The Collaborative International Dictionary of English v.0.48:

hallucinogen \hal*lu"ci*no*gen\ n.
   A substance capable of producing hallucinations when
   ingested; a hallucinogenic substance; as, LSD is a powerful
   hallucinogen.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

hallucinogen
    n 1: a psychoactive drug that induces hallucinations or altered
         sensory experiences [syn: hallucinogen, hallucinogenic
         drug, psychedelic drug, psychodelic drug]

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "hallucinogen":
   DET, DMT, Indian hemp, LSD, Mary Jane, STP, THC, acid,
   antidepressant, ataractic, belladonna, bhang, cannabis,
   diethyltryptamine, dimethyltryptamine, gage, ganja, grass,
   hallucinogens, hash, hashish, hay, hemp, henbane, hyoscyamus,
   jimsonweed, joint, kava, marijuana, mescal, mescal bean,
   mescal button, mescaline, mind-altering drug, mind-blowing drug,
   mind-expanding drug, morning glory seeds, peyote, pot, psilocin,
   psilocybin, psychedelic, psychic energizer, psychoactive drug,
   psychochemical, psychotomimetic, reefer, roach, stick, stramonium,
   tea, tranquilizer, weed