Pharmacogenomic Testing, Treatment Resistance and Where TMS Fits

Pharmacogenomic (PGx) testing looks at genes that affect how your body processes medications. For antidepressants, its most reliable use is narrow: identifying people who break down certain drugs unusually slowly or quickly, which can explain side effects at low doses or a lack of effect at normal ones. It cannot tell you which antidepressant will work, it does not diagnose depression, and it says nothing about whether TMS will help, because TMS is not a drug and is not metabolized.

In two large randomized trials, GUIDED and PRIME Care, giving clinicians a PGx report produced small improvements in remission and no significant improvement on the primary symptom measure in one of them. The 2023 CANMAT guidelines do not recommend routine testing but see a role when someone has had severe or unusual side effects or has not responded to adequate doses. That is exactly the situation of many people considering TMS, which is why the two topics meet.

What these tests measure

Commercial panels for psychiatry mostly report variants in genes for liver enzymes that metabolize medications, chiefly CYP2D6 and CYP2C19. Some add genes for the serotonin transporter and serotonin receptors. From the enzyme genes a laboratory infers a metabolizer type, from poor to ultrarapid, and sorts medications into color-coded or tiered lists.

The two kinds of gene are not equally useful. The Clinical Pharmacogenetics Implementation Consortium (CPIC), an international group that writes peer-reviewed gene-drug prescribing guidelines, gives recommendations for using CYP2D6, CYP2C19 and CYP2B6 results when prescribing SSRIs and related antidepressants. For the serotonin transporter gene (SLC6A4) and the serotonin-2A receptor gene (HTR2A), it concludes that existing data do not support clinical use. A separate CPIC guideline covers CYP2D6 and CYP2C19 and the dosing of tricyclic antidepressants.

In other words, the pharmacokinetic part of a report (how much drug reaches your bloodstream) rests on reasonable evidence. The pharmacodynamic part (how your brain is predicted to respond) does not.

What the trials found

Two points help in reading these numbers. First, a relative figure such as "1.41 times more likely" sounds larger than it is when remission rates are low to begin with. CANMAT's summary of the meta-analyses puts the absolute gain in response at 4.6 to 7.2 percentage points and notes that the trials carry significant risks of bias, primarily from a lack of blinding. Second, in GUIDED, patients who entered the trial on a medication the test flagged and switched off it by week 8 improved more than those who stayed on a flagged medication, a comparison that was not randomized. That fits the modest claim that the tests help avoid poorly matched drugs, not the stronger claim that they pick winners.

What FDA has said

FDA has addressed these tests twice in ways that matter to patients.

In a safety communication issued on October 31, 2018, the agency warned that many genetic tests claiming to predict response to specific medications had not been reviewed by FDA and might lack supporting evidence, using antidepressant tests as its example. It stated that "the relationship between DNA variations and the effectiveness of antidepressant medication has never been established" and told patients not to change or stop any medicine based on a genetic test report they had obtained on their own, but to discuss the results with their health care provider.

FDA also maintains a Table of Pharmacogenetic Associations listing gene-drug pairs for which it considers the evidence sufficient that some patients will have altered drug metabolism. Several antidepressants appear on it. For citalopram in CYP2C19 poor metabolizers the table gives a maximum recommended dose of 20 mg because of higher drug levels and QT-prolongation risk, and for vortioxetine in CYP2D6 poor metabolizers a maximum of 10 mg. The agency adds that inclusion in the table does not mean it advocates testing before prescribing, that most listed associations have not been evaluated for their effect on clinical outcomes, and that genetic makeup is only one of many factors affecting drug levels and response. The table is available on the FDA website.

Taken together, the regulatory position matches the scientific one: metabolism findings for specific drugs are real and sometimes actionable, while broad claims about predicting antidepressant response are not established.

