A widely used class of antidepressants might also help the body combat cancer, according to new research.

Scientists have found that selective serotonin reuptake inhibitors (SSRIs)—commonly prescribed for mood disorders—could shrink tumours by more than 50%, potentially enhancing the body’s immune response against cancer.

SSRIs work by increasing levels of serotonin, often referred to as the brain’s "happiness hormone." While primarily used to treat conditions like depression and anxiety, researchers wanted to investigate whether their influence on serotonin could also affect other systems in the body, including those involved in tumour suppression.

The findings, published in the journal Cell, were inspired by earlier observations that some cancer cells had high concentrations of enzymes that break down serotonin. This prompted scientists to explore whether blocking this breakdown with SSRIs could empower the body’s natural defences—especially T cells, the immune system’s primary fighters against disease.

In experiments involving mice with tumours and human cancer cells grown in lab dishes, SSRI treatment led to an average reduction in tumour size of around 50%. The treatment appeared to enhance the effectiveness of T cells, making them more capable of destroying cancerous cells.

"SSRIs don’t just improve mood—they also reinvigorate T cells by increasing their access to serotonin," said Dr. Lili Yang, senior author of the study from the University of California. "This makes them more effective at targeting and killing cancer cells."

One significant advantage of SSRIs is their long-established safety profile. Dr. Yang noted that repurposing these existing drugs for cancer treatment could be far faster and more practical than developing entirely new therapies.

The study also revealed that SSRIs could boost the performance of existing immunotherapies. When combined with an anti-PD-1 antibody—a treatment designed to unmask tumours and help the immune system attack them—SSRIs helped significantly reduce tumour sizes in mice. In some cases, the combination led to complete remission.

Despite the promising results, researchers caution that the study was based on laboratory and animal models. The team is now planning to study real-world data, examining whether cancer patients who are already taking SSRIs fare better than those who are not. Early estimates suggest about 20% of cancer patients may be on SSRIs, offering a substantial group for follow-up studies.

However, it's important to weigh potential risks. Although SSRIs are widely used and generally considered safe, previous research has linked long-term use to possible side effects such as sexual dysfunction.

In the UK, antidepressants are among the most commonly prescribed medications, with around 8.7 million people—roughly 15% of the population—receiving prescriptions. In the United States, about one in eight people are currently taking an antidepressant, according to the CDC.

With over 400,000 cancer diagnoses annually in the UK alone, the potential to repurpose a common and accessible drug like SSRIs could mark a significant advancement in cancer treatment—pending further human trials. Photo by Lanfear's Bane at English Wikipedia.