"Methadone's half-life is approximately 24 hours and leads to a long duration of action and once-a-day dosing. This feature, coupled with its slow onset of action, blunts its euphoric effect, making it unattractive as a principal drug of abuse."
National Institutes of Health, Office of the Director, "NIH Consensus Statement: Effective Medical Treatment of Opiate Addiction," (U.S. Department of Health: Bethesda, MD, November 1997) Vol. 15, No. 6., p. 14.