Codeine sulfate is an opioid agonist relatively selective for the mu-opioid receptor, but with a much ... Avoid concomitant mixed agonist/antagonist opioids (eg, butorphanol, nalbuphine ...

The precise mechanism of the analgesic action is unknown although fentanyl is known to be a mu opioid receptor agonist ... Have an opioid antagonist and other resuscitative measures available.