Start with the claim, because a version of it is probably why you are here: that roughly a third of people have a gene that stops antidepressants working, and that a test before treatment would spare them two useless courses. The first half is a garbled description of something real. The second half is not supported by the evidence, and the tests that promise it have been the subject of a formal FDA warning.
Here is the accurate version. Two liver enzymes, CYP2D6 and CYP2C19, do most of the work of breaking down antidepressants, and the genes coding for them vary a lot between people. In Europe that variation is not a 30% minority: by the reference frequency tables of the Clinical Pharmacogenetics Implementation Consortium (CPIC), around 47% of people of European ancestry are something other than a "normal" CYP2D6 metaboliser and around 60% are something other than a normal CYP2C19 metaboliser. What that variation predicts, though, is drug concentration — how much of a particular medicine ends up in your blood at a standard dose. It does not predict whether the underlying depression will respond to serotonergic medicines in general, and there is no variant that makes someone resistant to almost all antidepressants.
TL;DR Genes change dose, not destiny. CYP2C19 and CYP2D6 status is common, real and comes with published dosing guidance from CPIC and the Dutch DPWG — mostly "halve this dose" or "use a different drug from this list". The pharmacodynamic genes that commercial panels use to promise the "right" antidepressant are not clinically usable. Testing has the clearest value after a bad experience — unusual side effects at low doses, no effect despite good adherence, or before a tricyclic — rather than as a screen before your first prescription. In Germany expect a self-pay test and a guideline that prefers blood-level monitoring; in the UK expect to go private; in the Netherlands it is routine care.
How it actually works, in plain language
Every antidepressant has to be broken down by the liver. Most of them run through one or both of these two enzymes. Depending on which versions of the genes you inherited, you fall into one of four groups: poor metaboliser (the enzyme barely works), intermediate, normal, or ultrarapid (the enzyme works faster than usual). CYP2C19 adds a fifth, "rapid", between normal and ultrarapid.
The consequences are mechanical rather than mysterious. A poor metaboliser taking a standard dose accumulates more of the drug than the trials assumed — so side effects arrive early, at doses that look low on paper, and are easily misread as "this medicine doesn't agree with me". An ultrarapid metaboliser clears the drug so quickly that a standard dose may never reach a working concentration — so the medicine looks like a failure when it was arguably never properly delivered. That second scenario is the grain of truth inside the "resistance gene" story. It is drug-specific, not universal: your CYP2C19 status matters for escitalopram and sertraline, your CYP2D6 status for paroxetine and the tricyclics, and for some drugs — mirtazapine and CYP2C19, duloxetine and CYP2D6 — the Dutch working group found no interaction worth acting on at all.
The table above sets out each status, how common it is in Europe, and what the guidelines say to do about it. Two things in it are worth pausing on. First, the largest groups are the intermediate ones, where the practical advice is mild — titrate more slowly, aim lower. Second, the last row: SLC6A4 and HTR2A, the serotonin transporter and receptor genes, are what most consumer-facing panels actually lean on when they promise to name the antidepressant that will work for you. CPIC reviewed the evidence for both in its 2023 guideline and concluded the data do not support their use in prescribing; there are not even standard genotype-to-phenotype categories, so two labs can report the same DNA differently.
What the outcome evidence really shows
There is a difference between "this gene changes drug levels" (well established) and "testing people improves how they get better" (contested). The honest summary of the second question:
The two biggest trials did not deliver the headline. The GUIDED trial (1,167 patients, US, commercial combinatorial test) missed its primary endpoint: symptom improvement at week 8 was 27.2% with test-guided care versus 24.4% with usual care, a difference that could easily be chance. Its secondary outcomes did favour testing — remission 15.3% versus 10.1% — which is the number the marketing quotes. The PRIME Care trial (1,944 patients, US Department of Veterans Affairs, published in JAMA in 2022) showed clearly that giving clinicians the results changes prescribing — far fewer prescriptions with a predicted gene-drug interaction — but the effect on remission was, in the authors' own words, small and non-persistent: an advantage across 24 weeks, and no significant difference at week 24 itself.
