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")