Much of medicine (especially mental health related) is rather primitive. We are literally reverse engineering discoveries that seem to work on some other animals, and then figuring out how (to the best of our limited ability).

I liken SSRIs to carpet bombing - also their mechanism to increase seratonin in the brain is by making something else not use it (reuptake inhibition). We’re guessing that other thing isn’t a big deal. But who knows. Serotonin is produced in the gut and it controls melatonin, which controls our sleep. It’s all connected in a bizarre way.