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):

methadon
    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]