Naloxone is a pure competitive antagonist at all opioid receptors, notably the p- and k- receptors; it has no agonist activity. Naloxone antagonises both agonist and partial agonist opioids (although it may not be sufficient to reverse the effects of buprenorphine in overdose, so tenaciously does the latter drug bind to receptors). It induces an acute withdrawal syndrome in opioid-dependent subjects.
Naloxone undergoes high presystemic elimination when swallowed and is not used by this route; some 70% of a dose appears in the urine as metabolites (tV2 75 min).
Given i.v., it causes reversal of opioid-induced respiratory depression in 1-2 min; reversal of analgesia and depressed consciousness can be slower. A prompt marked improvement in respiration has diagnostic value in opioid overdose, but poor or no response may occur because insufficient has been given, or with burenorphine (above), or due to cerebral hypoxia or severe hyothermia.
Naloxone acts for about one hour after an i.v. injection of 100-200 micrograms, though the peak effect on depressed respiration may be as brief as 10
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