[While Christian Europeans shun ritual drug use (outside of the Eucharist we suppose), the ancient culture they admired most, the ancient Greeks, maintained an annual rite of initiation into the cult of Demeter & Persephone which featured a drink, Kykeon, which is thought to have been entheogenic. Nobody knows for sure, but it seemed to do the trick since participants experienced “revelatory states” and the ritual was popular over a period of 2,000 years.]

“Numerous scholars have proposed that the power of the Eleusinian Mysteries came from the kykeon’s functioning as a psychedelic agent. Use of potions or philtres for magical or religious purposes was relatively common in Greece and the ancient world. The initiates, sensitized by their fast and prepared by preceding ceremonies (see set and setting), may have been propelled by the effects of a powerful psychoactive potion into revelatory mind states with profound spiritual and intellectual ramifications. In opposition to this idea, other pointedly skeptical scholars note the lack of any solid evidence and stress the collective rather than individual character of initiation into the Mysteries. Indirect evidence in support of the entheogenic theory is that in 415 BC Athenian aristocrat Alcibiades was condemned partly because he took part in an “Eleusinian mystery” in a private house.

Many psychoactive agents have been proposed as the significant element of kykeon, though without consensus or conclusive evidence. These include the ergot, a fungal parasite of the barley or rye grain, which contains the alkaloids lysergic acid amide (LSA), a precursor to LSD, and ergonovine. However, modern attempts to prepare a kykeon using ergot-parasitized barley have yielded inconclusive results, though Alexander Shulgin and Ann Shulgin describe both ergonovine and LSA to be known to produce LSD-like effects.

Psychoactive mushrooms are another candidate. Terence McKenna speculated that the mysteries were focused around a variety of Psilocybe. Other entheogenic fungi, such as Amanita muscaria, have also been suggested. A recent hypothesis suggests that the ancient Egyptians cultivated Psilocybe cubensis on barley and associated it with the deity Osiris.

Another candidate for the psychoactive drug is an opioid derived from the poppy. The cult of the goddess Demeter may have brought the poppy from Crete to Eleusis; it is certain that opium was produced in Crete.

Another theory is that the psychoactive agent in kykeon is DMT, which occurs in many wild plants of the Mediterranean, including Phalaris and Acacia. To be active orally it must be combined with a monoamine oxidase inhibitor such as Syrian Rue (Peganum harmala), which grows throughout the Mediterranean.”

Pasted from <https://en.wikipedia.org/wiki/Eleusinian_Mysteries#Entheogenic_theories>