Green’s Dictionary of Slang

twitcher n.2

[SE twitch/twitching n. (2)]

1. (US drugs) a drug addict.

[US]Marion Star (OH) 3 Apr. 17/6: We can tell ‘coke’ or heroin fiends [...] We call them ‘twitchers’ due to their constant twitching or hitching of their shoulders [...] their eyes are wild and stary.
[US]Anslinger & Tompkins Traffic In Narcotics 316: twitcher. A drug addict.

2. a crack cocaine addict who is afflicted by nervous twitching.

[US]T. Williams Crackhouse 152: twitchers users of crack-cocaine-freebase who develop idiosyncratic tics or excessive movements.