Meta-analyses land on "moderate, and shaky". A 2025 systematic review in BMJ Mental Health pooling 13 randomised trials found higher remission with test-guided treatment at eight weeks (risk ratio 1.37) but not at twelve, and found that bigger gene panels added essentially nothing over small ones (Zhang et al., 2025). Almost none of these trials could blind the treating clinician, which is exactly the kind of design weakness that inflates soft outcomes.
The professional bodies have not moved. The American Psychiatric Association's workgroup reviewed the newer trials in 2024 and kept its position: the evidence does not support using currently available combinatorial pharmacogenomic tools to select treatment in major depression (Bousman et al., Am J Psychiatry). Germany's national depression guideline reaches the same conclusion in its own words — it makes no recommendation for pharmacogenetic testing either before the first antidepressant or after non-response, on the grounds that the evidence on patient-relevant endpoints is not convincing (NVL Unipolare Depression, version 3.2).
Where testing has clearly earned its place is safety. The European PREPARE study — nearly 7,000 patients across seven countries, published in The Lancet in 2023 — tested a 12-gene panel pre-emptively and found around 30% fewer clinically relevant adverse drug reactions among patients with an actionable variant. That is a real, large, European result. Note what it measures: fewer bad reactions to medicines, across 39 drugs, not faster recovery from depression.
So who is this test actually for?
Reading the evidence honestly, the case is strongest in these situations:
- You have had unusual side effects at low doses, or intolerable side effects on more than one drug in the same family. This is the pattern poor metaboliser status produces, and it is the situation in which even sceptical guidelines see a role.
- A drug did nothing at all despite good adherence and an adequate dose and duration. Ultrarapid metabolism is one candidate explanation among several — and it is worth checking specifically when the drug in question was one that runs through the enzyme concerned.
- A tricyclic is being considered. Tricyclics have a narrow margin between a working dose and a toxic one, and CPIC's guidance here is more assertive than for the SSRIs — for CYP2D6 poor metabolisers it advises halving the starting dose or choosing something else.
- Your blood level of an antidepressant has come back outside the therapeutic range and non-genetic explanations have been ruled out. This is precisely the situation in which the German guideline says genotyping may be considered.
- You are already at the treatment-resistant stage. Subgroup analyses in the 2025 meta-analysis found the remission benefit in difficult-to-treat depression, not in first-line patients — which is the mirror image of the founder-pitch use case.
And the weakest case is the one the marketing leads with: a healthy-curiosity test before your first prescription, in the expectation that it will name the drug that works. That is the claim regulators have objected to, and the trials do not support it.
How to get tested, country by country
Germany — self-pay, and the guideline says do something else first. Pharmacogenetic testing is available essentially only as an Individuelle Gesundheitsleistung (IGeL, self-pay); statutory insurance covers a short list of unrelated gene-drug pairs (DPYD before fluorouracil, CYP2C9 before siponimod, UGT1A1 before irinotecan and a couple more) but not psychiatric panels. Published self-pay price lists give the order of magnitude: one German laboratory's January 2025 list charges €127.35 for CYP2C19 alone (the relevant test for citalopram or escitalopram), €375.36 for CYP2D6 (paroxetine, fluvoxamine, venlafaxine, vortioxetine), €442.38 for the tricyclic panel and €542.92 for the full SSRI/SNRI panel — plus the doctor's own fee under the GOÄ schedule (MVZ price list, PDF). Under the Gendiagnostikgesetz a genetic test has to be ordered by a doctor with documented consent and counselling (GenDG), so there is no legal do-it-yourself route.
