WordNet (r) 3.0 (2006):

drug addict
    n 1: a narcotics addict [syn: drug addict, junkie, junky]

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "drug addict":
   LSD user, acidhead, addict, alcoholic, chain smoker,
   cocaine sniffer, cokie, cubehead, dipsomaniac, dope fiend, doper,
   drug abuser, drug user, drunkard, fiend, freak, glue sniffer,
   habitual, head, heavy smoker, hophead, hype, junkie,
   marijuana smoker, methhead, narcotics addict, pillhead, pothead,
   snowbird, speed freak, tripper, user