Hallucinogenic plants have been used by humans for ages; here is Wiki on one such Indian use in Vedic times, for example:
Soma (Sanskrit: सोमः), or Haoma (Avestan), from Proto-Indo-Iranian *sauma-, was a ritual drink of importance among the early Indo-Iranians, and the later Vedic and greater Persian cultures. It is frequently mentioned in the Rigveda, which contains many hymns praising its energizing or intoxicating qualities. In the Avesta, Haoma has an entire Yasht dedicated to it.
It is described as prepared by pressing juice from the stalks of a certain mountain plant, which has been variously hypothesized to be a psychedelic mushroom, cannabis, peganum harmala, or ephedra. In both Vedic and Zoroastrian tradition, the drink is identified with the plant, and also personified as a divinity, the three forming a religious or mythological unity.
Via B-squared, I now understand that the Golden Guide to Hallucinogenic Plants is available online. And, in case you are curious, the Golden Guide thinks that Soma is Amanita Muscaria–the mushroom, which, I first saw in Bavarian alps:
Recent studies suggest that this mushroom was the mysterious God- narcotic soma of ancient India. Thousands of years ago, Aryan conquerors, who swept across India, worshiped some, drinking it in religious ceremonies. Many hymns in the Indian Rig-Veda are devoted to soma and describe the plant and its effects.
PS:- By the way, if you followed the link above, and wondered about the mention of urine drinking in Rg Veda, this page gives some more information on the issue.