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, and 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, it will not produce the desired effect. Moreover, if you are currently "dependent" on any narcotic, taking naltrexone will typically produce withdrawl 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 quesitons which I will attempt to answer in the remainder of this article. These questions are:

  • What prescription drugs contain naltrexone?
  • How exactly does naltrexone work (ie 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.


The following prescription drugs have naltrexone as at least 1 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):

  • 20mg/0.8mg
  • 30mg/1.2mg
  • 50mg/2.0mg
  • 60mg/2.4mg
  • 80mg/3.2mg
  • 100mg/4mg

Embeda is manufactured by King Pharmaceuticals out of Bristol, Tennessee.

VIVITROL INJECTION: Vivitrol is an intramuscular (IM) injection given once monthly and contains 380mg of naltrexone. Vivitrol must be administered by a healthcare professional. The injection will be given in one of your largest guessed it...the buttocks. For more information on Vivitrol CLICK HERE.

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. For a very educational video on how Relistor works, CLICK HERE.


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" 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.


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:


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.


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!


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 " 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 does nothing to interfere with the pain relieving effects!  Now that is cool pharmaceutical science at work!!!


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 of medications currently used to treat Alcohol Addiction: CLICK HERE!

For general information on narcotic prescription medications used for pain CLICK HERE!

For a VIDEO on the uses of naltrexone CLICK HERE!


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Comments 16 comments

blabbyjen 6 years ago

HI Im on Suboxone for almost a month for chronic pain. Im not sure I quite understand how it works but a few days ago I was having an upper endoscopy done and in registration process the nurse mentioned fentanyl and versed would be given to me. I told them I was on Suboxone and that I shouldn't be given Fentanyl...I was not sure if it would just not work or put me in withdraw...The nurse wasn't familiar with this drug and Im not sure the doctor was either because it was decided at the last minute that I wouldn't be given the fentany. Is there anything I need to be aware of as far as what I should or shouldn't be taking.My Dr always explains things really fast and I think I understand what's hes told me then once I leave I feel like I forgot everything...Hes a confusing Dr. Any valuable information would be greatly appreciated:-) Thanks

pharmacist profile image

pharmacist 6 years ago from Whitinsville, MA Author

Hi blabbyjen,

Most importantly you should avoid certain "macrolide" drugs like erythromycin, antifungals like ketoconazole, HIV drugs known as "protease" inhibitors, and benzodiazepines like alprazolam, lorazepam and others. You did the right thing to explain to them that you are on this. Detailed prescribing information can be found here:

Best wishes,

jim 5 years ago

Blabbyjen , please listen to me and get off that crap as fast as you can ! I was put on Suboxone for chronic pain and help for depression . But after a year of taking it I developed bad side affects such as weight gain , sex drive gone , etc . I decided that I wanted to quit the medication ! I did everything the doctor told me to do ,tapering down in dosage but I was unable to stop !I tried several times using different methods . Unbelievable withdrawal , not to mention the aches and pains!It was much worse than the pains I had before taking the medication .The doctor told me that I may not be able to stop and have to take it for life . I was pissed ! Long story short I ended up going to the colemean institute !They had me off the meds in 3 days , $8000 dollars and a month to recover . Dont make the same mistake I made !!!!!!! Most , of these doctors( NOT ALL) couldn't care less !It's all about the buck !I went back to the doctor that put me on this crap to let him know how I was able to get off of suboxone and he refused to see me !!! Please research suboxone for yourself and you will see that 99% of the people that go on it cant get off of it ! they are referred to as lifers . The only positive thing I can say about the drug is that it did get rid of my pain and helped with my depression !But to me it wasn't worth it !Ive cured most of my problems with diet and exercise ! Google it and you will see what I'm saying is true . Goodluck !

John 5 years ago

Hi thanks for the info. Im about to start Naltrexone treatment for opiate abuse. Im hoping im not complicating things by getting on medication. Also I asked my Dr to run the nalaxone challenge test but he said its not needed. Would it be safe to start without the test? Im also very nervous in case of an emergency what would happen if its not known that Im taking this drug? I understand Ill need to carry ID card but what if its not found in an emergency? what else should I be aware of in terms of medication/anastasia if I ever need a prucedure? thanks

pharmacist profile image

pharmacist 5 years ago from Whitinsville, MA Author

Hi John,

If the doctor has all the facts about your history of opiate usage, they may not need the challenge test. Just be sure to let any healthcare worker know that you are taking this drug. Glad the article helped you!

