Naloxone, sold under the brandname Narcan among others, is a medication used to block the effects of opioids, especially in overdose. Naloxone may be combined with an opioid (in the same pill) to decrease the risk of opioid misuse. When given intravenously, naloxone works within two minutes, and when injected into a muscle, it works within five minutes; it may also be sprayed into the nose. The effects of naloxone last about half an hour to an hour. Multiple doses may be required, as the duration of action of most opioids is greater than that of naloxone.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 by the Food and Drug Administration in 1971. It is on the World Health Organization's List of Essential Medicines, the most effective and safe medicines needed in a health system. Naloxone is available as a generic medication. Its wholesale price in the developing world is between $0.50 and $5.30 per dose. Vials of naloxone are not very expensive (less than $25) in the United States. The price for a package of two auto-injectors in the US, however, has increased from $690 in 2014 to $4,500 in 2016. The 2018 price for the NHS in the United Kingdom is about £5 per dose.