While the mechanism of action of naloxone is not fully understood, the preponderance of evidence suggests that naloxone antagonizes the opioid effects by competing for the same receptor sites, especially the opioid mu receptor. Recently, naloxone has been shown to bind all three opioid receptors (mu, kappa and gamma) but the strongest binding is to the mu receptor.
Naloxone is an opiate antagonist and prevents or reverses the effects of opioids including respiratory depression, sedation and hypotension. Also, it can reverse the psychotomimetic and dysphoric effects of agonist-antagonists such as pentazocine. Naloxone is an essentially pure narcotic antagonist, i.e., it does not possess the "agonistic" or morphine-like properties characteristic of other narcotic antagonists; naloxone does not produce respiratory depression, psychotomimetic effects or pupillary constriction. In the absence of narcotics or agonistic effects of other narcotic antagonists, it exhibits essentially no pharmacologic activity. When given intravenously, the onset of action is apparent within 2 minutes. The onset of action is slower if given subcutaneously or intramuscularly. The duration of action also differs between sites of injection and dose.
Naloxone is hepatically metabolized and primarily undergoes glucuronidation to form naloxone-3-glucuronide.
LD50, IV administration, mouse = 150 ± 5 mg/kg; LD50, IV administration, rat = 109 ± 4 mg/kg;