Isoboramine

Isoboramine (also spelled as isoboromine) was a neurotransmitter in Trill physiology which regulated the connections between the host and the symbiont. Low isoboramine levels were harmful and could be fatal to the host and/or symbiont, so if isoboramine levels dropped below 40%, then the symbiont was removed. This would kill the host, but any joined Trill would willingly sacrifice themselves in order to save their symbiont.

Official records at the Trill Symbiosis Commission stated that in 2285, when Torias Dax was in a shuttle accident, he was placed in a coma. He was left in a vegetative state until his isoboramine levels eventually fell below 40% and the symbiont had to be removed. In actuality, the symbiont had been transferred to Joran Belar.

In 2371, Jadzia Dax's isoboramine levels fell dangerously low when a chemical memory blocker was wearing off in the Dax symbiont. 

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Isoboramin