µ receptor δ receptor κ receptor Morphine +   + Levorphanol +     Methadone +     Fentanyl +     Buprenorphine +   - Pentazocine +   + Butorphanol +   + Nalbuphine -   + Naloxone - - - Naltrexone - - - + binding as agonist, - binding as antagonist, Modified from [1]

Table 2: Classification of opioids as agonist, agonist-antagonist and antagonist with specific receptors.