What to say to your psychiatrist, in the order the German guideline itself suggests: "Können wir zuerst den Serumspiegel bestimmen (TDM)? Und falls der Spiegel außerhalb des therapeutischen Bereichs liegt und nicht-genetische Ursachen ausgeschlossen sind — wäre dann eine CYP2D6-/CYP2C19-Genotypisierung sinnvoll?" Therapeutic drug monitoring is covered by statutory insurance, catches non-genetic causes too (interactions, smoking, irregular intake), and is what the guideline recommends as the simpler instrument.
United Kingdom — private, for now. Pharmacogenomic testing for antidepressants is not part of routine NHS prescribing. The NHS has implemented pharmacogenomics narrowly so far — DPYD in oncology, MT-RNR1 in neonatal care, and CYP2C19 before clopidogrel after stroke, which NICE recommended as the first pharmacogenomic test for a common condition and which is still working its way into routine commissioning (NICE HTG724). Some mental health trusts and specialist clinics do offer testing case by case, so it is worth asking. Privately, psychiatric panels run to several hundred pounds — one UK laboratory service lists £786 including VAT for its psychiatric panel, and private psychiatry clinics that bundle test plus consultations charge considerably more. If you buy one, check that the laboratory is accredited (UKAS/ISO 15189) and that the report cites CPIC or DPWG recommendations rather than a proprietary algorithm; the NHS Genomics Education CYP2D6 and CYP2C19 pages are the reference your prescriber is most likely to recognise.
A fair question to put to a GP or psychiatrist before spending anything: "If a CYP2D6/CYP2C19 result came back abnormal, would it change what you prescribe or how you dose it?" If the answer is no, the money is better spent elsewhere.
Netherlands — the one country where this is ordinary care. The Dutch Pharmacogenetics Working Group (DPWG) of the pharmacists' association KNMP writes dosing recommendations per gene-drug pair, and those recommendations are built into the national G-Standaard drug database — meaning that once a result is in your pharmacy record, the software flags a gene-drug interaction automatically the next time something is prescribed (KNMP). Testing is ordered through your GP, a hospital or a pharmacy; Erasmus MC publishes tariffs of around €86 per determination, with CYP2D6 nearer €191 because it needs more variants, and states that where there is a medical question — side effects or lack of effect — insurers in principle reimburse, though the cost may come out of your eigen risico (Erasmus MC). Ask for farmacogenetisch onderzoek (CYP2D6/CYP2C19) and make sure the result is recorded at your pharmacy, which is where its value actually accrues.
Elsewhere in Europe. Spain took part in the PREPARE study and testing is being built out through hospital services and regional programmes, but availability differs greatly between autonomous communities; in Poland we could not verify any publicly funded route, which leaves private laboratories. In most other countries the practical answer is the same: hospital psychiatry or clinical pharmacology if you are already in specialist care, private otherwise.
The limits — and where the hype lives
A genotype is not a phenotype. Your genes describe the enzyme you were born with; what matters clinically is how fast it is working today. Fluoxetine and paroxetine inhibit CYP2D6 strongly enough to turn a genetically normal metaboliser into a functional poor one, and smoking, other medicines and irregular intake all shift levels as well. This is precisely why the German guideline prefers measuring the actual blood level.
Panels are not interchangeable. Different tests interrogate different variants, so the same person can be assigned different predicted phenotypes by different labs — and the "combinatorial" algorithms that convert genotypes into colour-coded drug lists are proprietary, which is why CPIC explicitly places them outside what it can evaluate.
A result is not a verdict on a drug. "Poor metaboliser" does not mean a medicine won't work; it usually means it should be dosed lower. "Normal metaboliser" does not mean the next antidepressant will work; most non-response has nothing to do with these two enzymes. And no result on any panel justifies stopping or changing a medicine on your own — dose changes and switches are prescriber decisions, and abrupt stops cause withdrawal effects.
Be sceptical of anything sold on the promise of skipping the trial-and-error. That is the specific claim the FDA has acted against, and the specific claim the trials have failed to demonstrate. The FDA maintains a table of pharmacogenetic associations it considers supported by evidence — a useful sanity check against any test report that claims far more.
