboratory robot

How accurate is pharmacogenetic testing?

Veronika Litinski, MSc Medicinal Chemistry, MBA3 Nov 2026

Pharmacogenetics (PGx) uses your genetic profile to predict how your body is likely to break down and respond to certain medications, so that your prescriber can choose a drug and dose that are more likely to suit you. The field began more than 70 years ago, and there are now well-established prescribing guidelines for doctors and pharmacists to follow. But how accurate is a pharmacogenetic test, and what should you look for? The short answer: a clinical-grade test performed in an accredited laboratory reads the variants it tests very reliably. How useful the result is depends on which variants and genes the test includes, how the results are interpreted, and whether they are reviewed together with your other medications.

What does “accurate” mean for a pharmacogenetic test?

The accuracy of a pharmacogenetic test depends on three things:

Laboratory accuracy. Does the laboratory correctly identify the gene variants it tests? Accredited clinical laboratories validate their methods, use reference samples and take part in quality-control programs.
Coverage. Does the test include the variants that matter, including those common in people of different ancestries and changes in the number of copies of a gene?
Interpretation. Are your results translated into a metabolizer status (poor, intermediate, normal, rapid or ultrarapid) and into recommendations according to recognized clinical guidelines?

A test can be highly accurate in the laboratory and still miss important information if it does not include the right variants, so all three matter.

How does genetic testing for medications work?

You collect a DNA sample at home and send it to a certified clinical laboratory. The laboratory extracts the DNA and identifies which versions of each gene you carry. Your results are then compared with a list of known gene variants that change how medications are broken down or how they act in the body, and translated into a report based on clinical guidelines.

Which genes and variants should be tested?

The human genome contains about 20,000 genes, and different genetic tests look at different parts of it for different purposes. Some consumer tests use a single variant in a single gene to predict things like caffeine tolerance or the best type of exercise, which is rarely reliable. Broad ancestry tests scan hundreds of thousands of variants to trace family history, but they are not designed for prescribing and often miss key variants that affect medication response.

A clinical pharmacogenetic test focuses on the genes that affect medications, and on the specific variants in those genes that change how enzymes work. Expert groups, led by the Association for Molecular Pathology, publish recommended minimum lists of variants that clinical laboratories should test for genes such as CYP2C19 and CYP2D6. These lists were chosen with variant frequencies in many populations in mind. A test that leaves out recommended variants may label someone a normal metabolizer when they are not.

Why does CYP2D6 need special attention?

CYP2D6 is one of the most important and most complex drug-metabolizing genes. Some people are missing a copy of the gene entirely, others carry extra copies, and some carry hybrid versions. A test that reads only single-letter changes in the DNA cannot detect these differences, so a clinical-grade test must also measure the number of CYP2D6 copies.

This matters in practice. For example, codeine, an ingredient in some prescription pain medications such as Tylenol No. 3, works only after the liver converts it into morphine using CYP2D6. People with extra working copies of the gene can be ultrarapid metabolizers: they turn standard doses of codeine into too much morphine, too fast, which can cause extreme sleepiness, slowed breathing and, rarely, death. People with no working copies, poor metabolizers, get little pain relief. The Clinical Pharmacogenetics Implementation Consortium (CPIC) recommends that both groups avoid codeine and tramadol.

Does ancestry affect the accuracy of a pharmacogenetic test?

It can. Some important variants are much more common in certain populations. For example, the proportion of CYP2D6 ultrarapid metabolizers ranges from a few percent in some populations to more than 10% in others, and some reduced-function CYP2D6 and CYP2C19 variants are common in people of African or East Asian ancestry but rare in people of European ancestry. A test designed mainly around variants common in people of European ancestry may give less accurate results for others. A panel that includes variants found across many ancestries, along with gene copy number, means that more people get accurate results for more medications.

What can’t a genetic result tell you?

