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Methadone is a synthetic opiate receptor agonist that has been available for more than 40 years. Although its main use has been in the maintenance treatment of opioid addicts, it has excellent analgesic effects and low cost. Its use is limited by its long and unpredictable half-life and by the limited knowledge of the most appropriate method for titration and interval of administration. Most reports on this drug are uncontrolled and limited to a small number of patients receiving low doses of methadone. Methadone should be titrated carefully and individualized doses and intervals should be determined for each patient. Future research should attempt to determine the equi-analgesic dose for chronic use, its effectiveness and tolerance when used in high doses, and its absorption and tolerance using alternative routes, e.g., rectal and subcutaneous.
Fainsinger et al. (1993) studied this question.