Which drug and how much should be administered to a casualty who is overdosed on opioid analgesics?

The appropriate intervention for a casualty who has overdosed on opioid analgesics is to administer Naloxone, specifically 0.4 mg either intravenously (IV) or intramuscularly (IM). Naloxone is an opioid antagonist that acts by competitively binding to the opioid receptors in the brain, reversing the effects of opioids. This is crucial in cases of overdose, where the respiratory depression caused by excessive opioids can be life-threatening. The quick administration of Naloxone can restore normal breathing and consciousness, which is vital for the casualty's survival.

Other options listed do not address overdose on opioids effectively. Fentanyl is itself an opioid and would exacerbate the problem, while Atropine is used primarily for bradycardia and has no effect on opioid toxicity. Midazolam, a sedative, could further depress the respiratory system and is not a suitable choice for opioid overdose management. Therefore, Naloxone's specific mechanism of action and its role in reversing opioid overdose make it the correct choice.