Can a doctor legally prescribe a placebo?

Haven't heard of any: Haven't heard of any law on that. "placebo" means different things to different people. There are many things that people take that actually have a huge placebo effect. In many cases, if you believe that something will help you, it can actually make you feel better. This is one reason there is a huge "vitamins and supplements" business out there.

An ethical dilema: Legally docs don't prescribe placebo, only in research setting. Nevertheless, placebo effect-prescribing happens all the time--for example, patient with a viral-cold is insistent that she needs an antibiotic and doc can't convince her otherwise ends up writing a script of antibiotic knowing that taking it or not she will get better--prety much a placebo script. Docs are only human. Good luck.