If antidepressants haven't worked for you
Genetics is one of several explanations for a treatment that hasn't helped, and rarely the first one worth checking. If two or more antidepressants have failed at an adequate dose and duration in your current episode, that situation has a name and a defined set of next options — switching, augmentation, psychotherapy, rTMS, ECT, and the EU-approved esketamine route. Our guide to what to do when depression treatment isn't working walks through all of them, and our eligibility check will tell you which are realistically open to you where you live.
Frequently asked questions
Is it true that about 30% of people are genetically resistant to antidepressants?
No. The underlying fact is that variants affecting antidepressant metabolism are common — in Europe, more than 30% for either gene. But metaboliser status changes the concentration of a specific drug in your blood, which affects dosing and side effects; it does not confer resistance to antidepressants as a class, and no such "resistance gene" is known.
Will the test tell me which antidepressant will work for me?
No, and that is the claim to be most careful about. What a well-conducted test tells your prescriber is which drugs may need a lower or higher dose, and which are best avoided given your enzyme status. The genes that would be needed to predict response — the serotonin transporter and receptor genes — were assessed by CPIC in 2023 and found not usable for prescribing.
Is it covered by insurance?
In Germany, generally not: it is a self-pay IGeL service, while therapeutic drug monitoring is covered. In the UK it is not part of routine NHS antidepressant prescribing, so it is usually private. In the Netherlands insurers in principle reimburse when there is a medical reason such as side effects or lack of effect, subject to your eigen risico.
Is a consumer DNA kit good enough?
Usually not for this purpose. CYP2D6 in particular involves gene deletions and duplications that many broad consumer arrays do not capture, third-party "interpretation" reports are not regulator-authorised, and in Germany genetic testing legally requires a doctor's order and consent under the GenDG. If you test, use a clinical laboratory whose report cites CPIC or DPWG.
Do I need to stop my medication before testing?
No. Your genotype does not change, so the sample — usually a cheek swab or blood — can be taken at any time. That is different from a blood-level (TDM) measurement, which has to be taken while you are on the drug, usually just before the next dose.
How often should I repeat it?
Never. The result is lifelong. Keep the report, put it in your pharmacy or GP record, and bring it to any new prescriber — that is where the value of the test compounds, across every future medicine that runs through the same enzymes.
Sources
- CPIC guideline (2023) — CYP2D6, CYP2C19, CYP2B6, SLC6A4 and HTR2A and serotonin reuptake inhibitor antidepressants
- CPIC guideline — CYP2D6, CYP2C19 and tricyclic antidepressants
- CPIC frequency tables — CYP2C19 and CYP2D6 phenotype frequencies by biogeographical group
- DPWG (KNMP) guideline — CYP2C19, CYP2D6 and SSRIs
- Greden et al. (2019), J Psychiatr Res — the GUIDED randomised trial
- Oslin et al. (2022), JAMA — the PRIME Care randomised trial
- Swen et al. (2023), The Lancet — 12-gene pharmacogenetic panel (PREPARE)
- Systematic review and meta-analysis (2025), BMJ Mental Health — PGx-guided versus unguided antidepressant treatment
- Bousman et al. (2024), American Journal of Psychiatry — APA workgroup update on pharmacogenomic tools in depression
- NVL Unipolare Depression, version 3.2 — German national depression guideline (pharmacogenetics and TDM sections)
- FDA — Table of Pharmacogenetic Associations
- FDA — warning letter over a genetic test claiming to predict patients' medication responses
- KNMP — pharmacogenetics and the G-Standaard (Netherlands)
- NHS Genomics Education — CYP2D6 and CYP2C19 knowledge hub
This guide is for general information only and is not medical advice, a diagnosis, or a recommendation for or against any test or treatment. Do not start, stop or change a medicine on the basis of a genetic test result — those are decisions for a licensed prescriber who knows your history. Prices and coverage rules were checked on the date above and change; confirm with the laboratory and your insurer. If you are in crisis, contact your local emergency number (112 in the EU) or a crisis line immediately.