Common antidepressant drugs help the immune system to attack tumors in mice A class of drug called monoamine oxidase inhibitors is commonly prescribed to treat depression; the medications work by boosting levels of serotonin, the brain's "happiness hormone." A new study by UCLA researchers suggests that those drugs, commonly known as MAOIs, might have another health benefit: helping the immune system attack cancer. Their findings are reported in two papers, which are published in the journals Science Immunology and Nature Communications. MAOIs had not been linked to the immune system's response to cancer before. What's especially exciting is that this is a very well-studied and safe class of drug, so repurposing it for cancer isn't as challenging as developing a completely new drug would be."