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Opioids work due to their effects on specific targets in the brain called “mu” receptors.

Drugs that can get attached to these “mu” receptors can have 3 types of effects on them:

  1. Full Activation (caused by drugs such as Morphine or Methadone)
  2. Partial Activation (effects reaches a plateau) so more drug will not have more effect; example would be Buprenorphine.
  3. Blockade; drug binds to the “mu” receptor & blocks it from effects of other drugs. These chemicals/drugs are called “Antagonists”.

Two most commonly used Opioid Antagonists are:

Naloxone & Naltrexon

Both are Opioid Antagonists, both block the effects of opioids and both have important clinical uses.

After taking by mouth, both are extensively metabolized by the liver.(Up to 98% of Naloxone is converted to an inactive molecule by the liver before it can reach the brain and have any effect)
Therefore, Naloxone is given as an Injection or Nasal Spray.When given by an injection, Naloxone starts working in a couple of minutes and its effects typically last for 30-90 minutes.
When given intramuscularly, Naloxone has a more prolonged effect as compared to IV injection.

Naloxone is used primarily as an Emergency Antidote to opioid overdoses.
The effects of the opioids can last much than those of Naloxone. It is possible to have the effects of Naloxone wear off and the opioid overdose effects takes over again. Therefore, it is important that someone stays with the person until the risk is over just in case another dose of Naloxone is needed.

Naltrexone is also metabolized in the liver.
However, its metabolite (6-β-naltrexol) is itself an active compound with similar effects on “mu” receptors as the parent drug.
Naltrexone is used as a medication to prevent relapse in alcoholism and opioid addiction.
Naltrexone is available in oral form and is used to treat cravings due to alcohol and/or opioid dependence.
Naltrexone is contraindicated when there is hypersensitivity to the drug (including Naloxone), acute hepatitis, liver failure, pregnancy, lactation, age under 18 years.
Naltrexone prevents the development of physical dependence to morphine, heroin and others.
After oral intake, Naltrexone starts working in 1-2 hours.
A 50 mg Naltrexone dose would block the effects of opiates including heroin for 24 hours, a double dose (100 mg) is effective up to 48 hours, and 150 mg will last for 3 days.
Naltrexone is available in the form as capsules and tablets as well as a skin implant which lasts up to a month.

Both Naloxone & Naltrexone can cause uncomfortable withdrawal symptoms which can be bad enough for the person to want to use again just to stop the highly uncomfortable feelings.


  • Both Naloxone & Naltrexone counteract the effects of opioids; i.e. they are both Opioid Antagonists
  • Naltrexone is longer acting and more suitable for long term treatment of alcohol and opioid addiction.
  • Naloxone is shorter acting, and can quickly counteract the effects of opioid overdose in emergency situations.