Used in a ritual context, that simple act of ingesting a psychoactive chemical can have powerful effects on the magick that one is performing. Good rituals evoke powerful emotions, and really there is no easier way to evoke powerful emotions than to take a drug. It goes without saying that caution should be exercised when monkeying around with drugs (yet I'm saying it) and I assume that anyone on this site and reading this post is intelligent enough to figure that out. The same should be noted about magick however. Bad things can happen if you are stupid, and even if you are smart and take every precaution you can think of, things still may go wrong.