Other medications can change your metabolizer status. Some medications, such as fluoxetine, paroxetine and bupropion, strongly block CYP2D6. Someone who is a normal metabolizer by genetics may then act like a poor metabolizer. This is called phenoconversion, and it is one reason your results should be reviewed together with all the medications you take.
Genes are not the only factor. Age, kidney and liver function, other health conditions, diet, smoking and alcohol also affect drug response. In women, hormonal treatments matter too. Estrogen-containing birth control pills can markedly slow CYP2C19, so a woman taking them may break down medications such as some antidepressants and proton pump inhibitors more slowly than her genetic result suggests. They can also lower levels of the seizure and mood medication lamotrigine by about half. Pregnancy changes the activity of several drug-metabolizing enzymes as well. Tell your pharmacist if you use birth control, menopausal hormone therapy or other hormonal treatments.
Rare or untested variants. If no tested variants are found, the gene is usually reported as normal. Very rare variants that are not part of the test can occasionally change enzyme function.
Whether a medication will work. A pharmacogenetic test predicts how your body handles a medication, not whether your condition will respond to it.

Do all drugs have pharmacogenetic guidelines?

No. About a dozen liver enzymes from the cytochrome P450 (CYP) family break down 70 to 80% of medications in clinical use, and genetic variants strongly affect several of them, including CYP2D6, CYP2C19 and CYP2C9. Clinical guidelines from expert groups such as CPIC and the Dutch Pharmacogenetics Working Group, together with drug labels from Health Canada and the U.S. Food and Drug Administration (FDA), give genotype-based recommendations for a growing list of medications. For other medications, there is not yet enough evidence to guide prescribing based on genes.

How do I get the most benefit from my pharmacogenetic results?

Pharmacogenetic testing is a companion tool for prescribers, not a replacement for clinical judgment. Share your results with your doctor, nurse practitioner and pharmacist. Clinicians experienced in pharmacogenetics often find that results explain why some medications did not work or caused side effects in the past, which can help you feel confident trying a medication that better suits your genes. Learn more about how pharmacogenetic tests support prescribers.

Before your appointment, it helps to prepare a list of:

Medications you take now, including over-the-counter products and supplements.
Medications you have taken in the past, and any side effects or lack of benefit.
Symptoms you are experiencing now.

Do not stop or change a medication on your own based on your results.

How can a Pillcheck test help?

A Pillcheck test analyzes the genes that affect medication response, including CYP2D6 copy number. A clinical pharmacist reviews your results together with all your medications, including possible drug interactions, and prepares written recommendations for your prescriber about which medications and doses are most likely to suit you. Read more about how to get a pharmacogenetic test in Canada.

Pillcheck testing is performed in CAP- and COLA-accredited clinical genetic laboratories located in Canada. Pillcheck reports on 24 genes, increasing coverage with new guidelines and drug labels (see the genes tab), and covers 355 medications, which account for over 70% of dispensed prescriptions. The Pillcheck test panel ensures high accuracy for people of non-European and admixed ancestries. Your results last a lifetime, and report updates are free of charge.

Summary

A clinical-grade pharmacogenetic test performed in an accredited laboratory reads the variants it tests very reliably.
How useful the result is depends on which variants are included. Expert groups publish recommended variant lists that account for many ancestries.
CYP2D6 requires measuring the number of gene copies, which affects drugs like codeine and tramadol.
Other medications can change how your enzymes work, so results should be reviewed together with all your medications.
Clinical guidelines from CPIC and others, and Health Canada and FDA drug labels, guide recommendations for a growing list of medications.
Share your results with your prescriber and pharmacist, and do not change medications on your own.

Frequently asked questions

Can pharmacogenetic results be wrong?

Laboratory errors are rare in accredited clinical laboratories. More often, a result is incomplete rather than wrong: the test may not include a rare variant, or another medication may be changing how an enzyme works. A pharmacist review helps account for these factors.

Is an ancestry DNA test enough to guide my medications?

No. Ancestry tests are designed to trace family history. They often miss variants and gene copies that are important for medications, and they do not provide recommendations based on clinical guidelines.

Do I need to repeat the test?

No. Your genes do not change, so your results can be used for life. Your report can be updated as new guidelines are published.

Will my results change if I start a new medication?

Your genetic results do not change, but some medications can temporarily change how your enzymes work. Tell your pharmacist about any new medication so that your recommendations can be reviewed.