Methadone
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.
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