Kykeon (Gr. κυκεών, from κυκάω, "to stir, to mix") was an Ancient Greek drink made mainly of water, barley and naturally occurring substances. Mysteries say it was derived from Ergot.

The Greeks could have used simple water extraction from Ergot to make an LSD-like substance that would account for the strong effects of the kykeon. It was used at the climax of the Eleusinian Mysteries to break a sacred fast, but it was also a favourite drink of Greek peasants. Kykeon is mentioned in Homeric texts: the Iliad describes it as consisting of barley, water, herbs, and ground goat cheese (XI, 638–641).

In the Odyssey, Circe adds some honey and pours her magic potion in it (X, 234). In The Homeric Hymn to Demeter, the goddess refuses red wine but accepts kykeon made from water, barley and pennyroyal. It was supposed to have digestive properties.

Hermes recommends it in Aristophanes' Peace (v. 712) to the hero who ate too much dry fruit and nuts. Aristocrats shunned it as a peasant drink.

Theophrastus depicts in his Characters (IV, 2–3) a peasant whose thyme breath inconveniences his neighbours at the Ecclesia. 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.

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