Naloxone, sold under the brand name Narcan among others, is a medication used to reverse or reduce the effects of opioids. It is used to counter decreased breathing in opioid overdose. Effects begin within two minutes when given intravenously, and within five minutes when injected into a muscle. The medicine can also be administered by spraying it into a person's nose. Naloxone blocks the effects of opioids for 30 to 90 minutes. Multiple doses may be required, as the duration of action of some opioids is greater than that of naloxone. Emergency medical services data from Massachusetts found that 93.5% of people given naloxone survived their overdose.

Administration to opioid-dependent individuals may cause symptoms of opioid withdrawal, including restlessness, agitation, nausea, vomiting, a fast heart rate, and sweating. To prevent this, small doses every few minutes can be given until the desired effect is reached. In those with previous heart disease or taking medications that negatively affect the heart, further heart problems have occurred. It appears to be safe in pregnancy, after having been given to a limited number of women. Naloxone is a non-selective and competitive opioid receptor antagonist. It works by reversing the depression of the central nervous system and respiratory system caused by opioids.

Naloxone was patented in 1961 and approved for opioid overdose in the United States in 1971. It is on the World Health Organization's List of Essential Medicines. Naloxone is available as a generic medication. In April 2021, the US Food and Drug Administration (FDA) approved a higher dose naloxone hydrochloride nasal spray product (Kloxxado) intended to treat opioid overdose from fentanyl and its analogues, which are many times stronger than heroin.