A two-separate-motivational-systems hypothesis of opioid addiction.

There has been a long debate as to whether opioids are sought for withdrawal relief or for their ability to serve as incentives in their own right. We suggest that independent motivational systems mediate the rewarding effects of opioids in the nondependent state and in the physically dependent/withdrawal state. In the opioid-dependent state and the presence of opioid withdrawal, the rewarding effects of withdrawal relief inhibit or mask the acute rewarding effects initially exerted in the nondependent state, but the acute rewarding effects are unmasked after the alleviation of withdrawal.