Are you implying that the doctor should act to trigger a placebo effect, while still making a true statement? Because in the least convenient version of the dilemma, you would have to choose one or the other.

Erased my previous comment. It missed the real point.

If you think the doctor should say, "This is the best painkiller I have," that suggests you want to believe you are getting a potent painkiller of some kind. You want to believe that it is a potent painkiller, which is false, as opposed to it is the most potent of the zero painkillers he has, which is true. The fact that the doctor is not technically lying does not change the fact you want to believe something that is false.

If the IV contains a saline solution, the Way may want me to believe the IV contains a saline solution, but I sure as Hell want to think it contains a potent painkiller.

(Yes, I realize the irony in using the expression "sure as Hell.")

The doctor should say "This is the best painkiller I have" and administer it.

The doctor can do a heck of a lot better than that, even without lying. Ericksonian hypnosis, for example, involves a lot of artfully-vague statements like, "you may notice some sensation happening now", and amplifying them to lead a person to believe more specific suggestions (such as pain-relief suggestions) that follow. A lot of it can also be done covertly, such that the patient is never consciously aware that a hypnotic procedure is under way.

(Of course, statistics say that relatively few people are able to undergo major surgery with hypnoanesthesia. But if that's the only painkiller you have, it'd be silly not to use it.)