Pharmacogenetic tests: an effective decision support tool for prescribers

Pharmacogenetic (PGx) testing shows how a patient's genes affect the way they break down and respond to medications. For prescribers, it is a decision support tool: it does not pick the perfect drug, but it flags patients who are likely to clear a medication much faster or slower than usual, and it points to guideline-based dosing or alternatives. This article reviews the main concerns clinicians raise about PGx testing, what the latest evidence and guidelines say, and how to use testing in practice.

Why is pharmacogenetic testing still underused?

Pharmacogenetics is not new. Arno Motulsky described the link between genes and drug response in 1957, and Werner Kalow published "Pharmacogenetics. Heredity and the response to drugs" in 1962. Today there are well-established prescribing guidelines and affordable tests, yet PGx testing is still underused outside psychiatry. Clinicians most often raise three concerns:

1. Reliability: different tests can give different recommendations for the same patient.
2. Clinical benefit: drug levels do not always predict how a patient will respond.
3. Cost: patients may have to pay out of pocket.

Which pharmacogenetic tests are reliable?

The value of a PGx test depends on two things:

• What the test detects. A panel must include the gene variants that are common in different ancestries to predict metabolism accurately in a diverse population like Canada's. Reputable providers disclose which genes and variants they test.
• How results are interpreted. A 2018 comparison of four commercial psychiatric tests found that recommendations for the same patients agreed only 84% of the time for mood stabilizers, 56% for antidepressants, 56% for anxiolytics and hypnotics, and 55% for antipsychotics. Many proprietary tests are "black box" tools that do not show how genetic results become recommendations, and some combine genes such as SLC6A4 and HTR2A that the 2023 CPIC antidepressant guideline does not support for prescribing.

Pillcheck uses transparent interpretation based on CPIC guidelines, other clinical guidelines and drug labels, so you can check every dosing alert against its source. 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.

What does the evidence show?

Earlier psychiatric trials were small and mixed, but the evidence base has grown:

• Depression. A 2025 meta-analysis of 13 randomized trials found that PGx-guided treatment improved response (risk ratio 1.23 at 8 weeks and 1.29 at 12 weeks) and remission at 8 weeks (risk ratio 1.37) compared with usual care. A 2024 meta-analysis of 15 trials found similar gains, while noting that the effects are modest and weaker in the trials with the lowest risk of bias.
• Adverse drug reactions across medical specialties. In the PREPARE trial of 6,944 patients in seven European countries, genotype-guided prescribing with a 12-gene panel reduced clinically relevant adverse drug reactions by 30% (odds ratio 0.70).

PGx testing does not predict the single best medication for an individual. It predicts how quickly a drug is likely to be cleared. Poor metabolism is a strong, though not perfect, predictor of side effects and intolerance, and much faster clearance lowers the chance of a response at usual doses. Pharmacogenetics is a companion, rather than a competitor, to protocol-based care.

How does PGx testing fit with psychotherapy?

PGx testing is a key tool for guiding the choice and dose of antidepressants and antipsychotics, and it complements psychotherapy rather than replacing it. A network meta-analysis of 101 trials found that combining medication with psychotherapy led to higher response and remission rates than either treatment alone. A meta-analysis of 17 trials found that optimizing medication first, then adding psychotherapy once patients respond, lowered the risk of relapse and recurrence, leading to a more sustainable response. Getting the medication right early, with the help of PGx testing, helps patients benefit fully from talk therapy.

What do Canadian and international guidelines say?

• CPIC publishes peer-reviewed gene-drug guidelines, including the 2023 guideline for CYP2D6, CYP2C19 and CYP2B6 and serotonin reuptake inhibitor antidepressants.
• CANMAT 2023 (Canadian depression guidelines) does not recommend routine PGx testing for every patient, because the average benefit is modest. It notes that testing may be useful when patients have side effects at low doses or a poor response at usual doses, may help choose treatments in difficult-to-treat depression, and recommends interpreting results with CPIC guidelines.
• The International Society of Psychiatric Genetics supports using CYP2D6 and CYP2C19 results to guide antidepressant prescribing.
• Therapeutic drug monitoring (TDM) remains valuable. The AGNP consensus guidelines recommend TDM for many psychiatric drugs and describe how pharmacogenetic results complement drug levels. A PGx test predicts metabolism for hundreds of drugs from a single sample, while TDM measures actual levels and is especially useful when nonadherence is suspected.

