Could a Common Antidepressant Help Cancer Immunotherapy Work Better?

Depression and anxiety are common companions to a cancer diagnosis, and doctors regularly prescribe either SSRIs or benzodiazepines to help patients cope while they go through treatment. A new study in PLOS Medicine asks a question that hasn't gotten much attention until now: does the choice between these two medication types matter for how well immunotherapy works?

The answer, based on a large analysis of electronic health records, is that it might — though the researchers frame their results as an association rather than proof of a cause-and-effect relationship.

Why serotonin might matter to the immune system

The study builds on laboratory research showing that the serotonin transporter, a protein called SERT (produced by the SLC6A4 gene), can act like a brake on the immune cells that fight tumors — specifically CD8+ T cells. In mouse experiments, SSRIs, which work by blocking this transporter, appeared to release that brake and strengthen the effect of anti-PD-1 immunotherapy drugs. Whether this laboratory finding carries over to actual patients had remained an open question.

Comparing two groups of real-world patients

To investigate, the research team used a method called target trial emulation, essentially designing the kind of comparison a randomized trial would make, but applying it to existing health records instead of enrolling new patients. They drew on the TriNetX network, which covers data from more than 187 million patients, and identified adults with solid tumors who had depression or anxiety and were starting immune checkpoint inhibitor therapy between 2015 and 2025.

Patients taking an SSRI were compared against patients taking a benzodiazepine, with statistical matching applied across 49 different baseline characteristics to make the two groups as comparable as possible. This produced 1,567 well-matched pairs, totaling 3,134 patients.

What the data showed

Over two years of follow-up, 23.5% of SSRI users died compared with 34.4% of benzodiazepine users — a difference that translated into roughly a 37% lower relative risk of death in the SSRI group. All five SSRIs that could be examined individually showed a similar pattern, with risk reductions in a comparable range.

The SSRI group also showed fewer newly recorded cases of cancer spreading to other parts of the body, hinting at a possible immune-related mechanism rather than just a general health difference between groups. Three "negative control" conditions that shouldn't plausibly be affected by either drug — kidney stones, gallstones, and cataracts — showed no difference between groups, which supports the overall analysis, since it suggests the matching process was working as intended.

SSRI users experienced somewhat more immune-related side effects overall, but this was driven almost entirely by thyroid dysfunction, a known and generally manageable effect of immune checkpoint inhibitors. There was no corresponding rise in more serious immune complications like liver inflammation, lung inflammation, or colitis.

Tumor gene activity tells a consistent story

In a separate analysis using tissue samples from more than 8,000 tumors spanning 20 cancer types, drawn from The Cancer Genome Atlas, the researchers looked at how active the SLC6A4 gene was inside tumors. In 13 of the 20 cancer types, higher activity of this gene was linked to fewer immune cells infiltrating the tumor. The organs where this pattern was strongest roughly matched the organs where immune-related side effects showed up in the clinical data, adding a layer of biological plausibility to the pattern seen in patients.

What this doesn't prove

The authors are upfront about the limits of this kind of study. Health records don't capture a measure called performance status, essentially how functionally well or unwell a patient is at the start of treatment, and this factor strongly predicts cancer survival on its own. Benzodiazepines also tend to be prescribed in different clinical situations than SSRIs, often for acute distress tied to worsening illness, which means some of the survival difference could reflect how sick people already were rather than an effect of the medication itself. The tumor gene activity analysis, meanwhile, used samples from patients who were not receiving immunotherapy, so it can't directly confirm what's happening inside tumors during treatment.

Where this leaves patients and doctors

The researchers describe their findings as a rationale for testing SSRIs directly in a randomized clinical trial alongside immunotherapy, not as evidence that patients should change their current medications. Depression and anxiety treatment decisions involve many factors beyond this one study, and this research does not provide medical advice. Anyone taking or considering psychiatric medication during cancer treatment should talk with their own oncologist or physician about what's appropriate for their individual situation.

Source: PLOS Medicine, DOI: 10.1371/journal.pmed.1005262, based on research by Chen, Dai, Huang, Chen, Kao, Hsieh, Jhou, and Lee.