"Methadone is a long-acting µ-opioid receptor agonist, introduced in the 1960s, after being developed in Germany at the end of World War II.60 It has an onset of action within 30 minutes61-63 and an average duration of action of 24 to 36 hours. Its oral bioavailability is excellent and approaches 90%. These unique pharmacologic properties ideally lend themselves to once-daily dosing for maintenance therapy, although, when used to treat chronic pain, methadone is generally dosed 3 times daily. When the dosage is judiciously titrated, methadone treated patients generally do not experience euphoria or sedation, nor do they suffer impairment in the ability to perform mental tasks. One of the most important advantages of methadone is that it relieves narcotic craving, which is the primary reason for relapse. Similarly, methadone blocks many of the narcotic effects of heroin,64 which helps reinforce abstinence. Once a therapeutic dose is achieved, patients frequently can be maintained for many years with the same dose.65
"Methadone hydrochloride is available in 5- and 10-mg tablets as well as a 40-mg dispersible wafer. However, it is most frequently used for maintenance in a 10-mg/mL liquid concentrate. An intravenous solution is also available but has been linked with bradycardia when administered for sedation."
Mori J. Krantz, MD; Philip S. Mehler, MD, "Treating Opioid Dependence: Growing Implications for Primary Care," Archives of Internal Medicine, (Chicago, IL: American Medical Association, February 2004), Vol. 164, p. 279.