When is pharmacogenetic testing most useful?

• Patients with side effects at low doses or no response at usual doses.
• Patients who have tried several medications without success.
• Drugs with a narrow therapeutic index or with CPIC or Health Canada label guidance.
• Patients taking several medications, where gene-drug and drug-drug interactions combine.

Timing matters: the earlier, the better. When testing is limited to people who have already failed several therapies, many opportunities to intervene are missed: months of ineffective treatment, side effects, lost work time and lost confidence in treatment. Testing at the start of treatment, or at the first sign of intolerance or poor response, gives the results the most chance to make a difference. In the PREPARE trial, patients were tested before their first prescription of the drug. Pillcheck's B2B benchmarking for employers, insurers and disability management partners can show how the impact differs by type and timing of referral.

Results also help conversations with patients. Explaining why past medications caused side effects or failed can reassure patients that their experience was real, and give them confidence to try a medication that better fits their genetics. Because genes do not change, a single test can inform prescribing for life. Read more in Encountering pharmacogenetic test results in the psychiatric clinic.

How is testing paid for?

PGx testing is not covered by provincial health plans, but many working Canadians can claim it through private health benefits or a Health Spending Account. Pillcheck is used by Canada Life, Medavie Blue Cross, Alberta Blue Cross, Pacific Blue Cross and Manitoba Blue Cross. Pillcheck is also a preferred vendor for Bell Canada health benefits, and for Organizational Solutions and its customers, including trade unions.

Insurance and disability management providers also use PGx testing in medication management programs. If your patient is on disability leave, they can ask their case manager to order a test through the secure Pillcheck Portal. The case manager only facilitates the referral and forwards the pharmacist's summary to the patient's physician; they see the pharmacist's recommendations, not the full genetic report. The Portal supports compliance with genetic non-discrimination and privacy laws.

How do Pillcheck pharmacists support prescribers?

Every Pillcheck test includes a medication review by a clinical pharmacist, who assesses the patient's current medications in light of their genetic results. The summary is shared with the patient and the referring clinician. For these reviews, Pillcheck partners with Northwest Telepharmacy, which employs clinical pharmacists across Canada. Each Pillcheck pharmacist letter includes the pharmacist's licence number, phone number and email, so you can contact them directly with questions.

Where can clinicians learn more?

The American Medical Association offers the online CME series ASHP-AMA Pharmacogenomics Virtual Summit Series. The University of Florida offers a collection of pharmacogenomics courses, including implementation guides for hospitals.

Summary

• PGx testing predicts how patients metabolize medications and helps reduce side effects; it supports, rather than replaces, clinical judgement.
• Meta-analyses show modest improvements in depression response and remission, and the PREPARE trial showed 30% fewer adverse drug reactions with a 12-gene panel.
• PGx testing guides antidepressant and antipsychotic prescribing and complements psychotherapy: combining medication with talk therapy improves remission, and optimizing medication first, then adding talk therapy, leads to a more sustainable response.
• CANMAT recommends testing for selected patients, such as those with intolerance or poor response, and interpreting results with CPIC guidelines.
• Refer early: testing at the start of treatment or at the first sign of intolerance offers more opportunities to improve outcomes than waiting until several therapies have failed.
• Choose tests with transparent, guideline-based interpretation. Pillcheck follows CPIC guidelines and drug labels and reports on 24 genes.
• Testing can be covered through workplace benefits, Health Spending Accounts and disability management programs.
• A clinical pharmacist's medication review saves prescribers time interpreting results.

Frequently asked questions

Is pharmacogenetic testing evidence-based?

Yes. CPIC publishes peer-reviewed gene-drug guidelines, Health Canada and FDA labels include pharmacogenetic information for many drugs, and randomized trials show fewer adverse drug reactions and modest improvements in depression outcomes with PGx-guided prescribing.

Do Canadian guidelines recommend PGx testing?

