Tolerance to the analgesic effect of buprenorphine, butorphanol, nalbuphine, and cyclorphan, and cross-tolerance to morphine.

Purpose: The increased use of opioids in the chronic treatment of pain, especially with oncologic patients, encourages the search for drugs with potent analgesic activity, but with minimal induced tolerance and cross-tolerance to morphine.

Methods: Four agonist-antagonist opioid derivatives (buprenorphine, butorphanol, nalbuphine, and cyclorphan) were examined. Tolerance to the analgesic effect of the four drugs and their cross-tolerance effects with morphine were evaluated in ICR albino mice by the "hot plate method".