Two caveats - first, like most things, antibiotic resistance becomes a problem rapidly; second, it turns out crossing the blood-brain barrier reliably is tricky

Two caveats - first, like most things, antibiotic resistance becomes a problem rapidly; second, it turns out crossing the blood-brain barrier reliably is tricky, and not incidentally triggering a severe immune response in destroying whatever you came there to attack is nasty too. You know, you say that should be a rapid discovery, but "whoops this medication has even stronger effects in a very different domain" is a thing that periodically happens, and because treatment facilities aren't necessarily looking for or even asking the right questions to notice extremely different side effects, you might not see it. - We realized ketamine had psychiatric benefits from after-the-fact research trying to figure out why some people had better PTSD outcomes than others - Rogaine was being tested as an ulcer treatment when they noticed its now synonymous effects - a number of classes of antibiotics have been seeing research publications of "hey I think this might help {depression,bipolar,anxiety,schizoaffective} symptoms" (cycloserine is one I know of offhand) and have all had known possible psychiatric side effects (albeit undesirable ones) on their boxes for decades - Listerine was originally targeted as a general antiseptic and gonorrhea cure - dextromethorphan was originally discovered during research for codeine alternatives - Prozac was discovered from people attempting to improve on observed antidepressant properties in an existing antihistamine (and thus, in turn, turning attention to the entire chain of SSRI and related drug classes) - MAOIs (the oldest class of antidepressant drugs) were discovered from noticing a tuberculosis drug relieved some depressive patients. - TCAs (the "less harsh" family of antidepressants compared to MAOIs) were discovered around the same time as MAOIs by spontaneously noticing a synthetic antihistamine relieved depressive symptoms during testing - One of my favorite serendipitous discoveries is a woman who realized she could smell Parkinson's when her partner developed a weird smell beginning almost a decade before he got a diagnosis, and then noticed all the patients at the support group they attended had the smell too...so she went to some researchers, and now we know there's a dozen or so molecules that are ~uniquely extremely heavily concentrated on the skins and sweat of Parkinson's patients.