Methylnaltrexone treats constipation caused by use of certain pain medications.

Methylnaltrexone Overview

Reviewed: September 18, 2012

Methylnaltrexone is a prescription medication used to treat constipation caused by prescription pain medicines called opioids (narcotics). Methylnaltrexone belongs to a group of drugs called peripherally acting mu-opioid receptor antagonists, which reverse the effects of opioids on the bowels, without affecting the pain relief from opioid treatment.

Methylnaltrexone is available in tablet form and is typically taken once a day, with water. Take Relistor tablets on an empty stomach at least 30 minutes before your first meal of the day. 

This medication also comes in an injectable form and is given just under the skin once a day in the upper arm, stomach, or thigh.

Common side effects of methylnaltrexone include stomach pain, gas, and nausea.

How was your experience with Methylnaltrexone Bromide?

First, a little about yourself

Tell us about yourself in a few words?

What tips would you provide a friend before taking Methylnaltrexone Bromide?

What are you taking Methylnaltrexone Bromide for?

Choose one
  • Other

How long have you been taking it?

Choose one
  • Less than a week
  • A couple weeks
  • A month or so
  • A few months
  • A year or so
  • Two years or more

How well did Methylnaltrexone Bromide work for you?

Did you experience many side effects while taking this drug?

How likely would you be to recommend Methylnaltrexone Bromide to a friend?

Methylnaltrexone Cautionary Labels


Uses of Methylnaltrexone

Methylnaltrexone is a prescription medicine used to treat constipation that is caused by prescription pain medicines, called opioids, when other medicines for constipation, called laxatives, have not worked well enough.

Methylnaltrexone tablets and injection are used to treat constipation caused by opioids in adults with long-lasting pain that is not caused by cancer.

Methylnaltrexone injection is used to treat constipation caused by opioids in adults who are receiving treatment for advanced illness, after treating constipation with other treatments that have not been successful. 

It is not known if this medication is safe and effective if used for longer than 4 months in people with advanced illness. It is also not known if this medication is safe and effective in children.

This medication may be prescribed for other uses. Ask your doctor or pharmacist for more information.



Methylnaltrexone Brand Names

Methylnaltrexone may be found in some form under the following brand names:

Methylnaltrexone Drug Class

Methylnaltrexone is part of the drug class:

Side Effects of Methylnaltrexone

Serious side effects have been reported with methylnaltrexone. See the “Methylnaltrexone Precautions” section.

Common side effects of methylnaltrexone include:

  • abdominal (stomach) pain
  • gas
  • headache
  • swelling or a feeling of fullness or pressure in your abdomen
  • anxiety
  • muscle spasms
  • runny nose
  • chills
  • nausea
  • dizziness
  • diarrhea
  • sweating
  • hot flush
  • tremor

If you get diarrhea that is severe or does not stop while taking methylnaltrexone, stop taking it and call your healthcare provider.

If you get stomach pain that is severe or will not go away, or nausea or vomiting that is new or worse, stop taking methylnaltrexone and call your healthcare provider.

These are not all of the possible side effects of methylnaltrexone. Tell your healthcare provider if you have any side effect that bothers you or that does not go away.

Call your doctor for medical advice about side effects. You may report side effects to the FDA at 1‑800‑FDA‑1088.


Methylnaltrexone Interactions

Tell your doctor about all the medicines you take, including prescription and non-prescription medicines, vitamins, and herbal supplements. Especially tell your doctor if you take other opioid antagonists. This is because when taken together they increase the risk of opioid withdrawal.

This is not a complete list of methylnaltrexone drug interactions. Ask your doctor or pharmacist for more information.

