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Methylnaltrexone (Injection)

Methylnaltrexone (meth-il-nal-TREX-one)

Treats constipation caused by narcotic pain medicines.

Brand Name(s):


There may be other brand names for this medicine.

When This Medicine Should Not Be Used:

This medicine is not right for everyone. Do not use it if you had an allergic reaction to methylnaltrexone, or if you have a stomach or bowel blockage.

How to Use This Medicine:


  • Your doctor will prescribe your exact dose and tell you how often it should be given. This medicine is given as a shot under your skin.
  • This medicine should come with a Medication Guide. Ask your pharmacist for a copy if you do not have one.
  • You may be taught how to give your medicine at home. Make sure you understand all instructions before giving yourself an injection. Do not use more medicine or use it more often than your doctor tells you to.
  • You will be shown the body areas where this shot can be given. Use a different body area each time you give yourself a shot. Keep track of where you give each shot to make sure you rotate body areas.
  • Use a new needle and syringe each time you inject your medicine.
  • Stop using this medicine if you stop taking narcotic pain medicines.
  • Missed dose: Call your doctor or pharmacist for instructions.
  • If you store this medicine at home, keep it at room temperature, away from heat and direct light. Do not freeze.

Drugs and Foods to Avoid:

Ask your doctor or pharmacist before using any other medicine, including over-the-counter medicines, vitamins, and herbal products.

Warnings While Using This Medicine:

  • Tell your doctor if you are pregnant or breastfeeding, or if you have kidney disease or stomach or bowel problems (such as Crohn disease, Ogilvie syndrome, diverticulitis, or cancer).
  • This medicine may cause a tear in your stomach or bowels.
  • Stop taking other laxatives before you start treatment with this medicine.
  • Throw away used needles in a hard, closed container that the needles cannot poke through. Keep this container away from children and pets.
  • Keep all medicine out of the reach of children. Never share your medicine with anyone.

Possible Side Effects While Using This Medicine:

Call your doctor right away if you notice any of these side effects:

  • Allergic reaction: Itching or hives, swelling in your face or hands, swelling or tingling in your mouth or throat, chest tightness, trouble breathing
  • Chills, diarrhea, increased sweating, stomach pain, anxiety, irritability, yawning
  • Severe abdominal pain
  • Severe diarrhea

If you notice these less serious side effects, talk with your doctor:

  • Dizziness
  • Nausea, gas

If you notice other side effects that you think are caused by this medicine, tell your doctor

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

Last Updated: 10/4/2015

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