Isoboramine, sometimes known as isoboromine, was a Trill physiological neurotransmitter regulating connections between the symbiont and the host. Low isoboramine levels could be either harmful or fatal to both the host and symbiont. Trill Symbiosis Commission regulations stated if isoboramine levels dropped below 40%, a medical professional should remove the symbiont, killing the host, who might be willing to die in order to save the symbiont.
After USS 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")