WordNet (r) 3.0 (2006):

pothead
    n 1: someone who smokes marijuana habitually

Moby Thesaurus II by Grady Ward, 1.0:

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