Opioid intoxication presents with the unmistakable combination of constricted pupils, slowed or absent breathing, and obtundation. The life-threatening element is respiratory depression, so the antidote must restore ventilatory drive quickly.
Naloxone fulfils this role as a competitive antagonist at the mu-opioid receptor. Delivered intravenously, intramuscularly, or intranasally, it displaces the agonist and reverses both the apnoea and the depressed sensorium within minutes.
Practical caveats: its duration is shorter than that of long-acting opioids such as methadone, so dosing may need repetition or a drip; in opioid-dependent patients it can trigger an acute withdrawal state.
The other agents target different toxidromes - flumazenil for benzodiazepines, atropine for cholinergic/organophosphate poisoning, and N-acetylcysteine for paracetamol hepatotoxicity.
\[\boxed{\text{Opioid overdose antidote} = \text{Naloxone}}\]