Depends on your hospital policy how soon or late a medication can be given. Also depends why you are holding / giving a med later or earlier. I'm certainly going to wait in giving a narcotic if my patient is drowsy with slow respirations. And if they are in so much pain they need th narcotic now, I'd probably look into getting some breakthrough meds for the,
It always depends on the specifics of the situation. You can be a lot more flexible in the timing of administering a daily multivitamin than you can be in giving something for which the effectiveness and/or safety is dependent on maintaining a certain level of the drug in the body.
Also depends on your state board of pharmacy. In my state, narcotics may not be given early, regardless of whether they are ordered prn or routine.
Beware, Pamcory's other post about PRN's was very sketchy, to the point where an administrator told her to talk to her health care provider overseeing her medical treatment, and closed the post.