Methadone (Dolophine) has an analgesic profile and potency similar to that of morphine but a longer dura-tion of action and better oral bioavailability.
Methadone is a useful analgesic drug for the treat-ment of moderate to severe pain. Unlike morphine, it is generally not used epidurally because of its long dura-tion of action. It is also rarely or never used in PCA sys-tems or in pregnant women during labor. The side ef-fects and signs of overdose following methadone administration are similar to those observed with mor-phine. Overdose is treated with naloxone. Clearance of methadone is via the urine and bile as the cyclic N-demethylated drug. The ability to N-demethylate the drug decreases in elderly patients, prolonging the action of methadone. In such patients, dosing intervals should be longer than in younger patients. In addition, the pH of the urine has a major effect on clearance of the drug. Alkalinization of the urine or renal insufficiency de-creases excretion of the drug.
Drug interactions and precautions for the use of methadone are similar to those of morphine. In addi-tion, rifampicin and hydantoins markedly increase the metabolism of methadone and can precipitate with-drawal from methadone. Conversely, the tricyclic anti-depressants and certain benzodiazepines can inhibit metabolism of methadone, thereby increasing accumu-lation of the drug, prolonging its half-life, and intensify-ing its side effects. Continuous dosing with methadone may lead to drug accumulation and to an increased in-cidence of side effects; methadone is generally not used for PCA. In pregnant heroin-addicted women, substitu-tion of methadone for heroin has been shown to be as-sociated with fewer low-birth-weight newborns and fewer learning and cognition problems later in the life of the child.