When testing is and is not worth doing

CANMAT's position is a practical guide. It does not recommend routine PGx testing, because the benefits are too modest and inconsistent to justify delaying treatment while waiting for results. It considers testing potentially useful when there have been severe or unusual adverse effects at low doses, or a poor response at therapeutic doses, since these patterns can reflect atypical metabolism. It also lists pharmacogenetic variability among the factors that can contribute to a poor response to initial treatment.

• Reasonable to consider: several medications stopped for side effects at starting doses; no effect from more than one drug metabolized by the same enzyme; a planned trial of a tricyclic antidepressant, for which genotype-based dosing guidance exists.
• Unlikely to help: choosing a first antidepressant; deciding whether depression is "biological"; predicting response to psychotherapy or brain stimulation.
• Not a substitute for: a careful review of what was actually tried, at what dose and for how long.

Results also need context a genetic report cannot supply. Other medications can inhibit the same enzymes, so a person with normal genes may behave like a poor metabolizer while taking an interacting drug. Age, liver and kidney function, and adherence all affect drug levels as well.

Where TMS fits

PGx testing and TMS relate to each other in three ways.

The test does not predict TMS response

The enzyme genes on a PGx panel govern drug metabolism, and no drug is involved in TMS. Researchers have looked instead at genes involved in brain plasticity, particularly a common variant in the BDNF gene, with inconsistent results. In a 2026 naturalistic study of 75 patients completing a standard course of 10 Hz rTMS, response and remission did not differ by BDNF genotype. No genetic test is validated for selecting patients for TMS. The broader question of prediction is covered in who responds to TMS.

The test can clarify the medication history that leads to TMS

Most people reach TMS after medications have not worked or have not been tolerated, and insurers define eligibility in those terms. New York Medicare accepts either a lack of response to medication from at least two classes or intolerance shown by two trials; Optum's policy counts inability to tolerate medication alongside lack of response. A PGx result showing poor metabolism can make sense of a history of side effects at low doses. A result showing ultrarapid metabolism can raise the question of whether a "failed" trial ever reached an effective drug level.

Either finding can point in two directions. It may suggest one more medication trial with a better-matched drug or an adjusted dose. Or it may support moving to a non-drug treatment: CANMAT notes that rTMS can be recommended as a first-choice treatment when there are tolerability concerns with medication options. A test result alone does not change an insurer's medication-trial requirements, and coverage of the test itself varies by plan, so check both before ordering.

The test should not delay effective treatment

For someone who already meets criteria for treatment-resistant depression, waiting weeks for a genetic report before starting any next step has a cost. CANMAT cites that delay as a reason against routine testing. If a test is going to be done, it can run in parallel with the TMS evaluation and insurance authorization. Medication often continues during TMS (patients in the 2018 THREE-D trial stayed on stable antidepressant doses), so results may still be useful for adjusting it; see combining TMS with medication.

How a precision psychiatry practice uses genetic results

Dr. Lewis's practice lists genetic insights that can inform medication choice and dosing among the tools of its precision psychiatry approach, alongside hormone, inflammation and nutrient testing. Its own article on the subject is careful about the limits: genetic testing "doesn't choose the 'perfect' medication," but can help guide dosing, reduce side effects and explain past treatment challenges. That description is consistent with the CPIC and FDA positions above. The practice's pages on precision psychiatry and how genetic testing guides medication choices go into the specific genes.

Used this way, a PGx report is one input into a wider review that also includes the diagnosis, medical contributors and the adequacy of past trials. Ruling out medical causes before TMS covers that review, and every treatment-resistant depression option compared sets out what can follow it. The role of testing within an integrative approach is discussed in TMS in an integrative, precision psychiatry practice.

Questions to ask before ordering a test

1. Which genes does this panel report, and which of them have CPIC prescribing guidelines?
2. Will the result change what we do next, or would we make the same decision without it?
3. Am I taking anything that inhibits these enzymes and would override my genotype?
4. What will it cost me, and does my plan cover it for my situation?
5. Can the next treatment step begin while we wait for the result?

The rest of this series is collected in the TMS guide, and the TMS service page explains how to start a conversation about whether TMS is an option.

