Green’s Dictionary of Slang

(drugs) one who has abandoned drug use.

1959		J.E. Schmidt Narcotics Lingo and Lore.
1975		Hardy & Cull Drug Lang. and Lore.
2000		Observer Rev. 26 Mar. 11: Having described himself [...] as ‘the great quitter’, he died of severe abdominal haemorrhaging brought on by rampant alcoholism.