Unlike altered perceptions, which are triggered by some sort of external stimulus, hallucinations are sensations that people experience when there is no external stimulus. A hallucination can be experienced through any of the five senses. Hallucinations can sometimes be dramatic and complex, and as a result they can be quite frightening.

Those who have taken large doses of hallucinogens often report experiencing bizarre and impossible events. For example, some users witness inanimate objects or people morphing into animals, objects talking and moving around a room, or extraterrestrial beings visiting from outer space. Others claim to have interactions with dead people.

As bizarre as these drug-induced hallucinations can be, there are some features of hallucinations that are commonly experienced. Users often report walls flexing back and forth to the rhythm of music, straight lines curving and then straightening out, and objects appearing and then disappearing from view. In an interview, a college student recalled this LSD experience:

We went to the sink that had little droplets of water in the bottom of it. By “unfocusing” our attention, we could cause strange effects to occur. The sink became this rushing current of rapids pouring down into the drain. A blink of the eyes and it was the sink again… There was a poster around campus that week for a band known as Anonymous…. It was a picture of a punk rocker’s face with really strange shadings that had obvious [sic] been done with pencil. We watched the poster for a moment. The hair on the top of his head receded and disappeared while the shading on the face became more pronounced turning the face into that of a “wolfman.”… The face cycled back and forth between that of the punk rocker and the wolfman, back and forth like the waves on the shore… The kitchen was full of wonders. The doors on all the shelves bulged inward and outward. The hairs on our arms interweaved continually and the hairs on our legs grew straight out. The once plain walls were full of intricate little patterns as was the carpet.

Users of these drugs claim that they usually understand during their trip that the hallucinations are drug induced and that the bizarre experience will soon end. In this sense, they are very different from dreams or hallucinations induced by extreme stress.

Another common feature of hallucinogens is that many users also claim to have gained new religious insights as a result of their trips. Many report finding new purposes for their lives, a sense that all people, animals, and objects are spiritually connected, and some establish an emotional identification with one or more of history’s great spiritual leaders, such as Jesus or Buddha. One writer expressed these spiritual thoughts following an experience with mescaline:

I began to feel like I was so connected with all of life and nature. I think, at that moment, I never felt more alive. I think somehow during this trip, I also became more aware of my own immortality. I seem to remember thinking about dying, and for the first time, it didn’t really scare me because I seemed to be aware that my soul somehow transcended anything physical. I felt very thankful that God had put me on earth to see beauty.

Despite the common elements reported by those who have used hallucinogens, drug-induced hallucinations are highly unpredictable. The experience one has with the drug may be pleasurable one time and highly disturbing the next, or, as is sometimes the case with LSD, over the course of a single trip, both pleasurable and frightening elements can occur. Occasionally, the frightening effects of hallucinogens can intensify, causing the affected person to experience feelings of hopelessness and intense anxiety that persist long after the drug itself has left the body. If this depression does not subside, people have been known to behave in ways that endanger their lives. Such out-of-control behaviors occasionally require at least short-term hospitalization.