The first time they gave me a bunch of morphine post-op, a nurse kept wondering by. After a few checks, getting slightly more in explicit, I asked "why". She explained that there was a fair likelihood of hallucination with the drug, but if they warn people in advance it is significantly more common.

There is obviously more risk to unexpected hallucination, but always some risk if it happens. I still think it's a really interesting risk minimisation proposition.