The age of personalized medicine is fast approaching, and with it, a whole new strategy for prescribing antidepressants. Researchers report in the medical journal JAMA Network Open that they are on the way to cracking the code to prescribe our best course of Selective Serotonin Re-uptake Inhibitor (SSRI) to treat moderate depression based on our genes.
In their May 2026 paper, researchers from across the United States present the results of a clinical trial designed to test whether using a patient’s DNA to choose an antidepressant is better than medics prescribing based on their usual practice. The team of over 200 researchers led by Dr Josh Peterson, Department of Biomedical Informatics, Vanderbilt University Medical Center, say that prescribing SSRIs based on genotype improves outcomes, with significant upgrades in mental health index scores in as little as six months.
Psychiatrists have long known that not every antidepressant works for every patient, making the path to recovery a complicated journey. Why is it that am SSRI that is very effective in one person does nothing for another? While in some cases the distinctions might lie in underlying neurobiology, in others there is a more basic reason. Individual differences in how our liver processes drugs before they get to our brain could be behind this mystery. Now scientists are bringing together genetics, biochemistry and psychiatry to map the interactions between our genes and different SSRIs.
Different SSRIs Work for Different People
In the 1990s the explosion of SSRIs changed the face of psychiatry, but 30 years later, people are still arguing over whether or not they are effective. This is in part because there is almost always a subgroup of individuals for whom a particular SSRI doesn’t seem to work. These people, often called ‘non-responders’, can skew the results of clinical trials. A small set of people for whom a drug does nothing can pull down the average success rate, making the drug look like it is less useful than it is for most people.
Over the decades, researchers have created many different types of SSRI, giving doctors a range of options for their patients to try. Pharmacologists have also been working to figure out why drugs work better in some people than others. It turns out that the reason behind some of the variation in potency lies in the way our liver metabolizes drugs. In fact, the extent that the same drug works (and causes side effects) in different people seems to be connected to how quickly it is degraded and eliminated from our body.
What do our genes have to do with prescribing SSRIs?
Our liver is the detox organ. It is packed full of enzymes that process and disassemble all kinds of substances in our bodies. From alcohol, to medication, if it doesn’t seem right, our liver will break it down and take it out using specialized enzymes, for example members of the cytochrome P450 family. Pharmacologists have discovered that two specific cytochrome P450 enzymes, CYP2D6 and CYP2C19, affect the pharmacological properties of SSRIs. There are different versions of the genes for these enzymes. Variations of these CYP2D6 and CYP2C19 enzyme encoding genes are part of our natural differences, like our hair colour or how long our toes are, and hardwired into our DNA. The versions of these enzymes, and the specific combination that each of us have affect how quickly our liver breaks down and removes drugs.
Pharmacogeneticists have created a set of guidelines that doctors can use to look up which SSRIs work best with different genotypes. The challenge now is to figure out whether these guidelines work in a clinical setting.
Charting a course to using our genes to prescribe SSRIs
The team planned to test whether choosing SSRIs based on the genotype-specific prescribing guidelines would be more effective than prescribing based on the doctor’s usual trial and error routine.
They would recruit patients with depressive disorder and split them into two very similar groups. Then they would measure their scores on a standard depression assessment tool, Patient-Reported Outcomes Measurement Information System (PROMIS) Emotional Distress – Depression 8a and b survey, at different points throughout the trial. Clinicians would treat the patients for six months. At the end they would compare the changes in scores of people who got genotype-specific prescriptions against people who had usual treatment.
All patients would have their DNA tested and none of them would know how their physician decided to prescribe their SSRIs. The treating doctors would not know the results of the DNA tests until the patient arrived at the clinic for their prescribing appointment. Would doctors using our genes to prescribe SSRIs make a difference to outcomes?
All Aboard!
The researchers recruited 1460 participants with moderate depression, including adults and children from 8 years of age. The average age of adults was 40.1 years and children 14.5. They collected their DNA to look for different genetic variants of CYP2D6 and CYP2C19, and then randomly divided the patients into two groups.
