Methadone in man: Pharmacokinetic and excretion studies in acute and chronic treatment

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Abstract
The biologic disposition of methadone in acute and during chronic administration was studied in 12 human volunteers. In the acute study a biexponential methadone plasma level deeay was observed. The acute primary half-life (t½) of 14.3 hr in combination witn the acute seeondary t½ of 54.8 hr were longer than the single exponential chronic t½ of 22.2 hr determined in the same subjeets. The urinary and feeal excretion of methadone and its mono-Nvdemethylated metabolite increased from 22.2% in the acute to 62.0% in the ehronic phase of the study. The urinary metabolite 1 to methadone ratio tripled from the acute to the ehronie phase. The pupillary effeets of methadone monitored throughout 24 hr were nearly the same in magnitude in the acute and the chronic studies, whereas the plasma levels increased 3- to 8-fold following chronic methadone administration, These findings suggest that both dispositional and pharmacologic toleranee are involved in the development of toleranee following chronic administration of methadone.

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