Diurnal variation in fentanyl-induced hormone responses and side effects

The prolactin (PRL) and thyrotropin (TSH) secretory response to the opioid agonist fentanyl (0.1 mg IV) was investigated with serial blood sampling in ten healthy women at 9 AM and 9 PM on different days. In five subjects saline control trials were also performed. A repeated-measures analysis of variance yielded a highly significant effect of fentanyl administration both on PRL and TSH secretion. In every case there were elevated hormone responses in the evening, and more drug-related subjective symptoms were reported at this time than before noon. These findings indicate a diurnal variation of opioid responsiveness, with lower sensitivity in the morning.