The symbiote toxin is a toxin which Goa'uld and Tok'ra symbiotes release when they die. Upon a symbiote's death, they release a toxin into their host's bloodstream, which instantly kills the host. It is possible for the symbiote to not release the toxin, but this requires a conscious effort on the symbiote's part. The symbiote poison is too fast acting to allow a symbiote enough time to not release the poison. (SG1: "Summit", "Last Stand", "Endgame", "Threads", "Dominion")