Naloxone is a semisynthetic morphine derivative and a competitive opioid antagonist with very high affinity for opioid receptor sites. It can displace opioid agonists and partial antagonists.¹³

It can be used in suspected or known opioid overdoses to reverse the CNS and/or respiratory depression caused by natural and/or synthetic opioid toxicity. This can include administration to neonates born to mothers who have used opioids.

It can also be used in overdoses of methadone, dextropropoxyphene and the mixed agonist/antagonist analgesics nalbuphine and pentazocine.

Naloxone lacks the morphine like ‘agonistic’ effects of other opioids and has very little effect on its own, displaying virtually no pharmacological effects and with no tolerance or dependence developing. Even at ten times the usual therapeutic dose it will produce insignificant pain relief, only slight drowsiness, and no respiratory depression, psychotomimetic effects, circulatory effects or miosis (constricted pupils).

Once administered, it quickly enters body tissues and especially the brain, as it is highly lipophilic. It can be detected in systemic circulation within a minute of intranasal administration. The cerebral concentration 15 minutes after injection can be 50% higher than the serum concentration.

The half-life for naloxone is about 60-90 minutes, and it is usually effective for 45 minutes to 4 hours. Half-life may be longer if administered by nasal spray compared to intramuscular injection. The effects of opioids with a longer half-life (eg methadone) may start to show through (eg respiratory suppression returning) as naloxone’s blocking effects start to wear off, requiring another dose of naloxone.¹³