We had a debate about this where I work. I have no problem giving one Vicodin 2 hours later if the patient rec'd one and the order was for 1-2 Q4...sometimes the one just didn't help and others just seem to have their pain controlled better if they rec'd one Q2 instead of two Q4. Others said I couldn't do this. Finally called pharmacy and they said there was no problem giving one every two, as long as the patient didn't receive more than 2 in a 4-hour period. I've never had an MD object to this practice, either.
On a related note, I had a patient complain that her nurse wouldn't give her half of a Percocet when she didn't want to take a whole one, saying that she couldn't because the order was written as one to two tabs