What is a Generic Drug?

A generic drug is a copy of the brand-name drug with the same dosage, safety, strength, quality, consumption method, performance, and intended use. Before generics become available on the market, the generic company must prove it has the same active ingredients as the brand-name drug and works in the same way and in the same amount of time in the body.

The only differences between generics and their brand-name counterparts is that generics are less expensive and may look slightly different (eg. different shape or color), as trademarks laws prevent a generic from looking exactly like the brand-name drug.

Generics are less expensive because generic manufacturers don't have to invest large sums of money to develop a drug. When the brand-name patent expires, generic companies can manufacture a copy of the brand-name and sell it at a substantial discount.

Relistor (Methylnaltrexone)


Relistor (Methylnaltrexone)

Prescription Required

12mg / 0.6ml (vial)

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  • Product Details


    Relistor (Methylnaltrexone) is a medication classified as an opioid antagonist. It is designed to counteract constipation caused by opioid pain relievers without significantly affecting pain relief or causing withdrawals. Relistor works by blocking the opioid receptors in the intestines, allowing people to go to the bathroom more easily.

    This drug is mainly used to treat constipation caused by opioid pain medications in adults with long-lasting pain that is not due to cancer. It is also appropriate for patients suffering from chronic pain caused by previous cancer treatments or the cancer itself, who do not need weekly increases in their opioid dosage.

    Fact Table




    US DailyMed, EU EMA, US FDA


    Low bioavailability

    Legal status


    Chemical Name


    Elimination half-life

    8 hours

    Dosage (Strength)

    12mg / 0.6ml (vial)


    Consult Doctor



    Protein binding


    PubChem CID





    Not Available

    ATC code





    Not Available

    Routes of administration



    The recommended dosage for Relistor injections is 12 mg under the skin (subcutaneously) once daily for adults experiencing constipation with chronic non-cancer pain. In adults with advanced illness and opioid-induced constipation, the recommended dosage is based on weight: For people who weigh less than 38 kg, the dosage is 0.15 mg/kg; In the case of people weighing between 38 kg and less than 62 kg, the prescribed dosage is 8 mg; For those with a weight of 62 kg to 114 kg, the recommended dose is 12 mg; and for people who weigh more than 114 kg, the dosage is 0.15 mg/kg per day.

    Remember to always follow the healthcare provider's directions and ask for any clarification needed. Before administering the injection, make sure the area is clean and free of any visible signs of infection. The medication can be injected into the thigh, upper arm, or stomach area.


    Relistor contains the main ingredient methylnaltrexone bromide.


    Avoid taking or using Relistor if any of the following apply:

    • Known hypersensitivity to methylnaltrexone or any of its components
    • Presence of known or suspected gastrointestinal obstruction or a history of bowel obstruction


    • Before taking Relistor, it is important to inform your doctor about:
      • Any medications you are currently taking, including prescription drugs, over-the-counter medicines, and supplements
      • Any medical conditions you have, particularly if you have a history of gastrointestinal issues, such as inflammation, obstruction, or perforation.
      • If you are pregnant or breastfeeding, or if you are planning to become pregnant or breastfeed
      • If you have liver or kidney problems
    • If you have known or suspected lesions in your gastrointestinal (GI) tract, weigh the risks and benefits of using Relistor with your healthcare provider. Should you experience severe, persistent, or worsening abdominal pain while on this medication, contact your healthcare provider and discontinue treatment immediately.
    • If you experience severe or persistent diarrhea while on Relistor, it is advised to discontinue the medication and consult your healthcare provider for further guidance.
    • People with problems with the blood-brain barrier should consider the overall risk and benefit of using Relistor. Be vigilant for symptoms of opioid withdrawal, such as anxiety, nausea, and irritability, and consult your healthcare provider if such symptoms arise.
    • Make sure you are within close proximity to toilet facilities once the medication has been administered.
    • Stop using Relistor if you also discontinue your opioid pain medication.
    • People who have been on opioid medication for less than 4 weeks may find Relistor less effective.
    • Discontinue all maintenance laxative treatments before initiating Relistor therapy. If there's a suboptimal response to Relistor after 3 days, maintenance laxative therapy may be resumed.
    • It is vital for patients using Relistor to be regularly monitored by a healthcare provider to ensure the medication's effectiveness and to monitor for potential adverse effects.

    Side Effects

    While Relistor can be effective in managing opioid-induced constipation, there are potential side effects that may occur during treatment. The most common side effects of Relistor include:

    • Pain in the stomach area
    • Diarrhea
    • Headache
    • Feeling of fullness or pressure in the abdomen
    • Sweating
    • Anxiety
    • Muscle spasms
    • Runny nose
    • Chills

    While these side effects are usually mild and often resolve on their own, consult your healthcare provider if they persist or become severe.

    In addition to these common symptoms, Relistor can cause serious side effects, such as:

    • Gastrointestinal Perforation: Severe stomach pain could indicate a serious medical condition like a tear in your stomach or intestinal wall. If you experience worsening or persistent severe stomach pain, discontinue Relistor and seek emergency medical help.
    • Persistent or Severe Diarrhea: If you experience severe or ongoing diarrhea, stop taking Relistor and consult your healthcare provider.
    • Opioid Withdrawal Symptoms: During treatment, you may experience symptoms of opioid withdrawal like sweating, chills, diarrhea, stomach pain, anxiety, and yawning. Inform your healthcare provider if you experience any of these symptoms.

    These are not all the potential side effects of Relistor. For any concerns about using this medication, consult your healthcare provider.


    Relistor (Methylnaltrexone) Prescribing Information. Bridgewater, NJ: Salix Pharmaceuticals; 2018.

    IMPORTANT NOTE: The above information is intended to increase awareness of health information and does not suggest treatment or diagnosis. This information is not a substitute for individual medical attention and should not be construed to indicate that use of the drug is safe, appropriate, or effective for you. See your health care professional for medical advice and treatment.

    Product Code : 13777

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