CANMAT 2023 does not recommend routine testing for every patient with depression, but notes it may help patients with side effects at low doses, poor response at usual doses or difficult-to-treat depression, and recommends using CPIC guidelines to interpret results.

When should I refer a patient for PGx testing?

The earlier, the better. Testing at the start of treatment or at the first sign of side effects or poor response gives the most opportunity to improve outcomes. Waiting until several medications have failed misses chances to intervene.

Why do different PGx tests give different recommendations?

Tests differ in which genes and variants they analyze and in how they turn results into recommendations. Tests that follow published CPIC guidelines and drug labels give transparent recommendations you can verify.

Does a PGx test replace therapeutic drug monitoring?

No. PGx testing predicts metabolism for many drugs before treatment starts, while TDM measures actual drug levels. The two are complementary.

How can I refer a patient to Pillcheck?

Contact Pillcheck at info@pillcheck.ca or 1-877-409-3629. Patients can also order directly and share the pharmacist's summary with you.

If you are interested in referring your patients to Pillcheck, please contact us at info@pillcheck.ca or 1-877-409-3629.

References

Bousman CA, Dunlop BW. Genotype, phenotype, and medication recommendation agreement among commercial pharmacogenetic-based decision support tools. Pharmacogenomics J. 2018;18(5):613-622.

Bousman CA, Eyre HA. "Black box" pharmacogenetic decision-support tools in psychiatry. Braz J Psychiatry. 2020;42(2):113-115.

Bousman CA, Stevenson JM, Ramsey LB, et al. Clinical Pharmacogenetics Implementation Consortium (CPIC) Guideline for CYP2D6, CYP2C19, CYP2B6, SLC6A4, and HTR2A Genotypes and Serotonin Reuptake Inhibitor Antidepressants. Clin Pharmacol Ther. 2023;114(1):51-68.

Zhang Y, Gao Y, Zou Y, et al. Comparative effectiveness of pharmacogenomic-guided versus unguided antidepressant treatment in major depressive disorder: new insights from subgroup and cumulative meta-analyses. BMJ Ment Health. 2025;28(1).

Milosavljević F, Molden E, Ingelman-Sundberg M, Jukić MM. Current level of evidence for improvement of antidepressant efficacy and tolerability by pharmacogenomic-guided treatment: A systematic review and meta-analysis of randomized controlled clinical trials. Eur Neuropsychopharmacol. 2024;81:43-52.

Swen JJ, van der Wouden CH, Manson LE, et al. A 12-gene pharmacogenetic panel to prevent adverse drug reactions: an open-label, multicentre, controlled, cluster-randomised crossover implementation study. Lancet. 2023;401(10374):347-356.

Lam RW, Kennedy SH, Adams C, et al. Canadian Network for Mood and Anxiety Treatments (CANMAT) 2023 Update on Clinical Guidelines for Management of Major Depressive Disorder in Adults. Can J Psychiatry. 2024;69(9):641-687.

Hart XM, Gründer G, Hiemke C, et al. Consensus Guidelines for Therapeutic Drug Monitoring in Neuropsychopharmacology: Update 2026 - Pharmacokinetic, pharmacogenetic and clinical aspects. Pharmacopsychiatry. 2026;59(4):166-295.

Cuijpers P, Noma H, Karyotaki E, et al. A network meta-analysis of the effects of psychotherapies, pharmacotherapies and their combination in the treatment of adult depression. World Psychiatry. 2020;19(1):92-107.

Guidi J, Fava GA. Sequential Combination of Pharmacotherapy and Psychotherapy in Major Depressive Disorder: A Systematic Review and Meta-analysis. JAMA Psychiatry. 2021;78(3):261-269.

Bousman CA, Müller DJ. Pharmacogenetics in Psychiatry: A Companion, Rather Than Competitor, to Protocol-Based Care. JAMA Psychiatry. 2018;75(10):1090.

Bousman CA, Mukerjee G, Men X, et al. Encountering Pharmacogenetic Test Results in the Psychiatric Clinic. Can J Psychiatry. 2022;67(2):95-100.

International Society of Psychiatric Genetics. Genetic Testing Statement.