Methadone hydrochloride produces subjective changes similar to those produced by heroin and is approximately equipotent to morphine when both are administered subcutaneously and one half as potent when administered orally. Chronically administered methadone produces sedation, lethargic apathy, reduction in sexual interest and activity, hemodilution, and edema. Even when patients were receiving and tolerant to 100 mg/day, drug-seeking behavior was seen. Methadone hydrochloride in dose level of 100 mg orally produces physical dependence similar to that produced by morphine, except that onset of the abstinence syndrome is slower. The abstinence syndrome may be of moderate or severe intensity and is qualitatively similar to morphine abstinence syndrome. As in morphine-dependent subjects, the acute abstinence syndrome of methadone-dependent subjects is followed by a protracted abstinence syndrome.
Martin WR, Jasinski DR, Haertzen CA, Kay DC, Jones B, Mansky PA, Carpenter RW. Methadone—A Reevaluation. Arch Gen Psychiatry. 1973;28(2):286-295. doi:10.1001/archpsyc.1973.01750320112017