STP was the slang term for the psychedelic drug DOM. STP can stand variously for Serenety, Tranquility, Peace; Super Terrific Psychedelic or Stop The Police.
First synthesised by Alexander Shulgin this drug became the center of a national drugs scare in San Francisco in 1967. DOM takes alot longer to exhibit its effects than other drugs that people were used to such as LSD (2 hours to DOM's peak as opposed to LSD's 20-45 minutes). This was only part of the problem: the illegal lab that synthesised it and put twice the necessary amount of DOM in the pills it manufactured. This lead to users taking four to six times the amount they should have as they figured that they should have been flying after half an hour and took more to compensate for the percieved lack of potency.
Another problem was that the duration of the DOM trip is alot longer than other related drugs. So you can imagine the emergency rooms filling with trippers who don't thing they're ever going to come down off this drug that no-one could identify.
You may be familiar with the motor oil additive STP. One so called expert at the time actually stated that the psychedelic and the oil additive were one and the same. If anyone in either the law or the people had listened to him the results could have been hilarious or terrible respectively
This story is paraphrased from the great book PIHKAL by Alexander Shulgin