Methadone plasma level: sustained by a reservoir of drug in tissue.

Binding of methadone in body tissues limits the rise in plasma concentration and prolongs the pharmacological action in patients receiving a daily maintenance dose. An opiate with equal intrinsic activity but greater binding affinity would, therefore, be a better drug than methadone for maintenance treatment; it would be safer when taken in a single dose and longer in its duration of action when taken on a regular schedule.