The first antidepressant, an MAO inhibitor (iproniazid), was discovered serendipitously when it was observed to improve mood in tuberculosis patients.

“the first anti-depressant was actually a tuberculosis treatment they were giving it to patients on a tuberculosis Ward and an astute infectious disease doctor noticed some of these patients are really depressed but when I give them this tuberculosis treatment they perk up like within a few weeks they start looking a lot less depressed and I don't think it's a coincidence I think it's the medication I'm giving them do you recall what the drug was aoide it's the first MAO inhibitor and um I could be saying the name wrong but uh it's first MAO inhibitor and uh that became the first anti-depressant”