Methadone Dosing

"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."

Source: 

Effective Medical Treatment of Opiate Addiction. NIH Consensus Statement 1997 Nov. 17-19; 15(6): 4.