Kykeon was a popular peasant drink in the Ancient Mediterranean that could be made from water, barley, mint (pennyroyal), honey, and/or cheese.
In this book series, Kykeon consists of barley, honey, and mint. It's unknown if this specific Kykeon also contains a sedative, but it's possible considering that it caused both Rory and Sadie to fall unconscious after consuming it. This could also be explained by magic.
Rites of Demeter Edit
In the Rites, Kykeon is a drink consumed by participants. It is said to hold magical properties in this setting, causing the drinkers to see Demeter or Persephone, and to also lose their fear of death. Scientists and historians speculate the presence of ergot in the drink, a psychoactive fungus that grows on barley. Other theories include: the presence of psychedelic mushrooms, the chemical reaction between used herbs, and hallucinations caused by a previous nine day fast. In this series, however, it's most likely that the Rites were a truly magical ritual.
Practically everyone who was an anyone participated in the Rites during their two-thousand year golden age.