Not anti-depressants or anti-psychotics. Downers (aka depressant, or central depressant) can be administered intravenously (and get into the bloodstream quicker than taken orally). In emergency situations if a person is agitated or in pain they might be given a depressant intravenously to calm them down asap.

Psychotropic meds like anti-depressants and other meds that work in the brain take longer to have an effect because it takes longer for them to get into the brain. These meds have to get through the blood-brain barrier which can take a long time (weeks, etc.)