The Hoffan drug was a compound created by the Hoffan people in the Pegasus galaxy with the help of the Atlantis expedition. When administered to a human, it renders the human immune to the Wraith feeding process while simultaneously killing the Wraith who attempted the feeding.
The Hoffan drug works by inhibiting the chemical injected into the human body by the Wraith when the feeding process begins. Without this chemical interaction taking place, the Wraith cannot draw energy from their victims as they normally do. An unintended side-effect of the drug is that it mutates upon contact with the chemical, becoming a lethal toxin in the Wraith's body which causes complete organ failure within hours of exposure. By itself, the drug also has a 50% mortality rate in humans, regardless of their exposure to the Wraith. Those that survive also suffer from a lowered immune response. (SGA: "Poisoning the Well", "Outsiders")
When Michael Kenmore discovered and duplicated the drug, he refined it to reduce the mortality rate to only 30% while making its lethal effects on Wraith much more fast-acting with one Wraith being shown to die within a couple of minutes rather than the long time Steve suffered to death, although the refined version still causes an extremely painful death. (SGA: "The Kindred, Part 1")
The drug was created by the scientists of Hoff over many generations, due to the constant cullings by the Wraith. Working on their predecessors' knowledge, each generations of scientists continued to refine the drug. They were able to perfect the compound when the Atlantis expedition visited their world. With Dr. Carson Beckett's medical knowledge, decades of research was condensed into mere days. It was first tested on a terminally ill Hoffan, who volunteered to be fed on by a captured Wraith, called Steve. The drug proved successful, but had the unintended side effect of killing Steve through complete organ failure. Elated at the results, the Hoffans began distributing the drug to their entire population. However, in their rush, they had failed to wait long enough to realize that the drug had negative side effects for humans, too. 50% of their population was stricken terminally ill within hours. Regardless, the Hoffans pressed forward, resolute in their belief that the drug was their salvation. (SGA: "Poisoning the Well")
The Wraith eventually learned of the Hoffan drug and exterminated the Hoffan race for the threat it represented, leaving Hoff in ruins. Michael Kenmore looted their vaults for information pertaining to the drug and duplicated it with the help of a clone of Dr. Carson Beckett. With its lethality rate in humans reduced to 30% and its lethality to Wraith made much more fast-acting, Michael began randomly administering it to human worlds in the Pegasus galaxy as part of an opening volley in his personal war against the Wraith. At least seven worlds were poisoned; Balar was one such planet. With their feeding grounds poisoned in such a random manner, many Wraith were soon struck down by the drug. Todd negotiated with Atlantis for data on the drug in order to detect it, though he appears to have been unsuccessful in doing so. (SGA: "The Kindred, Part 1", "The Kindred, Part 2", "Outsiders", "The Queen")
The existence of the drug left Todd open to the possibility of a treatment being developed by Dr. Jennifer Keller that would eliminate the Wraith need to feed; with no way to predict if they would encounter a plague survivor during a culling, any future feeding would carry an element of risk, but eliminating the need to feed on humans would allow the Wraith to continue, albeit in an altered state with no idea about the potential side-effects to other aspects of Wraith physiology. (SGA: "The Queen", "Infection")
To remove the threat of this drug, at least one other Hive ship began exterminating human worlds known to have been contaminated and hunted down any possible survivors. They were also working on a means to detect the drug in humans. Beckett and Dr. Rodney McKay were kidnapped to aid in this research. Beckett successfully developed a means to detect the drug, which he then used on himself to determine that Michael had given him the drug while in captivity. Beckett tricked the Wraith Commander into feeding on him, then escaped with McKay after the Wraith died. The finished detection system was destroyed by Beckett and McKay, leaving the Wraith again unable to detect survivors. (SGA: "Outsiders")