Psilosynine was a neurotransmitter and one of the main chemicals involved in Betazoid telepathy.

In 2370, Lwaxana Troi suffered from low levels of psilosynine after prolonged telepathic communication with the Cairn. (TNG: "Dark Page")

Later the same year, Counselor Deanna Troi showed heightened levels of the chemical after experiencing a telepathic 'echo' in nacelle control. Dr. Beverly Crusher later gave her an inhibitor to suppress the levels of psilosynine, allowing Troi to enter the control room without being overwhelmed. (TNG: "Eye of the Beholder")

In ultimately unused dialogue from the second draft script of DS9: "Accession", Dr. Julian Bashir explained about psilosynine, "It's a neurotransmitter involved in psychic phenomenon [sic]. We all have a little of it, but people who've been exposed to things like... Vulcan mind melds... the Bajoran orbs, are often left with more."
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