Isoboramine

Isoboramine is a neurotransmitter in Trill that connects the synaptic functions of a host to their Trill. A reduction in isoboramine levels is harmful to both and a fall below forty percent of normal necessitates the removal of the symbiont. Benzocyatizine can be used to increased isoboramine levels. 

The parasites, genetically modified symbionts, also need isoboramine to link with a host. 

Isoboramine is used in the brine liquid the symbionts live in to allow a way for symbionts to communicate with each other and unjoined Trills. 

In 2285, after being injured in a shuttle accident, Torias Dax's isoboramine levels could not be increased, forcing Trill doctors to remove and give it to a new host, Joran Dax. 

In 2371, the memory block used by the Trill Symbiosis Commission to keep Joran's memories from resurfacing began to break down, lowering Jadzia Dax's isoboramine levels, threatening the removal of Dax and Jadzia's life. 

After Ezri Dax was joined with Dax in 2375, Doctor Fulghum of the USS Destiny inhibited her isoboramine levels to keep the memories of the past hosts of Dax from surfacing too quickly. (SS DS9 (Ninety-three Hours))

In 2376, Girani Semna discovered isoboramine in Shakaar Edon's body during his autopsy, showing he had been joined and revealing the connection to the Trill the parasites had. 

Later that year, Julian Bashir injected several dying joined Trill with high doses of isoboramine in attempt to keep the symbiosis between them and their symbionts.