Naloxone is a drug used to counter the effects of overdosing with opioids such as heroin and morphine. Specifically, naloxone is used in opioid overdoses for countering life-threatening depression of the central nervous system and respiratory system. The drug is derived from thebaine and has an extremely high affinity for µ-opioid receptors in the central nervous system.

Naloxone is a µ-opioid receptor competitive antagonist, and its rapid blockade of those receptors often produces rapid onset of withdrawal symptoms. Naloxone is injected, usually through intravenously route for fastest action. The drug acts after about two minutes, and its effects may last about 45 minutes. Naloxone has been shown to reduce rates of fatal overdose with heroin.