The time course of methadone in the plasma of 5 methadone maintenance subjects receiving a single daily oral dose of 100 or 120 mg. has been determined. The predose mean plasma level of methadone was 0.48 fig per milliliter. The peak plasma level occurred at 4 hours with an average value of 0.86 fig per milliliter. This level slowly declined so that 24 hours after administration the mean plasma level had reached a value (0.46 fig per milliliter) approximately equal to the predose level. The mean apparent half‐life of methadone in these sub;ects was 25 hours. Data on the renal clearance of methadone indicate that the drug is partially reabsorbed by the kidney and that this reabsorption is pH dependent.
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Inturrisi et al. (1972) studied this question.