Sleet is a designer drug, originating from Madripoor, where it was distributed by the Mutant Liberation Front. It was supposed to be a very intense high, but tests showed that it burned out too quickly to be popular as a street drug.
It did, however, have the side of causing both paranoia and murderous rage. Sleet users often died, and inevitably took others with them along the way. For this reason, the makers of the drug tried instead to use the drug as a chemical weapon. Fortunately, the intervention of the New Mutants and their allies Sunfire and Wolverine put an end to Stryfe’s plans to use deploy it in a number of world cities, saving millions of lives.