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Kykeon (Ancient Greek: κυκεών, kykeȏn; from κυκάω, "to stir, to mix") was an Ancient Greek drink of various descriptions. Some were made mainly of water, barley and naturally occurring substances. Others were made with wine and grated cheese. It is widely believed that kykeon usually refers to a psychoactive compounded brew, as in the case of the Eleusinian Mysteries. A kykeon was used at the climax of the Eleusinian Mysteries to break a sacred fast, but it is also mentioned as a favourite drink of Greek peasants.
Kykeon is mentioned in Homeric texts: the Iliad describes it as consisting of Pramnian wine, barley, and grated goat's cheese. In the Odyssey, Circe adds some honey and pours her magic potion into it. In the Homeric Hymn to Demeter, the goddess refuses red wine but accepts kykeon made from water, barley, and pennyroyal.
In an attempt to solve the mystery of how so many people over the span of two millennia could have consistently experienced revelatory states during the culminating ceremony of the Eleusinian Mysteries, it has been posited that the barley used in the Eleusinian kykeon was parasitized by ergot, and that the psychoactive properties of that fungus triggered the intense experiences alluded to by the participants at Eleusis.
For more on the possibilities of kykeon's psychoactive properties, see entheogenic theories of the Eleusinian mysteries.