The absolute bioavailability of oral melatonin tablets was studied in 12 no
rmal healthy volunteers. Subjects were administered, in a randomized crosso
ver fashion, melatonin 2 mg intravenously and 2 and 4 mg orally. Blood was
sampled over approximately eight (estimated) half-lives. Both the 2 and the
4 mg oral dosages showed an absolute bioavailability of approximately 15%.
No difference in serum half-life was seen in any of the study phases. Oral
melatonin tablets in dosages of 2 and 4 mg show poor absolute bioavailabil
ity, either due to poor oral absorption, large first-pass metabolism, or a
combination of both. Furth er studies examining larger doses, in an attempt
to saturate first-pass metabolism if it occurs, may be warranted. (C) 2000
the American College of Clinical Pharmacology.