Mechanism of action
By chemical structure, naloxone is a phenanthrene derivative and has a structure close to opioid molecules. During metabolism in the body, it undergoes reduction reactions (specifically, reduction of the carbonyl group).
The drug acts as a full competitive antagonist of all types of opioid receptors ($\mu$, $\delta$, $\kappa$). The mechanism of direct competitive antagonism is that the substance binds to the receptors and physically displaces full agonists (e.g., morphine, fentanyl, methadone, codeine, promedol) and partial agonists from them. The main target in saving a patient's life is the $\mu$-opioid receptors of the neurons in the respiratory center.
Pharmacological effects
The drug does not cause intrinsic effects characteristic of opioids, but merely eliminates the action of narcotic analgesics.
- Restoration of respiration: Reverses drug-induced respiratory depression, restoring the sensitivity of the respiratory center to carbon dioxide ($CO_2$).
- Elimination of central effects: Neutralizes analgesia, euphoria, and sedation caused by narcotic substances.
- Effect on brainstem reflex centers: Eliminates miosis (pupillary constriction). Miosis occurs due to excitation of the oculomotor nerve centers upon opiate intake and is a characteristic diagnostic sign of poisoning.
Indications for use
The primary indication is acute intoxication (overdose) with narcotic analgesics. In opioid poisoning, respiration first becomes slow and deep (therapeutic doses), and then very slow and shallow (toxic doses), up to paralysis of the respiratory center and death.
Naloxone is the drug of choice and is administered intravenously or intramuscularly for the emergency restoration of vital functions.
Comprehensive detoxification: In addition to administering the antidote, the therapeutic logic includes eliminating the poison from the body:
- Gastric lavage: 0.05% potassium permanganate solution (causes chemical oxidation of morphine) or activated charcoal suspension in warm water (toxin adsorption) is used.
- Bowel cleansing: administration of a saline laxative after lavage.
- Resuscitation measures: artificial lung ventilation (mechanical ventilation) is used in case of profound depression.
Features and limitations
- Short duration of action: The effect after intravenous administration lasts from 1 to 4 hours. Since many opioids act longer, careful monitoring of the patient and, if necessary, re-administration of the drug are required.
- Difficulties with buprenorphine: Naloxone is less effective in buprenorphine (agonist-antagonist) overdose because the latter forms a very strong bond with receptors.
- Differentiation of drugs: A clear distinction should be made between pure antagonists (naloxone, naltrexone) and agonist-antagonists (nalbuphine, pentazocine, butorphanol) and full agonists (omnopon). Unlike naltrexone, which acts for up to 36 hours and is used orally for long-term dependence therapy, naloxone is strictly an injectable emergency-care drug.
Prescription rules
The drug is available as a solution for injection. It is administered intravenously (I.V.) or intramuscularly (I.M.).
Example of prescription in opiate poisoning: I.V. 400 mcg over 2–3 minutes. The total dose must not exceed 10 mg.
Rp.: Sol. Naloxoni hydrochloridi 0,04% - 1 ml D.t.d. N. 10 in amp. S. Administer intravenously 1 ml (400 mcg) over 2-3 minutes in case of opiate poisoning.
Briefly: answers to topical questions
What side effects can occur with the administration of naloxone?
The following adverse events and clinical risks may occur when using the drug:
- Reduction of analgesia — the drug significantly decreases the analgesic effect of opiates. This occurs because it is a competitive antagonist and completely eliminates the effects of narcotic analgesics by displacing them from receptor binding.
- Rebound coma — since the drug's action is short-lived, there is a high risk of life-threatening intoxication symptoms returning, which requires repeated injections or continuous intravenous infusion.
Naloxone is a short-acting injection for rescue (acts for hours), Naltrexone is a pill for sobriety (acts for a day).
Class and exam questions
Why does morphine poisoning often require repeated administration of naloxone?
The duration of action of naloxone is only 1–4 hours, which is significantly shorter than the half-life of many opioids. If the dose is not repeated, respiratory depression may return after the antidote stops working.
Will naloxone help in benzodiazepine poisoning?
No, it is a specific opioid receptor antagonist. In overdose with benzodiazepines or Z-drugs, another antidote is used — flumazenil.
What is the difference between naloxone and nalbuphine?
Naloxone is a full competitive antagonist that only blocks receptors and has no intrinsic effects. Nalbuphine is an agonist-antagonist that possesses its own analgesic activity.
Does naloxone affect the cough reflex?
Opioids (e.g., codeine) depress the cough center, exerting an antitussive effect to which tolerance develops rapidly. Naloxone, by displacing opioids from receptors, also eliminates this effect.
What else to study on this topic
- Full opioid receptor agonists (morphine, fentanyl, methadone)
- Agonist-antagonists and partial agonists (buprenorphine, nalbuphine)
- Naltrexone: pharmacology and use in addiction therapy
- Mechanisms of drug-induced respiratory center depression
- Flumazenil as a specific antidote in benzodiazepine poisoning
- Methods of GI decontamination in acute poisonings (potassium permanganate, saline laxatives)
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