 The metabolic half life is 8 to 59 hours (approximately 24 hours for opioid-tolerant people, and 55 hours in opioid-naive people), as opposed to a half life of 1 to 5 hours for morphine.The length of the half life of methadone allows for exhibition of respiratory depressant effects for an extended duration of time in opioid-naive people.Levomethadone (the "L" enantiomer) is a μ-opioid receptor agonist with higher intrinsic activity than morphine, but lower affinity.