Status Updates From Bronze Age Eleusis and the ...

As far as the use is concerned, despite the association of Demeter with poppies in art and iconography, there are no references that would connectit to the Mysteries. [...] The Greej did not consider the use of opium a taboo, and there is no reason for which it would not have been listed, along with pennyroryal, as one of the ingredients of the Eleusinian kykeon.

It is wrong, however, to define Mystery Cults only on terms of their secrecy : secrecy was a characteristic of several other, non-mystic cults, but it was the initiation that made a cult what we call today a Mystery Cult.