Frequently asked questions

Can a genetic test tell me which antidepressant will work?

No. The tests can show whether you are likely to process certain drugs unusually fast or slowly, which helps with dosing and side effects. FDA has stated that a relationship between DNA variations and antidepressant effectiveness has not been established.

Does pharmacogenomic testing predict whether TMS will work?

No. These tests examine drug-metabolizing enzymes, and TMS is not a drug. No genetic test is currently validated for predicting TMS response.

Should I stop my antidepressant if the test puts it in the red column?

Not without talking to your prescriber. FDA has specifically told patients not to change or stop a medicine based on a genetic test report without discussing it with their health care provider. A medication that is working and well tolerated usually should not be changed because of a test category.

Do I need genetic testing before TMS?

No. It is not required for TMS and does not affect how TMS is delivered. It can be worth doing if your history of side effects or non-response suggests unusual drug metabolism, and it can be done alongside the TMS evaluation.

Will a test result help with insurance approval for TMS?

Not directly. Insurers base TMS approval on documented medication trials or intolerance. A genetic result may help explain that history, but it does not replace the documentation plans ask for.

Sources

1. Canadian Network for Mood and Anxiety Treatments (CANMAT) 2023 Update on Clinical Guidelines for Management of Major Depressive Disorder in Adults. Canadian Journal of Psychiatry, 2024. pmc.ncbi.nlm.nih.gov
2. Impact of pharmacogenomics on clinical outcomes in major depressive disorder in the GUIDED trial: A large, patient- and rater-blinded, randomized, controlled study. Journal of Psychiatric Research, 2019. pubmed.ncbi.nlm.nih.gov
3. Effect of Pharmacogenomic Testing for Drug-Gene Interactions on Medication Selection and Remission of Symptoms in Major Depressive Disorder: The PRIME Care Randomized Clinical Trial. JAMA, 2022. pubmed.ncbi.nlm.nih.gov
4. Pharmacogenomic Testing and Depressive Symptom Remission: A Systematic Review and Meta-Analysis of Prospective, Controlled Clinical Trials. Clinical Pharmacology & Therapeutics, 2022. pubmed.ncbi.nlm.nih.gov
5. Clinical Pharmacogenetics Implementation Consortium (CPIC) Guideline for CYP2D6, CYP2C19, CYP2B6, SLC6A4, and HTR2A Genotypes and Serotonin Reuptake Inhibitor Antidepressants. Clinical Pharmacology & Therapeutics, 2023. pubmed.ncbi.nlm.nih.gov
6. Clinical Pharmacogenetics Implementation Consortium guideline (CPIC) for CYP2D6 and CYP2C19 genotypes and dosing of tricyclic antidepressants: 2016 update. Clinical Pharmacology & Therapeutics, 2017. pubmed.ncbi.nlm.nih.gov
7. The FDA Warns Against the Use of Many Genetic Tests with Unapproved Claims to Predict Patient Response to Specific Medications: FDA Safety Communication. U.S. FDA (archived copy), 2018. web.archive.org
8. Table of Pharmacogenetic Associations. U.S. FDA, 2026. fda.gov
9. The Role of Brain Derived Neurotrophic Factor Polymorphism in Transcranial Magnetic Stimulation Response for Major Depressive Disorder. Journal of ECT, 2026. pubmed.ncbi.nlm.nih.gov
10. Transcranial Magnetic Stimulation LCD L33398 (v40). CMS, 2026. cms.gov
11. Behavioral Clinical Policy: Transcranial Magnetic Stimulation (BH803TMS112025). Optum Behavioral Health Solutions, 2026. providerexpress.com
12. Effectiveness of theta burst versus high-frequency repetitive transcranial magnetic stimulation in patients with depression (THREE-D): a randomised non-inferiority trial. Lancet, 2018. pubmed.ncbi.nlm.nih.gov

Educational information, not medical advice. Whether TMS or any treatment fits you is a decision to make with a psychiatrist who knows your history.