In the war against the Goa'uld, the Tok'ra devised a plan to wipe out the Goa'uld leadership at the summit of System Lords at Hassara, and to make a final decisive move in a wide-scale attack on the entire Goa'uld population using a synthetically created poison that targets symbiotes. Ren'al, of the Tok'ra, designed the poison which is equally effective against the Goa'uld and the Jaffa, as well as the Tok'ra themselves. Because a dying symbiote typically releases a toxin lethal to the host, the poison would result in the death of the host as well, however the poison is non-persistent and dissipates quickly, causing no permanent damage to the environment. The poison was not used at the Hassara summit as planned, however, during the invasion of the Tok'ra base on Revanna by Zipacna's forces, Lieutenant Elliot had become a host to Lantash, and together they sacrificed their lives to release the poison against Zipacna and his Jaffa.
The Tok'ra provided the SGC with the formula for the poison, and scientists at Area 51 continued to modify and manufacture it to be used as a defensive option in the event of a Goa'uld attack on Earth. After two years, the US military had access to a sufficient quantity to negate a sizable enemy force or to effectively neutralize and invade three Goa'uld-occupied planets, which would result in tens of thousands of casualties. The new formula, developed in conjunction with the Tok'ra, was more stable and easier to produce than before, and took the form of a blue liquid that could be released as an invisible gas which spreads quickly and causes instantaneous death upon exposure.
Production of the poison continued, and it was only ever intended to be used in defensive situations, however members of the Trust with access to Area 51 were able to acquire significant amounts of the poison. By commandeering the stargate and using a missile system to deliver the poison in aerosol form over large areas, the Trust launched a full-scale chemical attack against Goa'uld-occupied worlds with no regard for the millions of Jaffa lives that would be lost. The Trust targeted at least five Goa'uld planets, including P4S-161 and P3S-114, and left millions dead including Rebel Jaffa such as M'zel, and Tok'ra operatives such as Zarin. When SG-1 recovered the stargate, members of the Trust escaped in an alkesh, still in possession of enough poison to wipe out at least a dozen more Goa'uld worlds.
Following the defeat of the System Lords, Ba'al attempted to establish a power base on Earth by infiltrating the Trust, and symbiote poison provided an option to target Ba'al without inflicting collateral damage on human associates unaware of his identity. A cruise missile, code-named Alpha Two, was used as the delivery system and launched at Ba'al's compound, however, although the Jaffa at the scene were killed, Ba'al managed to escape before the attack. He had created multiple clones of himself which he used as part of a scheme to acquire the Ancient database of planets from the SGC. As the clones breached security on the base, the SGC released symbiote poison through the ventilation system of the facility as a means of killing the clones without harming base personnel. Some of the clones were killed instantly, but the rest were able to retreat and escape by beaming out of the facility onto a waiting ship in orbit. Ba'al himself acquired access to the symbiote poison and later used it to eliminate his own clones. He called a meeting of his clones on P3R-112 to discuss the Adria situation, but when the clones had assembled he beamed in canisters of the poison, killing the clones and their Jaffa. Not all of the clones were eliminated, however.
Cross Reference: Ba'al, Lieutenant Elliot, Goa'uld, Jaffa, M'zel, P3S-114, P4S-161, Rebel Jaffa, Ren'al, Tok'ra, Trust, Zarin, Zipacna
Episode Reference: Summit, Last Stand, Zero Hour, Endgame, Ex Deus Machina, Insiders, Dominion