The notion that hallucinogenic drugs played a significant part in the development of religion has been extensively discussed, particularly since the middle of the twentieth century. Various ideas of this type have been collected into what has become known as the entheogen theory. The word entheogen is a neologism coined in 1979 by a group of ethnobotanists (those that study the relationship between people and plants). The literal meaning of entheogen is "that which causes God to be within an individual" and might be considered as a more accurate and academic term for popular terms such as hallucinogen orpsychedelic drug. By the term entheogen we understand the use of psychoactive substances for religious or spiritual reasons rather than for purely recreational purposes.
Perhaps one of the first things to consider is whether there is any direct evidence for the entheogenic theory of religion which derives from contemporary science. One famous example that has been widely discussed is the Marsh Chapel experiment. This experiment was run by the Harvard Psilocybin Project in the early 1960s, a research project spearheaded by Timothy Leary and Richard Alpert. Leary had traveled to Mexico in 1960, where he had been introduced to the effects of hallucinogenic psilocybin-containing mushrooms and was anxious to explore the implications of the drug for psychological research.
On Good Friday 1962, two groups of students received either psilocybin or niacin (a nonhallucinogenic "control" substance) on a double-blind basis prior to the service in Boston University's Marsh Chapel. Following the service nearly the entire group receiving psilocybin reported having had a profound religious experience, compared to just a few in the control group. This result was therefore judged to have supported the entheogenic potential of hallucinogenic drug use. Interestingly, the experiment has subsequently been repeated under somewhat different and arguably better controlled circumstances and the results were substantially the same.