WordNet (r) 3.0 (2006):

speed freak
    n 1: addict who habitually uses stimulant drugs (especially
         amphetamines)

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "speed freak":
   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, pothead,
   snowbird, tripper, user