They are psychoactive drugs that cause changes in perception, thought, emotion, and consciousness. They put you in a dreamlike or dissociative state. Mushrooms and Acid are examples, as is marijuana. Marijuana is a mild hallucinogen where mushrooms and acid significantly alter your state of consciousness and one can experience a dramatic break down in reality. Most of the time, hallucinogens (excluding marijuana) aren't really taken by people who just wanna get "high." They are mind-expanding drugs that can either turn out to be a great time, or a horrible time.
Approximately 35%.