Isoboramine, sometimes known as isoboromine, is a Trill physiological neurotransmitter regulating connections between the symbiont and the host. Low isoboramine levels can be either harmful or fatal to both the host and symbiont. Trill Symbiosis Commission regulations state if isoboramine levels drop below 40%, a medical professional should remove the symboint, killing the host, who may be willing to die in order to save the symbiont.
In 2285, Trill Symboisis Commission records indicate medical professionals placed Tobias Dax in a coma after a shuttle accident. Left in a vegetative state until his isoboramine levels fell below 40%, the official record continues, Torias and the symboint were separated for the symboint's safety. In actuality, the Dax symbiont had been transferred to Joran Belar during this time.
After Cantabrian chief medical officer Marie Bourget joined Tawana Killan and the Jonar symbiont, Tawana reported feeling slightly disconnected from the symboint, almost as if it was hiding something from her. Bourget determined her isoboramine levels were lower than normal and issued a daily regiment of isoboramine injections to help stimulate her normal isoboramine production. (Star Trek: The Cantabrian Expeditions: "An Innocent Time", et al)