Methylnaltrexone Precautions

Serious side effects have been reported with methylnaltrexone including the following:

  • Tear in your stomach or intestinal wall. Stomach pain that is severe can be a sign of a serious medical condition. If you get stomach pain that is severe, does not go away, or gets worse, stop taking this medication and seek emergency medical attention.
  • Diarrhea that is severe or that will not go away. Stop taking this medication and call your healthcare provider if you get diarrhea that is severe or that does not go away during treatment with methylnaltrexone.
  • Opioid withdrawal. You may have symptoms of opioid withdrawal during treatment with methylnaltrexone including sweating, chills, diarrhea, stomach pain, anxiety, and yawning. Tell your healthcare provider if you have any of these symptoms.

Do not take methylnaltrexone if you:

  • are allergic to methylnaltrexone
  • have or may have a blockage in your intestines called a mechanical bowel obstruction.

Methylnaltrexone Food Interactions

Medications can interact with certain foods. In some cases, this may be harmful and your doctor may advise you to avoid certain foods. In the case of methylnaltrexone, there are no specific foods that you must exclude from your diet when receiving this medication.


Inform MD

Tell your healthcare provider about all of your medical conditions, including if you:

  • are allergic to Relistor or to any of its ingredients
  • have kidney problems
  • have liver problems
  • have any stomach or bowel problems such as stomach ulcer or Crohn’s disease
  • are pregnant or plan to become pregnant. It is not known if methylnaltrexone can harm your unborn baby. If you become pregnant while using methylnaltrexone, tell your healthcare provider right away.
  • are breastfeeding or plan to breastfeed. It is not known if methylnaltrexone passes into your breast milk.

Tell your healthcare provider about all medicines you take. Continue taking your other medicines for constipation unless your healthcare provider tells you to stop taking them.


Methylnaltrexone and Pregnancy

Tell your doctor if you are pregnant or plan to become pregnant.

The FDA categorizes medications based on safety for use during pregnancy. Five categories - A, B, C, D, and X, are used to classify the possible risks to an unborn baby when a medication is taken during pregnancy.

Methylnaltrexone falls into category B. There are no well-done studies that have been done in humans with methylnaltrexone. But in animal studies, pregnant animals were given this medication, and the babies did not show any medical issues related to this medication.

Taking methylnaltrexone during pregnancy may cause opioid withdrawal symptoms in your unborn baby. Tell your healthcare provider right away if you become pregnant during treatment with Relistor.

Methylnaltrexone and Lactation

It is not known if methylnaltrexone crosses into human milk. Because many medications can cross into human milk and because of the possibility for serious adverse reactions in nursing infants with use of this medication, a choice should be made whether to stop nursing or stop the use of this medication. 

Taking this medication while you are breastfeeding may cause opioid withdrawal in your baby. Do not breastfeed during treatment with methylnaltrexone. 

Methylnaltrexone Usage

Take methylnaltrexone exactly as your healthcare provider tells you.

Methylnaltrexone is available in tablet and injection forms. 

Do not take more than one dose in a 24-hour period.

  • Most patients have a bowel movement within a few minutes to a few hours after taking a dose of methylnaltrexone. That said, stay close to the toilet after taking methylnaltrexone. 
  • If you stop taking your prescription pain medicine, check with your healthcare provider before continuing to take methylnaltrexone. In most cases, you will need to stop taking methylnaltrexone if you stop taking your prescription opioid pain medication. 


  • Take methylnaltrexone tablets once a day, with water.
  • Take methylnaltrexone tablets on an empty stomach at least 30 minutes before your first meal of the day. 


  • Inject methylnaltrexone under the skin (subcutaneous injection) of the upper arm, abdomen, or thigh.

Step 1: Choosing and preparing an injection site

  1. Choose an injection site - abdomen, thighs, or upper arms. Do not inject at the exact same spot each time (rotate injection sites). Do not inject into areas where the skin is tender, bruised, red or hard. Avoid areas with scars or stretch marks. 

