Yes, METHADONE is a Scrabble word!

Letter values in Scrabble US: M 3, E 1, T 1, H 4, A 1, D 2, O 1, N 1, E 1.

Is it a valid word?

METHADONE is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

synthetic narcotic drug similar to morphine but less habit-forming; used in narcotic detoxification and maintenance of heroin addiction

Also: methadone hydrochloride, dolophine hydrochloride, synthetic heroin.