Opioid overdose results from excessive mu-opioid receptor activation in the brainstem respiratory centers (pre-Bötzinger complex and parabrachial nucleus), causing respiratory depression through reduced sensitivity to CO2 and decreased respiratory rate, tidal volume, and protective airway reflexes. The triad of opioid toxicity is: miosis (pinpoint pupils from parasympathetic oculomotor nucleus activation), respiratory depression (reduced rate and tidal volume), and CNS depression (decreased consciousness). Death occurs from hypoxic-ischemic injury due to respiratory arrest, often compounded by aspiration pneumonitis from loss of airway reflexes. Naloxone is a competitive mu-opioid receptor antagonist that rapidly displaces opioids from receptors without activating them. Its onset is 1-3 minutes IV, 3-5 minutes IM/SC, 8-13 minutes intranasal. Duration of action is 30-90 minutes - SHORTER than most opioids, creating risk of renarcotization (recurrence of opioid effect after naloxone wears off). Synthetic fentanyl analogues (illicit fentanyl, carfentanil) pose unique challenges: extreme potency (fentanyl 50-100x morphine, carfentanil 10,000x morphine) means lethal doses are physically tiny, and repeated high-dose naloxone may be required. Additionally, fentanyl's lipophilicity creates tissue reservoirs that can cause recurrent respiratory depression after naloxone wears off.