A neurotransmitter in joined Trill that carries signals between host and symbiont, whose falling level endangered Jadzia Dax in the Deep Space Nine episode Equilibrium.
Isoboramine is the chemical that lets a Trill host's brain and the symbiont communicate. If its concentration in the host falls too far, the joining is failing, and Trill doctors will remove the symbiont to save it even though the host will die.
In Equilibrium, Jadzia Dax begins to hallucinate and to remember music she never learned, and her isoboramine levels drop steadily. Sisko and Bashir take her to the Trill homeworld, where the Symbiosis Commission treats her with supplements but avoids their questions.
The cause is a suppressed memory. Dax was once briefly joined to Joran, a murderer whose existence the Commission erased from the records to hide the fact that far more Trill are suited to joining than it admits. Accepting Joran's memories restores her balance.