that's the question of the day.
if one were to take the exact same amount of opiates every day, then yes, i would assume good ol homeostasis would cause down regulation.
but it's not really direct inhibition (adrenocorticotropic hormone), and the exact mechanism behind it i just can't seem to find.
i've been an avid opiate user for years, daily. i've also never found myself sick in these times at all (except when im out, which is extremely rare). each time i use opiates, i feel the effects of them, it's not just to feel normal (minus the couple years of suboxone maintanance which i have finally broke away from). so if one feels the high, then there must be be enough opiates to cause the usual opiate effects, and apparently one of them is corticotropin inhibition...
...and with that comes decreased cortisol (and aldosterone) mostly, cortisol being a primary stress response hormone. stress is being linked to more and more problems every month it seems... high blood pressure, illness, depression, cancer, and many others.
it seems that opiates reduce stress, not only psychologically, but physiologically via hormone response.