Naloxone: short half-life and recurrent respiratory depression
Naloxone reverses overdose of opioids (drugs that relieve pain and slow breathing) but often wears off in 30 to 90 minutes, before the opioid does, so slow breathing can return.
Remember
Naloxone often wears off before the opioid does, so breathing can slow dangerously again (respiratory depression) after a good response: give repeated doses or an infusion and keep the patient monitored.
An opioid is a drug that relieves pain and also slows breathing. It does both through the mu opioid receptor. In overdose the danger is respiratory depression, which means dangerously slow, shallow breathing. Naloxone is the antidote. It is a competitive antagonist, which means it fights the opioid for the same binding site. It sits on the mu opioid receptor without activating it and displaces the opioid, so breathing improves within minutes.
The catch is its half-life, the time taken for the amount of drug in the blood to fall by half. For naloxone this is only 30 to 90 minutes. It does not remove the opioid from the body. Once the naloxone has been cleared, an opioid that lasts longer is still present and can occupy the receptors again, so drowsiness and respiratory depression return. Methadone is a long-acting opioid with a half-life of roughly 15 to 60 hours, and a modified-release (slowly absorbed) preparation is another example of an opioid that can outlast naloxone.
A good first response should therefore not end observation. The patient should be kept in a monitored area, and repeated boluses (single doses) or a continuous infusion may be needed if the opioid outlasts naloxone. If the person is dependent on opioids, full reversal can precipitate withdrawal, so the aim is adequate breathing rather than full wakefulness.
Key facts
- Mechanism
- naloxone is a competitive antagonist at the mu-opioid receptor.
- Use
- it reverses opioid-induced respiratory depression in overdose.
- Routes
- intravenous, intramuscular or intranasal.
- Naloxone half-life
- about 30 to 90 minutes.
- Methadone half-life
- about 15 to 60 hours, so methadone is long-acting.
- Management
- if the opioid outlasts naloxone, repeated boluses or an infusion may be needed, with monitoring.
Exam traps
- A competitive antagonist binds reversibly, so its effect fades as the drug is cleared and is not permanent.
- Six-hourly dosing of naloxone leaves gaps in antagonism, because intramuscular naloxone still has a shorter duration of action than methadone.
- Naloxone has no agonist activity, whereas buprenorphine partly activates the mu receptor; respiratory depression can return because naloxone wears off while a longer-acting opioid is still present.
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