What comes first- the belief in a treatment's effectiveness or it's effectiveness?
We will never know because the human being is a blended creature- a mind/body being.
There is no separate mind or body- both are part of the same system. Therefore we cannot separate the effect of a substance or treatment on one aspect of ourselves and not the other- both will depend on our beliefs concerning the substance or procedure.
This is why true objectivity in science (and medicine, in particular) is impossible and why unbiased evidence-based medicine is a misnomer, for there is no evidence separate from the consciousness of those observing it!