Heidi 5 years ago

Thank you soo much for the GREAT info "Pharmacist!" I just had a question(S)... I've been on Suboxone for 5 1/2 yrs now (I know..LONG TIME) for tx of opiate addiction. I have been doing TONS of research because I have finally narrowed my 30# wt gain over these years-even with diet exercise-to Suboxone use. To make a long story short..I now WANT OFF, but I've read a lot about how difficult it is to detox off it-even w/ a slow taper.

So, I am wondering if I could use Naltrexone to aid in the withdrawal s/s. I currently am taking 1/2 of an 8mg strip(4mg/d). Would this be an option for me??

Also..I was contemplating asking my Dr to decrease me to 2mg(my appt is Thurs.) but also give me a Rx for po Clonidine 0.1mg, so I don't get the withdrawals. Then go down to 1mg then off. I don't know..what do you think would be the most helpful?

SORRY..JUST HAD ANOTHER THOUGHT..Maybe you could help?? I know one of the above comments stated he gained wt as well. There is NO DOCUMENTATION ANYWHERE regarding wt gain as a S.E. of Buprenorphine or Naloxone. I have asked several Dr's as well, and they all state that if anything, it would cause decreased appetite therefore resulting in WT LOSS. SOO NOT THE CASE IN MY OPINION!! They also state that if a person does gain wt it is because they are now off opiates and their appetite has resumed. But, like I mentioned I have been on Suboxone for almost 6 yrs and this is obviously not the case with me-maybe in the beginning, but NOT NOW right??

I also know that Suboxone has been r/t an increase in testosterone levels-which could result in wt gain right? Not so sure about the actual chemistry of this drug and why it causes this increase thus resulting possibly in wt gain?

Have you heard of any of this before, or am I crazy? ANY INFO would be GREATLY APPRECIATED! THANK YOU IN ADVANCE!! /Heidi.

pharmacist profile image

pharmacist 5 years ago from Whitinsville, MA Author

Hi Heidi,

Sorry for the delay. I've been away. If you are still curious about these...shoot me an email. Thanks!

Ariella, RN 5 years ago


I have a few questions about low dose naltrexone. My doctor wants to start me on it for dyasthesias that I have s/p spinal cord surgery (C-spine). So far I've tried neurontin for 6 months and then topamax for 2 months, both of which haven't helped. My insurance won't approve lyrica. The only off-label use I could find on the internet for low dose naltrexone was for MS. Do you know anything about it being used for post-op neuralgia?



pharmacist profile image

pharmacist 5 years ago from Whitinsville, MA Author


Thank you for sharing. It is an honor to reply. Low dose naltrexone has been studied for many conditions. One study, published in the May/June edition of the 2009 journal "Pain Medicinie" looked at a very small study for low dose naltrexone in fibromyalgia patients. This results were very promising. This may correspond to your situation. However, I also encourage you not to give up on getting Lyrica (or something similar like Cymbalta or Savella) approved. Most insurance companies have a process by which you can appeal their decision to not cover.

Best wishes.

Pam W. 5 years ago

I have been receiving the Vivitrol injection for 5 months now and have had absolutely no side effects. It is just an added "check" for me in my recovery of opiate addiction. I wish it did a little more for the cravings but I guess you can't have everything. My cravings get really bad at times. Anyway, I totally recommend it.

My question is, I am a phlebotomist and required to take the flu shot. Should I? Will it work? I have heard NO to both???? Please advise.

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pharmacist 5 years ago from Whitinsville, MA Author

Hi Pam,

The current CDC recommendation is to give the flu vaccine to everyone over 6 months of age. As a healthcare worker, you may be in a position of greater exposure to get the flu, or to pass it on. The flu vaccine is never a 100% protection. There is always the chance you will get a strain of the virus not covered by the vaccine. But it is far less likely. My advice...get the shot. :)

Hurtingbad 4 years ago

So I have just gotten out of rehab for methadone use was put on it to get off oxys and now iv been addicted to methadone for three years? I don't understand why I just didn't detox off oxys myself I'm almost two weeks off methadone and im still in full withdrawal thanks to naltrexone? Which I started yesterday? Iv got a migraine that won't quit I can't eat or drink anything and I can't stop stretching! I'm worthless!!!! Should I stop taking the naltrexone ?

