What Is Naltrexone?
Naltrexone Chemical Structure
What is naltrexone? If you have tried to find a simple and straightforward answer to this question you may be somewhat frustrated by this point. As a pharmacist, I receive questions like this all the time. My goal in this article, as in all my pharmacy articles, is to provide clear and concise answers to questions about medications and prescription drugs. Here we go:
Naltrexone is a drug designed to block the effects of narcotics in the body. That's it. To be a bit more precise, it blocks the receptors which are stimulated by narcotics or similar substances. It is not a pain reliever. It does not have any other known benefits (at normal doses) beyond those produced by blocking the effects of narcotics (including heroin). If you have naltrexone in your system and you take a narcotic, the narcotic will not work and it will not produce the desired effect. Moreover, if you are currently "dependent" on any narcotic, taking naltrexone will typically produce withdrawal symptoms.
Maybe that is all you wanted to know. I have given you the "Reader's Digest" quick answer. However, although "what is naltrexone?" can be answered easily, the drug itself has a wide and interesting variety of uses. Therefore the more curious inquirer may now have several other questions, which I will attempt to answer in the remainder of this article. These questions are:
- Which prescription drugs contain naltrexone?
- How exactly does naltrexone work (i.e., how does it block the effects of narcotics)?
- What are some of the reasons for prescribing naltrexone or products with naltreone?
- What is the difference between naltrexone and a similar-sounding drug, naloxone?
In the remainder of this article I will seek to provide you with a clear answer to each of these questions.
Prescriptions Containing Naltrexone
The following prescription drugs have naltrexone as at least one of the ingredients:
Naltrexone Tablets: This would be the regular, generic naltrexone tablets which are manufactured by one of several generic manufacturers today. One such manufacturer is Mallinckrodt pharmaceuticals, and their product is pictured above. It is available as a 50mg tablet.
Revia: Revia is the brand name product of the above Naltrexone tablets. Revia is manufactured by Duramed Pharmaceuticals, which was bought out by Teva. Revia is available at 50mg tablets.
Embeda: Embeda is a relatively new combination product containg both morphine and naltrexone. It is available in an extended release capsule and comes in the following strengths (the left number is the amount of morphine/the right number is the amount of naltrexone):
Embeda is manufactured by King Pharmaceuticals out of Bristol, Tennessee.
Vivtrol Injection: Vivitrol is an intramuscular (IM) injection given once monthly and contains 380mg of naltrexone. This drug must be administered by a healthcare professional. The injection will be given in one of your largest muscles—yep, you guessed it—the buttocks.
Relistor Injection: While not technically "naltrexone" Relistor contains the very cool cousin of naltrexone known as methylnaltrexone. Relistor is supplied in a 12mg/0.6ml single dose vial and is given subcutaneously (SubQ) every other day in a dose that varies depending on a patient's body weight.
How Exactly Does Naltrexone Work?
For those interested in the chemistry and mechanism of action I shall offer a brief explanation of how Naltrexone works.
Narcotics, such as morphine or heroin, produce their effects by stimulating receptors known as "mu receptors" (mu is the Greek letter "M," and these receptors received their name due to studies involving the mechanism of morphine"... cool, huh?). Stimulation of mu receptors is responsible for the pain relief and pleasure response to these drugs. These receptors also seem to be involved in the pleasure response triggered during alcohol consumption. Mu receptors are located in the brain and central nervous system, and also in the intestines (this is why narcotic drugs cause constipation... because they stimulate mu receptors in the intestines causing their normal movement to slow down!).
Naltrexone blocks these receptors so they cannot be stimulated. Naltrexone does not stimulate these receptors, it just blocks them from being capable of stimulation. If you imagine that mu receptors are like the keys on a piano, then Naltrexone is the pharmacological equivalent of closing the cover over the keys.
With this basic understanding of the pharmacology, we are now in a position to discuss some of the unique and interesting ways that naltrexone is used as a prescription drug.
