Nish'ta is a biological compound employed by the Goa'uld as a powerful form of mind-control. Nish'ta is an organism delivered in a gaseous form and is absorbed by the body's tissues, rendering the subject's mind "extremely pliable" and open to suggestion.
An electrical shock, such as that delivered by a Zat'nik'tel discharge, will kill the organism and free the victim from its control. Once freed, the subject is immune from repeat infection.
Apophis used the mind-controlling agent, which Teal'c later identified as nish'ta, to brain-wash Teal'c's son Rya'c and turn him against him. SG-1 infiltrated the Goa'uld Setesh's compound on Earth, where he used nish'ta to control his "followers" -- including Colonel Jack O'Neill, Captain Samantha Carter and Dr. Daniel Jackson. (SG1: "Family", "Seth")
A similar but not identical version was used by Hathor. It was not mind-controlling as much as influencing and only took effect so long as the target was in close proximity. As it was not a permanent effect, a Zat'nik'tel shot does not prevent reinfection. Hathor's compound only seemed to work on male Humans, leaving the female staff of Stargate Command and Teal'c unaffected. The men were made extremely pliable to Hathor's will and also had their libidos increased, making them easily distracted by women. (SG1: "Hathor")