Variable bioavailability of oral melatonin.

To the Editor: It has been claimed that melatonin has many actions, mostly on the basis of uncontrolled studies of widely varying oral doses. It is commonly assumed that the administered dosage of melatonin bears a direct relation to its circulating concentration. We estimated the bioavailability of oral melatonin by determining the pharmacokinetics of intravenous bolus and oral doses of melatonin (Sigma Chemical, St. Louis) in four normal men (21 to 32 years old), who were given 20 μg intravenously on one occasion and 500 μg orally on a second occasion, with an interval of at least one month between . . .