Uses for Naltrexone
The ability to bind to mu receptors and block the effect of narcotics, along with the pleasure stimulation associated with alcohol abuse, has led to some unique uses and indications for naltrexone. The following are ways that naltrexone is currently used:
1) Treatment of Alcoholism or Drug Addiction
Drugs available include: Naltrexone tablets, Revia tablets (brand name naltrexone), and Vivitrol injection.
Due to naltrexone's ability to block the euphoric effects of these drugs, regular administration can help patients overcome the psychological addiction to these drugs. Because the tablets need to be taken daily (or sometimes 3 times per week), this approach does require the patient to actively participate in his/her own treatment. Vivitrol injection offers the advantage of being given once monthly. If a patient seeks to abuse narcotics, they will find their attempts to be fruitless. The cravings for alcohol abuse are also diminished while using these drugs.
2) To Prevent the Abuse of Narcotic Pain Relievers
Drugs for this purpose include: Embeda.
Embeda is the first in a unique approach to combine a powerful narcotic pain reliever (in this case, morphine) with an embedded supply of naltrexone. Now, if you have been paying attention, this may sound very strange to you. Morphine and Naltrexone? Didn't I say that naltrexone BLOCKS the effect of narcotic drugs? Yes, I did. However, it only blocks the effect of narcotic drugs IF it actually gets absorbed into the blood stream! In this case, Embeda has an "embedded" (ah...see where the name comes from?) matrix of naltrexone which, when taken orally, will pass straight through the intestines and never get absorbed. The morphine WILL get absorbed, and work to relieve pain as it should.
So, why include the Naltrexone? Well, sadly, there as some folks who would like to abuse morphine by dissolving it and injecting it. This approach gives a much more powerful effect. However, if you try this with Embeda, you will also be injecting the Naltrexone...and guess what? The morphine will now be completely ineffective. Pretty clever chemistry if you ask me!
3) To Relieve Constipation Caused by Opioids
One final use for naltrexone comes to us in the injectable drug Relistor. Relistor is injected under the skin and is used by patients currently being treated with narcotics for chronic pain, often for cancer pain. Hmmmm. Now you should be really confused. Injecting naltrexone and treating for pain with narcotics? Won't this block the pain relieving effects of the narcotic drugs? The answer is "yes...it would"... but this drug is actually methylnaltrexone, a unique cousin of naltrexone, which is unable to cross the "blood brain barrier" and therefore cannot interfere with the pain-relieving effects of narcotics. Methylnatrexone can, however, go to the receptors in the intestinal tract where narcotics often cause constipation. There, in the intestines, it blocks the constipating effects of the drugs...but as we just said...it does nothing to interfere with the pain-relieving effects. Now that is cool pharmaceutical science at work!
Naltrexone vs. Naloxone
One final quick piece of information for the curious reader who has gotten this far in the article. You may have heard about another drug, similar to naltrexone, known as "naloxone." What is naloxone?
Naloxone is an ingredient in several prescription drugs such as:
- Talwin NX (containing pentazocine and naloxone)
- Suboxone (containing buprenorphine and naloxone)
- Narcan and Vivitrol (injectable formulations of naloxone)
Naloxone basically has the same pharmacological effects as naltrexone, but it is shorter-acting, and thus makes it especially useful as a "revival" drug administered to patients who have intentionally or unintentionally overdosed on narcotics.
What about low-dose naltrexone?
Low-dose naltrexone (we are talking about doses of 4mg or 5mg) has been investigated for treating a variety of painful conditions such as fibromyalgia and Multiple Sclerosis, as well as treatments for cancer. The mechanism by which low dose naltrexone accomplishes this is not fully understood. One hypothesis is that it produces a "protective effect" on nerves by reducing the release of substances known as "pro-inflammatory cytokines." More studies are needed, but promising results have been observed.
Here is a great chart, created by the National Institute on Alcohol Abuse and Alcoholism, of medications currently used to treat alcohol addiction.
Here is a YouTube video about the uses of naltrexone.