psilocybin, the active ingredient in magic mushrooms. Here's the boring news first: ingesting psilocybin produces a mystical experience that can be quantified. "Noetic quality," for example, increased from 19.4 on a placebo to 70.6 on the highest dose used in the study. "Transcendence of space and time" increased from 18.3 to 78.2. Etc. You probably already knew that.
Here's the somewhat more interesting news: psilocybin can sometimes produces bad trips full of fear and anxiety, but the researchers have also figured out how to minimize this. Partly this was due to the experimental design: "The study was designed to optimize the potential for positively valued experiences by providing 8 hours of preparation, administering psilocybin in a pleasant, supportive setting, and instructing volunteers to focus explicitly on their subjective or inner experience." They used soothing music, too. But they also tried various dosages of psilocybin on their subjects, and it turns out that nearly all of the episodes of anxiety happened at the highest dose. Crank it down one notch and you're still likely to get most of the benefits but with significantly less chance of a bad experience.
http://www.alternet.org/story/151331/how_magic_mushrooms_can_improve_your_life_in_the_long-term?akid=7132.200602._lCmu2&rd=1&t=8