In the intervention group, clinicians would choose the drug based on a patient’s genetic profile using the Clinical Pharmacogenetics Implementation Consortium (CPIC) guideline for CYP2D6, CYP2C19, CYP2B6, SLC6A4, and HTR2A genotypes and serotonin re-uptake inhibitor antidepressants. In the control group, the doctors treated patients according to their usual routine.
At the start of the trial, all volunteers took the Patient-Reported Outcomes Measurement Information System (PROMIS) Emotional Distress – Depression survey, then again at the three-month midpoint and finally at the 6-month trial end point.
Of the 1460 patients, only 692 had genotypes covered by the pharmacogenetic guidelines, and some of those dropped out during the trial. By the end there were 329 people in the intervention group and 320 in the usual care group.
Across both groups, participants had an average PROMIS depression T score of 61.5 and personal health questionnaire-8 (PHQ-8) depression score of 12.5.
Did the pharmacogenetic guidelines help?
The results after 3 months showed no significant difference between groups in PROMIS depression scores and slightly larger for PHQ-8scores. In contrast, after 6 months the difference was significant, with the intervention group achieving depression remission in 48.3% of patients, compared to 39.4% in the control group according to PROMIS scores. The difference in PHQ-8 scores was also significant, with 29.9% of patients in the intervention group showing remission, versus 21.0% in the control group. The burden of side effects was similar between groups.
Overall this shows that doctors using the Clinical Pharmacogenetics Implementation Consortium (CPIC) guidelines to prescribe SSRIs might help some patients to find an effective treatment sooner.
Finding a Path Forward
At the end of the day, the difference was pretty small. Why was that? Was it just because the guidelines aren’t as good as the creators had hoped? It turns out there was more to this. The researchers designed the study to test therapeutic guidelines based on genetic profiles, but the clinicians actually choosing the therapy for each patient didn’t have to follow those recommendations strictly.
Doctors could make changes to prescriptions in case of severe side effects, like sleepiness, anxiety, and fatigue. At the same time, they could adjust the therapy for the patients in the control group, as they normally do in clinical practice; these changes could have affected the results.
In fact, at the end of the study, they examined the agreement between the therapies suggested by genetic profiles and the ones actually administered at 3 months in both groups. Of the intervention group, 15.5% of patients switched to a medication other than that suggested by the guidelines. In the control group, 25.7% had switched medications.
Pharmacogenetic guidance effectively made a difference only for about one patient in ten in the study. More than three quarters of the time doctors were picking the right drug even without the help of guidelines. Perhaps a notable finding of the trial was the therapeutic skills of the clinicians in identifying what works best for each patient.
New Horizons
While there was only a roughly 10% difference in finding an SSRI that worked by the 3-month point between groups, the results are still encouraging. The fact that the pharmacogenetic guided patients improved their depression scores compared to the control group hints that while doctors were choosing appropriate SSRIs 74% of the time, regardless of genotype, the genotype guided selections might have worked better.
Pharmacogenetic approaches to depression are still in their early days, but this trial shows we might be heading in the right direction. How long before your doctor uses your genes to prescribe SSRIs?
Sources
Blambila L, Sabei P, Raulino VG, Herkenhoff ME. Pharmacogenetics of antidepressant response: a focused review on CYP2C19, CYP2D6, SLC6A4, and HTR2A polymorphisms. Front Pharmacol. 2026;17:1773677. doi:10.3389/fphar.2026.1773677
Leveroni M, Jones S, Bell S. Depression Updates and Selective Serotonin Reuptake Inhibitor Management. Prim Care. 2026;53(2):153-165. doi:10.1016/j.pop.2026.01.002
Blake KV, Hines LJ, Liu M, et al. Genotype-Guided Antidepressant Prescribing for Patients With Depression: A Randomized Clinical Trial. JAMA Netw Open. 2026;9(5):e2610609. doi:10.1001/jamanetworkopen.2026.10609
