This is one of my favorite medical ethics questions. When I spring it on people who're waiting for their interview into medical school, they're often boggled.
I went looking, and I found that different countries have different medical standards. AFAICT, it's something that needs to be decided by a committee, because there's no 'correct' answer. Do you inform your patient you're giving them a placebo? I just don't know!
My biggest objection (other than the fact that it limits information exchange) is that it might cause a masking of other symptoms. If a patient undergoes a deterioration that CAN be influenced with medicine, you'd want to notice as fast as possible.
OTOH, reducing suffering is very important. A white lie could have a large aggregate effect of reducing pain.
Man, I just don't know. But finding out about the power of placebos kinda changed my mind on 'alternative medicines'. If they're actually reducing suffering, then they *do* have a material benefit, instead of just being a net loss.