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pharmacist 4 years ago from Whitinsville, MA Author


What you are experiencing may be (I hope) the worst of the detoxing effects. Methadone has a long half-life, meaning it sticks around in our bodies a long time. The naltrexone is basically kicking the last bit of methadone out...and it hurts. Some people find intense exercise to help. Keep your eye on the goal. I think the worst is behind you. Keep in touch with your MD. Hang in there.

self medicated 4 years ago

Ok so I'm coming off a 3 year oxy habit (anywhere btw 90 and 900mg's a day; depending on the financial situation.) Had a girlfriend and her mom happened to be a doctor...well after a bit of research online and dozens of independent attempts of quitting (tapering, suboxone, cold turkey) I had the ingenious idea of having her prescribe Vivitrol to me (which as mentioned in the article is natlrexone in a pretty little needle form that goes in your butt.) The warning on the box clearly states to not take Vivitrol if you have had opiates i you in the past 10 days. Considering the fact that I can attest that a person can pass a piss test within 4 days of doing roxies, I gave it 33 hours, the exact amount my stash of xanax had me asleep, and decided to roll the dice. !SNAKE EYES! Well I've played doctor before and have done intramuscular injections on myself and others so I was pretty confident with my syringe skills. Needless to say I will not be doing that again. The Vivitrol is so thick that it feels like its going to actually take a year for all of it to go in. The thing is also one doesn't want to push it in too fast as there have been complaints of doctors messing this procedure up and clots having to be surgically removed. Finally I was done. I took the needle out! 1 min - nothing 2 min - nothing 3 min - feels like someone started kicking me in the stomach with steel toe boots; there is no pain I can compare this to and I've broken both my ankles, my arm and my hip. I'll spare you the details but lets just say I felt like the toilet was becoming to be a physical part of me because I was on it so much. I was actually crying from the pain - seriously think about that, when was the last time something hurt you so much physically that tears came out of your eyes? I sit here about 30 hours later still feeling like a brain is on overdrive but my body cant make itself do anything at all. There just seems to be so many more hours in a day now and I don't know where to begin and what to do. I'm planning on taking some Clomid so my brain produces more LH and thus more Testosterone as we all know what opiate abuse can do to the hormonal balance in people. I am not sure which way to go though because some people do 100/100/50/50 (that is mg per day of Clomid and the "/" seperates weeks) while others do 8 weeks of 20. I want my testosterone to rebound fast but it inevitably turns into estrogen which does not reach the brain due to the Clomid. So while yes it does jump start the process it can also turn you into a depressed menopausal 60 yr old which is kind of what I'm feeling like right now. BTW Hurtingbad, if you can get your hands on some suboxone or subutex, take a half it will instantly make you feel better. I highly discourage the use of suboxone as I have heard horror stories about it but since you are going through precipitated withdrawals this is the only thing that can help you. DO NOT TRY TAKING OPIATES! They will not work and you will be wasting your time. Anyway good luck to everyone and if anybody has knowledge regarding the revitalization of testosterone and some magical cure for the hopelessness/depression/lack of motivation please feel free to respond.

constipated and confused 4 years ago

I have been on narcotic therapy for 10 years after sustaining an incomplete spinal cord injury. After years of misery (and pain) due to opioid induced constipation, I have finally been prescribed Relistor.

I have read, from several different sources, that if you take your oral narcotics at the same time, or shortly after, a dose of Relistor, the pain relieving effects of that dose will be blocked. However, the comments above indicate that it won't interfere with the pain meds you just took. I ask because my painkillers don't seem to provide as much relief if I take them in close proximity to the Relistor.

Any clarification would be appreciated.

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pharmacist 4 years ago from Whitinsville, MA Author


the ingredient in Naltrexone cannot get to where the pain meds go to treat your pain. However, other factors could be involved. You may need to look into adjusting the timing of your pain meds to avoid the Relistor problem.

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