The Collaborative International Dictionary of English v.0.48:

methadon \methadon\, methadone \methadone\n.
   a narcotic drug similar to morphine but less habit-forming;
   used in treating heroin addiction.

   Syn: methadone hydrochloride.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

methadone
    n 1: synthetic narcotic drug similar to morphine but less habit-
         forming; used in narcotic detoxification and maintenance of
         heroin addiction [syn: methadone, methadone
         hydrochloride, methadon, dolophine hydrochloride,
         fixer, synthetic heroin]

Moby Thesaurus II by Grady Ward, 1.0:

83 Moby Thesaurus words for "methadone":
   Amytal, Amytal pill, Demerol, Dolophine, H, Luminal, Luminal pill,
   M, Mickey Finn, Nembutal, Nembutal pill, Seconal, Seconal pill,
   Tuinal, Tuinal pill, alcohol, amobarbital sodium, analgesic,
   anodyne, barb, barbiturate, barbiturate pill, black stuff, blue,
   blue angel, blue devil, blue heaven, blue velvet, calmative,
   chloral hydrate, codeine, codeine cough syrup, depressant,
   depressor, dolly, downer, goofball, hard stuff, heroin, hop, horse,
   hypnotic, junk, knockout drops, laudanum, liquor, lotus,
   meperidine, morphia, morphine, narcotic, opiate, opium, pacifier,
   pain killer, paregoric, pen yan, phenobarbital,
   phenobarbital sodium, purple heart, quietener, rainbow, red, scag,
   secobarbital sodium, sedative, shit, sleep-inducer, sleeper,
   sleeping draught, sleeping pill, smack, sodium thiopental,
   somnifacient, soother, soothing syrup, soporific, tar,
   tranquilizer, turps, white stuff, yellow, yellow jacket