WordNet (r) 3.0 (2006):

drug user
    n 1: a person who takes drugs [syn: drug user, substance
         abuser, user]

Moby Thesaurus II by Grady Ward, 1.0:

73 Moby Thesaurus words for "drug user":
   LSD user, acidhead, addict, alcoholic, alternating personality,
   antisocial personality, chain smoker, cocaine sniffer, cokie,
   cubehead, dipsomaniac, disordered personality,
   disturbed personality, dope fiend, doper, double personality,
   drug abuser, drug addict, drunkard, dual personality,
   emotionally unstable personality, escapist, fiend, freak,
   glue sniffer, habitual, head, heavy smoker, hophead,
   hostile personality, hype, hypochondriac, hypochondriast, idiot,
   imaginary invalid, immature personality, inadequate personality,
   inferior personality, junkie, malade imaginaire,
   maladjusted personality, marijuana smoker,
   mentally defective personality, methhead, moral insanity,
   multiple personality, narcotics addict, neuropath, neurotic,
   neurotic personality, paranoid personality, perverse personality,
   pillhead, pothead, psychoneurotic, psychopath,
   psychopathic personality, psychotic, psychotic personality,
   schizoid, schizoid personality, seclusive personality,
   sexual psychopath, shut-in personality, snowbird, sociopath,
   speed freak, split personality, tripper, user, valetudinarian,
   valetudinary, weak personality