This invention relates to a method of selectively enhancing the
analgesic potency of
morphine and other clinically used bimodally-acting
opioid agonists and simultaneously attenuating development of physical dependence, tolerance and other undesirable side effects caused by the chronic administration of said bimodally-acting
opioid agonists comprising the co-administration of a bimodally-acting
opioid agonist which activates both inhibitory and excitatory
opioid receptor-mediated functions of neurons in the nociceptive (pain) pathways of the
nervous system and an
opioid receptor antagonist which selectively inactivates excitatory
opioid receptor-mediated side effects. This invention also relates to a method of using excitatory opioid
receptor antagonists alone to block the undesirable excitatory side effects of endogenous bimodally-acting opioid agonists which may be markedly elevated during
chronic pain. This invention further relates to a method of long-term treatment of previously detoxified
opiate, cocaine and
alcohol addicts utilizing said excitatory opioid
receptor antagonists, either alone or in combination with low-
dose methadone, to prevent protracted physical dependence, and to compositions comprising an excitatory opioid
receptor antagonist of the invention and a bimodally-acting
opioid agonist.