Pharmacology · Opioids and antagonists
Naloxone !
An opioid antagonist: it pushes opioids off their receptor and reverses an overdose within minutes. It does almost nothing when no opioid is present.
Updated 2026-11-01
The figure shows naloxone in the inactive mouse μ receptor; the structure also holds a nanobody and an allosteric modulator, which are hidden.
Mechanism of action
Naloxone is a pure competitive antagonist at opioid receptors, with the highest affinity for the μ receptor. It takes the agonist's site, displaces it and holds the receptor in its inactive shape without activating the G protein. It thus reverses respiratory depression, sedation and miosis within minutes; in a dependent person it can trigger acute withdrawal.
- Competes with the opioid for the μ receptor and displaces it
- The receptor stays inactive: the Gi protein is not switched on
- The breathing center regains its drive within minutes
- It wears off sooner than many opioids: sedation can return
Pharmacokinetics
- Absorption
- Given intravenously, intramuscularly, subcutaneously or intranasally.
- Metabolism
- Liver, by glucuronidation (mainly naloxone-3-glucuronide).
- Elimination
- Kidney, as metabolites.
- Half-life
- About one and a half hours.
- Onset and duration
- Within about two minutes intravenously; a little longer intramuscularly or subcutaneously.
Indications
- Opioid overdose (coma, respiratory depression and miosis), including fentanyl
- Reversal of opioid-induced respiratory depression after anesthesia
- Combined with sublingual buprenorphine to deter injection
Adverse effects
Common
- Withdrawal symptoms in dependent people: agitation, sweating, vomiting, pain
- Tachycardia and hypertension
Contraindications
- Known hypersensitivity to naloxone
Key interactions
- Opioid agonists (morphine, fentanyl)
- Reverses their effect; in dependent people it precipitates withdrawal.
- Buprenorphine
- Its high affinity makes reversal harder and may need more naloxone.
- Long-acting opioids (methadone)
- They outlast naloxone: respiratory depression can return.
What is monitored
- Respiratory rate and level of consciousness for several hours after waking
- Signs of withdrawal
Test yourself
Think of the answer, then tap the question to see it.
What kind of ligand is naloxone at the μ receptor?
A pure competitive antagonist: it takes the agonist's site without activating the receptor and displaces the agonist.
Why does naloxone not work for a benzodiazepine overdose alone?
It acts only on opioid receptors; benzodiazepines act on the GABA-A receptor. Naloxone does almost nothing when no opioid is present.
Why is buprenorphine combined with naloxone in sublingual tablets?
Taken sublingually, naloxone is barely absorbed and does not interfere; if someone dissolves and injects the tablet, naloxone acts and precipitates withdrawal.
Related drugs
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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: Naloxone Injection: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2026); Opioid overdose (World Health Organization, 2025); NALOXONE HYDROCHLORIDE INJECTION, USP, AUTO-INJECTOR- naloxone hydrochloride injection, solution (DailyMed (U.S. National Library of Medicine), 2026); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026).
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Representative model, for educational purposes only.