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The scanners are a small group of people, about 230, that possess telepathic and telekinetic abilities. They are the offspring of pregnant women who were part of a trial that tested an experimental drug, Ephemerol. The result was children who developed higher brain functions that allow them to control other people's minds and even kill other people using telekinesis, as happens in the earliest scene with Revok. Edit (Coming Soon)
It's a fictional drug invented for the film that was given to pregnant women about 30 yrs before the story opens. The chief function was to quell morning sickness. It caused the children of these women to develop the psychic powers they now possess. It is also used to pacify/sedate scanners, suspending their powers to control others.
Though it is a fictional drug, Cronenberg was probably inspired to create it for his film by another well-known drug called thalidomide, which was also given to pregnant women in 1957. Children born to these women had malformed limbs. More info on thalidomide can be read here. Edit (Coming Soon)