  2. Clean the injection site with an alcohol swab and let it air-dry. Do not touch this area again before giving the injection

Step 2: Preparing the injection for pre-filled syringe

  1. Find a quiet place. Choose a flat, clean, well-lit working surface.

  2. Wash your hands with soap and warm water before preparing for the injection.

  3. Look at the pre-filled syringe of methylnaltrexone. Make sure that the dose prescribed by your healthcare provider matches the dose on the pre-filled syringe label. Look at the plunger rod of the syringe. If the dose prescribed by your healthcare provider is 8 mg, the plunger rod will be yellow; if the prescribed dose is 12 mg, the plunger rod of the syringe will be purple.

  4. The liquid in the pre-filled syringe should be clear and colorless to pale yellow, and should not have any particles in it. If not, do not use the pre-filled syringe, and call your healthcare provider.

  5. Firmly hold the barrel of the pre-filled syringe and pull the needle cap straight off. Do not touch the needle or allow it to touch any surface.

Step 3: Injecting methylnaltrexone

  1. Pinch the skin around the injection site 

  2. Insert the full length of the needle into the skin at a 45-degree angle with a quick “dart-like” motion. Let go of skin and slowly push down on the plunger with your thumb until the pre-filled syringe is empty. This will release the needle safety device.

  3. Continue to hold pressure on the plunger with your thumb and quickly pull the needle out of the skin. Be careful to keep the needle at the same angle as it was inserted. Then remove your thumb from the plunger to allow the protective sleeve to cover the needle. There may be a little bleeding at the injection site.

  4. Hold a cotton ball or gauze over the injection site. Do not rub the injection site. Apply an adhesive bandage to the injection site if needed.

Step 4: Disposing of supplies

  • Do not re-use the pre-filled syringe or recap the needle. Place used pre-filled syringe in a closeable, puncture-resistant container. You may use a sharps container (such as a red biohazard container), a hard plastic container (such as a detergent bottle), or metal container (such as an empty coffee can). Ask your healthcare provider for instructions on the right way to throw away (dispose of) the container. There may be state and local laws about how you should throw away used needles and syringes.
  • Avoid touching the trigger fingers of the methylnaltrexone pre-filled syringe to keep from activating the safety device too soon. The safety device (needle guard) is activated by pressure from the plunger on the trigger fingers.
  • If you have any questions, talk to your healthcare provider or pharmacist.

Methylnaltrexone Dosage

Take methylnaltrexone exactly as prescribed by your doctor. Follow the directions on your prescription label carefully.

The methylnaltrexone dose your doctor recommends will be based on the following:

  • other medical conditions you have
  • other medications you are taking
  • how you respond to this medication
  • your liver function
  • your weight

The recommended dose of Relistor (methylnaltrexone) tablets is 450 mg once daily.

The recommended dose of Relistor (methylnaltrexone) injection is 8 mg subcutaneously for opioid-induced constipation and advanced illness adult patients weighing 38 kg to less than 62 kg or 12 mg subcutaneously for patients weighing 62 kg to 114 kg. 

Adult patients whose weight falls outside of these ranges should be dosed at 0.15 mg/kg.

Methylnaltrexone Overdose

If you take more methylnaltrexone than prescribed, talk to your healthcare provider right away or seek emergency medical attention.


Other Requirements


  • Store methylnaltrexone tablets at room temperature between 68°F to 77°F (20°C to 25°C).
  • The bottle of methylnaltrexone tablets contains 2 desiccant canisters to help keep your medicine dry. Do not remove the desiccant canisters from the bottle.


  • Store methylnaltrexone vials or pre-filled syringes at 68 to 77°F (20 to 25°C).
  • Do not freeze methylnaltrexone.
  • Keep methylnaltrexone away from light until you are ready to use it.
  • If methylnaltrexone has been drawn into a syringe and you are unable to use the medicine right away, keep the syringe at room temperature for up to 24 hours. The syringe does not need to be kept away from light during the 24-hour period.

Keep methylnaltrexone and all medicines, needles and syringes out of the reach of children.