Influence of the time of administration of dexamethasone 0.25 mg on cortisol secretion in normal humans.

Single low doses (0.25 mg) of dexamethasone were given at 11 p.m., 2, 5 and 8 a.m. on separate days to five normal subjects. The concentrations of cortisol in plasma on the next day were significantly decreased compared to results after placebo administration, and cortisol suppression was maximal after dexamethasone had been given at 8 a.m. Our findings suggests that the postulated phase-advance of circadian rhythms is not a major cause of cortisol non-suppression in depressives given dexamethasone.