Screamers is the slang for hallucinogens -
Hallucinogens are a diverse group of drugs that alter perception (awareness of surrounding objects and conditions), thoughts, and feelings.
They cause hallucinations, or sensations and images that seem real though they are not. Hallucinogens can be found in some plants and mushrooms (or their extracts) or can be human-made. People have used hallucinogens for centuries, mostly for religious rituals.
In the movie, Fear and Loathing in Las Vegas, Duke and Gonzo brought along with them a drug suitcase which contain the following:
We had two bags of grass, seventy-five pellets of mescaline, five sheets of high-powered blotter acid, a saltshaker half-full of cocaine, and a whole galaxy of multi-colored uppers, downers, screamers, laughers... Also, a quart of tequila, a quart of rum, a case of beer, a pint of raw ether, and two dozen amyls. Not that we needed all that for the trip, but once you get locked into a serious drug collection, the tendency is to push it as far as you can. The only thing that really worried me was the ether. There is nothing in the world more helpless and irresponsible and depraved than a man in the depths of an ether binge, and I knew we'd get into that rotten stuff pretty soon.
Understanding slang words for drugs gives parents a key tool in monitoring their teens for drug abuse. See WebMD's Teen Drug Slang: Dictionary for Parents for more info.