Opioid-induced abnormal pain sensitivity.

Opioid analgesics are effective for treating many pain conditions. Opioid analgesic tolerance is a pharmacologic phenomenon that could affect the clinical use of opioid analgesics. Recent studies have shown that neural plasticity associated with the development of opioid tolerance may activate a pronociceptive mechanism that could counteract the analgesic effects of opioids. Thus, exposure to opioids could lead to two seemingly unrelated cellular processes (ie, the development of opioid tolerance and opioid-induced pain sensitivity). Their converging effects may be part of the mechanisms leading to the reduced opioid analgesic efficacy in chronic opioid therapy.