The psychopharmacologic and prolactin response after large doses of naloxone in man

Large doses of naloxone (150–300 mg), placebo, and morphine (15–30 mg) were given intramuscularly to human volunteers and compared using measures of subjective feeling states, physiological measures and discriminative features. Plasma prolactin responses after naloxone 210 mg and placebo were compared. The subjective measures and discriminative features of naloxone revealed that the drug is subtly psychoactive but the stimulus is vague and cannot be identified clearly as an opioid agonist or antagonist in nondependent opioid-using volunteers. The physiologic and prolactin responses closely resembled opiate agonist activity. We conclude that naloxone in this dose range may act as an opiate agonist in man.