What is the antidote for opioid overdose?

Naloxone is the specific antidote for opioid overdose because it is an opioid antagonist that works by rapidly binding to the same receptors that opioids target, thereby displacing the opioid molecules and reversing their effects. This action can rapidly restore normal breathing and consciousness in someone who has experienced an overdose, which is critical given that respiratory depression is often the most life-threatening complication of opioid overdose. Naloxone's effectiveness and quick onset of action make it the drug of choice in emergency situations involving opioid toxicity.

Other substances mentioned, such as flumazenil, are contraindicated for opioid overdoses as they specifically target benzodiazepines, while atropine is used in cases of bradycardia and does not address opioid effects. Activated charcoal is sometimes used for poisonings but is not a direct antidote and is only effective if administered shortly after the ingestion of the opioid. In a life-threatening overdose situation, quick administration of naloxone can save a life, which underscores its importance as the correct answer.