Opioid drugs and their receptors. A summary of the present state of knowledge.

New developments in the field of opioid pharmacology have necessitated the reclassification of drugs formerly lumped together as 'narcotic analgesics'. Various developments pertaining to the characterization of opioid receptors, endogenous 'opioid ligands' and the subclassification of opioid receptors are reviewed. At the moment no entirely satisfactory classification of opioid analgesics appears to be possible; however, these drugs can be divided into 'agonist' and 'agonist-antagonist' analgesics on the basis of their interactions with mu-, kappa- and sigma-receptors. Individual agonist and agonist-antagonist analgesics vary in their analgesic potencies. However, by selecting the correct dosage analgesic effects approximately equivalent to those of 10 mg morphine can be achieved with most of these drugs. The agonist analgesics produce morphine-like 'drug-seeking behaviour' and have a high potential for abuse; the agonist-antagonist analgesics generally have a lower potential for abuse. The agonist-antagonist analgesics are not a homogeneous group; some of these compounds produce psychotomimetic and adverse haemodynamic effects, this probably being due to interaction with sigma-receptors. Nalbuphine, an agonist-antagonist analgesic devoid of 'sigma effects', appears to be an interesting new drug in the development of opioid analgesics with more selective actions.