As the bumper stickers say, "Tweakers Suck". Coke and speed seem to bring out the worst qualities of people's personalities. Large doses induce a mental state indistinguishable from paranoid schizophrenia. The only way they can differentiate stimulant abuse from "natural" schizophrenia is by observing the disappearance of symptoms as the tweaker returns to normal when the drug leaves his system. Prolonged usage over years can make this condition more permanent.

I used to smell the speed labs as a teenager when I drove through areas of SE portland. Another time I moved in next door to a speed cook in the Santa Cruz mountains. It was easy to tell when he had finished a new batch. He had an old hulk of a car he was "restoring" (for the last 5 years). He would just grind on it and sand it for hours on end. Finally he sanded through the sheet metal in a few places. Didn't seem to phase him though.