What genetic (pharmacogenomic) testing can and can't tell you

What the test actually measures

The best-established part of pharmacogenomic testing looks at liver enzymes — chiefly CYP2D6 and CYP2C19 — that determine how quickly your body processes certain antidepressants. For these specific enzyme-drug pairs there are published, evidence-based dosing guidelines (from a group called CPIC). Many commercial panels also test serotonin-transporter and -receptor genes (SLC6A4, HTR2A) and fold them into a single recommendation — but the evidence does not support using those genes to guide prescribing, and the guidelines say so explicitly. Not all of what's on the report carries the same weight.

What the trials actually found

The evidence is narrower than many marketing claims suggest. In the GUIDED trial, testing did not meet its primary symptom outcome. The larger VA PRIME Care trial found that testing reduced prescriptions for drugs with predicted gene interactions and produced a small, short-lived improvement in remission. A 2018 American Psychiatric Association work group judged the evidence insufficient to support routine use, and the FDA has cautioned against claims that these tests predict which drug will work best.

Where testing genuinely helps

The evidence points to narrower, real uses — which is where it earns its place. Testing can help when you've had unexplained side effects or non-response on a medication that the tested enzymes actually process; when dosing certain older antidepressants (tricyclics) or other drugs with a narrow margin of safety; and when something in your history has already raised a specific flag. That's a different proposition from 'test everyone first, then pick.'

Your history leads — and can override the genes

Genotype is one input among several, and it isn't even always the deciding one. Other medications you take can inhibit the very same enzymes and effectively override your genetics — a phenomenon called phenoconversion. Your actual prior response to medications, your side effects, your medical context, and your goals stay central to the decision. Testing adds context to clinical judgment; it doesn't replace it.

What it can't do, plainly

A test can't pick the right medication by itself, can't guarantee that a drug will work or be tolerated, and can't substitute for a real evaluation and follow-up. Coverage and cost vary widely. Foundry discusses those limits — and the cost — before anything is ordered, and offers testing when the clinical picture suggests it would genuinely inform the decision, not as a routine step.

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