Coming to the party late, but here I am.

TBH, I've never been inpatient (yet) or worked inpatient psych, but I know that many drugs are given IM (injected into a large muscle), not IV. The IM route is used when fast-acting medication is needed in the case of severe agitation, threatening behaviors etc. and/or the patient refuses or spits out oral medications.

IM meds are also used to bring blood levels of certain drugs up to therapeutic levels, thus stabilizing the patient more quickly. Most antipsychotics and anxiolytics come in injectable form (years ago when I worked in a nursing home, we used to line up the psych residents and swack 'em up with a shot of Haldol IM every night). They're usually more effective because of the route of administration, which gets the drug into the bloodstream faster since it bypasses the stomach.

I've not heard much about IV psychiatric meds; personally I've never seen it---it's so much faster and easier to slam 5 mg of Haldol into an arm muscle than try to start an IV and then push the medication verrrrrrry.......slooooooowly to avoid speed shock.