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.