08 Apr Why Some Antidepressants Work Differently For Different People
Editor’s note: This piece discusses mental health issues. If you have experienced suicidal thoughts or have lost someone to suicide and want to seek help, you can contact the Crisis Text Line by texting “START” to 741-741 or call the Suicide Prevention Lifeline at 800-273-8255.
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Two people can take the same antidepressant at the same dose and have very different experiences. One may improve within weeks, while another may develop side effects or feel little benefit. This variation is well documented and is one reason clinicians increasingly look at biology, not just symptoms, when treatment is not going as expected.
Large treatment studies have also shown that only about one-third of patients achieve remission after a first antidepressant trial — which helps explain why medication selection often involves careful adjustment rather than a one-size-fits-all approach.
Medication Response Is Not Just About the Diagnosis
Antidepressant response can be influenced by many factors, including age, liver function, other medications, adherence, and underlying medical conditions. But one of the most important biological factors is how the body metabolises a drug.
Many medications are processed by a family of liver enzymes known as cytochrome P450 enzymes, which are responsible for most prescription drug metabolism. Within that system, a small number of enzymes account for the majority of clinically important drug breakdown. This matters because a medication has to reach the right concentration in the body to work well. If it is cleared too slowly, side effects may become more likely. If it is cleared too quickly, the drug may not remain at a therapeutic level long enough to help.
Why CYP2D6 Gets So Much Attention
One of the best-known enzymes in pharmacogenomics is CYP2D6 — a gene that helps determine how active the CYP2D6 enzyme is in the body. Peer-reviewed research has found that CYP2D6 is involved in the metabolism of 20–25% of commonly used medications, including several antidepressants, antipsychotics, beta-blockers, and certain pain medications.
People can inherit different CYP2D6 activity patterns, often described as poor, intermediate, normal, or ultrarapid metabolisers. These differences can affect how quickly some drugs are activated or cleared. That is why clinicians and patients sometimes explore CYP2D6 and antidepressant response when treatment seems unusually ineffective or poorly tolerated. In practical terms, these inherited genetic differences in how medications are processed may help explain why one person experiences sedation, nausea, or no improvement on a dose that works well for someone else.
What the Guidelines Say
The Clinical Pharmacogenetics Implementation Consortium (CPIC) has published formal prescribing guidance for several antidepressants based on CYP2D6 and related genes, including updated recommendations for SSRIs, SNRIs, and serotonin modulators. These guidelines help clinicians interpret a patient’s genetic test result and understand whether it may support a dose adjustment, closer monitoring, or consideration of an alternative medication.
In addition, the FDA’s pharmacogenetic associations table includes multiple CYP2D6-linked medications where genotype may affect systemic drug levels, adverse effects, or dosing considerations. For example, the FDA notes CYP2D6-related considerations for drugs such as venlafaxine, paroxetine, and vortioxetine. That does not mean a gene test can predict exactly which antidepressant will “work” — it means genetic information can sometimes help explain altered exposure, tolerability, or the need for dose adjustment.
A Useful Tool, Not a Standalone Answer
Pharmacogenomics can be helpful, especially when a patient has had multiple medication failures, unexpected side effects, or complex polypharmacy. However, it is only one part of treatment planning. Clinical decisions still depend on diagnosis, symptom severity, medical history, drug interactions, and close follow-up.
The safest takeaway is that if an antidepressant response seems unusual, the explanation may be biological — and sometimes genetics is part of that story.
Conclusion
Differences in antidepressant response are real, measurable, and often biologically complex. Pharmacogenomic information, especially around CYP2D6, can help clarify why some medications are less effective or harder to tolerate. Used alongside clinical judgment, it can support more individualised prescribing rather than trial-and-error alone.
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Last Updated on April 9, 2026 by Marie Benz MD FAAD