Psilosynine
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. ()

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. ()

de:Psilosynin
Biochemical compounds