Ask A Toxicologist > What is naloxone?

What is naloxone?

posted on 7:49 AM, December 5, 2016
Opioids are drugs derived from the opium poppy, including such agents as morphine and codeine that have a number of effects on the body. These include reduction of pain, as well as the side effects of sleepiness, constipation and itching. More serious and concerning effects include slow breathing and slow heart rate. Naloxone, also called ‘NarcanTM’, blocks and reverses these effects, and can be used to treat an overdose of opioids by increasing a person’s breathing back to a normal rate, and by waking the person back up. Naloxone can be used to reverse the effects of most types of opioids, such as morphine, hydromorphone, oxycodone, hydrocodone, fentanyl, methadone, and heroin. In individuals who take or use opioids daily, naloxone can cause withdrawal symptoms such as nausea, vomiting, diarrhea, sweating, yawning, anxiety, runny nose, and watery eyes. In severe cases where too much naloxone is given to a person who is a heavy user of opioids, the rapid reversal of the opioids can cause lung inflammation and abnormal heart rhythms. For this reason, low doses of naloxone should be used to safely reverse opioid effects.

How and why is naloxone given?

Naloxone can be injected into a vein, muscle, or under the skin, and can also be given by spraying the medication into the nose or through a breathing mask. It can also be taken by mouth or through a feeding tube. Some forms of oral opioid medications contain naloxone, designed to help keep people safe by preventing them from abusing the opioids pills by snorting crushed pills, or injecting pills dissolved in liquid. Naloxone is available in hospitals, and is carried by medical personnel such as emergency medical technicians and paramedics, and some police officers. In some parts of the US, naloxone is available to individuals who abuse opioids due to addiction, as well as their friends and family. Quick access to naloxone can save a life by helping a person to start breathing again after an overdose.

How does naloxone work?

Opioids create their effects in the body when they attach to a receptor, similar to the interaction between a lock and key, where the opioid is the key, and the receptor is the lock. Once the key enters the lock, the body experiences the effects of the opioid. Opioids act in the brain by repeatedly attaching and then detaching from these receptors. Naloxone works by preventing the opioid ‘key’ from entering the receptor ‘lock’ - a process called ‘competitive antagonism’. Once an opioid can’t attach to the receptor, it can’t cause opioid effects.

Are there any opioids that cannot be reversed by naloxone?

There are a few cases where naloxone is not effective in reversing opioid effects. Since naloxone works by preventing opioids from attaching to the opioid receptors in the brain (called ‘mu’, ‘kappa’, and ‘sigma’), it doesn’t work as well against those that are very slow to detach once bound to the receptor (a characteristic called ‘slow dissociation’). For example, buprenorphine, which is used to treat opioid addiction, has a very strong ‘lock and key’ interaction (also called ‘high affinity’), which doesn’t reverse very well with naloxone unless it is given in large enough doses to ‘out compete’ buprenorphine for the receptors. Opioids with very strong or long-lasting effects, such as methadone, sometimes require higher or repeated doses of naloxone to reverse effects.


Naloxone is a safe, effective and potentially life-saving medication that reverses the dangerous effects of opioid overdose by increasing a person’s breathing back to a normal rate, and by waking the person back up.

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