The opposite of Placebo is... ?

"Nocebo"!

That means that while it is likely that a positive expectation can produce an imagined effect that actually has real physical effects, it is just as likely that negative expectations and skepticism can cause a complete lack of effects or experiences, EVEN if the tested substance or situation is PROVEN to work beforehand.

That is why I find it very interesting when someone with a very skeptical expectation of something is turned around and a positive effect is experienced. 

This is on my mind because many times now I have seen people who are very skeptical to what I do (my relaxation sounds etc.), saying that the sounds will not work on them. Then after testing it many of these people come back "confessing" that it actually worked and they think it is very weird.

So let's say someone is doing a blind study (or double blind).
If the substance tested is NOT a placebo (of course the person does not know if it is a placebo or not) and the person is very skeptical/negative that it will work, then if the outcome is that the person STILL experiences a positive effect instead of a "nocebo" effect, it means that the tested substance is very potent, because it was so powerful that it overpowered the mental ability to "lock out" any positive effects.