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Medically reviewed by Last updated on Aug 19, 2020.


(meth il nal TREKS one)

Index Terms

  • Methylnaltrexone Bromide
  • N-methylnaltrexone Bromide

Dosage Forms

Excipient information presented when available (limited, particularly for generics); consult specific product labeling.

Solution, Subcutaneous:

Relistor: 8 mg/0.4 mL (0.4 mL); 12 mg/0.6 mL (0.6 mL) [contains edetate (edta) calcium disodium]

Tablet, Oral, as bromide:

Relistor: 150 mg [contains edetate (edta) calcium disodium]

Brand Names: U.S.

  • Relistor

Pharmacologic Category

  • Gastrointestinal Agent, Miscellaneous
  • Opioid Antagonist, Peripherally-Acting


An opioid receptor antagonist which blocks opioid binding at the mu receptor, methylnaltrexone is a quaternary derivative of naltrexone with restricted ability to cross the blood-brain barrier. It therefore functions as a peripheral acting opioid antagonist, including actions on the gastrointestinal tract to inhibit opioid-induced decreased gastrointestinal motility and delay in gastrointestinal transit time, thereby decreasing opioid-induced constipation. Does not affect opioid analgesic effects.


Vdss: ~1.1 L/kg


Metabolized to methyl-6-naltrexol isomers, methylnaltrexone sulfate, and other minor metabolites


Urine (~44% to 54%, primarily as unchanged drug); feces (~17%, primarily as unchanged drug)

Time to Peak

SubQ: 30 minutes; Oral: ~1.5 hours (delayed by 2 hours with high fat meal)

Half-Life Elimination

Terminal: ~15 hours (oral)

Protein Binding

11% to 15%

Special Populations: Renal Function Impairment

A single SubQ dose in patients with varying degrees of renal impairment resulted in a 1.3- to 1.9-fold higher AUC0-∞ of methylnaltrexone.

Special Populations: Hepatic Function Impairment

A single oral dose in patients with mild, moderate, and severe hepatic impairment resulted in a 1.7-, 4.8-and 3.8-fold higher Cmax of methylnaltrexone, respectively. AUC0-∞, increased ~2.1-fold in moderate and severe hepatic impairment.

Special Populations: Elderly

In elderly subjects (mean age 72 years old), mean clearance was about 20% lower (56 L/h versus 70 L/h) and AUC was 26% higher.

Use: Labeled Indications

Opioid-induced constipation with advanced illness (injection only): Treatment of opioid-induced constipation in adults with advanced illness or pain caused by active cancer who require opioid dosage escalation for palliative care.

Opioid-induced constipation with chronic non-cancer pain (tablets and injection): Treatment of opioid-induced constipation in adults with chronic non-cancer pain, including patients with chronic pain related to prior cancer or its treatment who do not require frequent (eg, weekly) opioid dosage escalation.


GI obstruction (known or suspected); patients at increased risk of recurrent GI obstruction.

Canadian labeling: Additional contraindications (not in US labeling): Hypersensitivity to methylnaltrexone or any component of the formulation

Dosing: Adult

Opioid-induced constipation with advanced illness: SubQ: Dosing is according to body weight: Administer 1 dose every other day as needed; maximum: 1 dose/24 hours

<38 kg: 0.15 mg/kg (round dose up to nearest 0.1 mL of volume)

38 to <62 kg: 8 mg

62 to 114 kg: 12 mg

>114 kg: 0.15 mg/kg (round dose up to nearest 0.1 mL of volume)

Opioid-induced constipation with chronic non-cancer pain: Note: Discontinue all maintenance laxatives prior to initiation of therapy; if response is not optimal after 3 days, laxative therapy may be reinitiated.

Oral: 450 mg once daily

SubQ: 12 mg once daily

Dosage adjustment for concomitant therapy: Significant drug interactions exist, requiring dose/frequency adjustment or avoidance. Consult drug interactions database for more information.

Dosing: Geriatric

Refer to adult dosing.


SubQ: Administer by subcutaneous injection into the upper arm, abdomen, or thigh. Rotate injection sites with each dose.

Tablet: Oral: Administer with water on an empty stomach at least 30 minutes before the first meal of the day.


Injection: Store intact vials and prefilled syringes between 20°C and 25°C (68°F and 77°F); excursions are permitted between 15°C and 30°C (59°F and 86°F). Do not freeze. Protect from light. Solution withdrawn from the single use vial is stable in a syringe for 24 hours at room temperature. Do not remove the prefilled syringe from the tray until ready to administer.

Tablets: Store at 25°C (77°F); excursions permitted to 15°C to 30°C (59°F to 86°F).

Drug Interactions

Naldemedine: Opioid Antagonists may enhance the adverse/toxic effect of Naldemedine. Specifically, the risk for opioid withdrawal may be increased. Avoid combination

Naloxegol: Opioid Antagonists may enhance the adverse/toxic effect of Naloxegol. Specifically, the risk for opioid withdrawal may be increased. Avoid combination

Opioid Antagonists: Methylnaltrexone may enhance the adverse/toxic effect of Opioid Antagonists. Specifically, the risk for opioid withdrawal may be increased. Avoid combination

Adverse Reactions

The following adverse drug reactions and incidences are derived from product labeling unless otherwise specified.

>10%: Gastrointestinal: Abdominal pain (14% to 29%), flatulence (13%), nausea (9% to 12%)

1% to 10%:

Central nervous system: Dizziness (7%), headache (4%), anxiety (2%), chills (1%)

Dermatologic: Hyperhidrosis (3% to 6%)

Endocrine & metabolic: Hot flash (3%)

Gastrointestinal: Diarrhea (5% to 6%), abdominal distention (4%), vomiting (2%)

Neuromuscular & skeletal: Muscle spasm (2%), tremor (1%)

Respiratory: Rhinorrhea (2%)

<1%, postmarketing, and/or case reports: Abdominal cramps, diaphoresis, flushing, gastrointestinal perforation, increased body temperature (Thomas 2008), malaise, opioid withdrawal syndrome, pain, syncope (Portenoy 2008)


Concerns related to adverse effects:

• Diarrhea: Discontinue treatment for severe or persistent diarrhea.

• Gastrointestinal perforation: Gastrointestinal perforations have been reported in patients with advanced illnesses associated with impaired structural integrity of the GI wall (eg, Ogilvie’s syndrome, peptic ulcer disease, diverticular disease, infiltrative GI tract malignancies, or peritoneal metastases). Use with caution in these patients or in patients with other conditions that may result in impaired integrity of the GI wall (eg, Crohn disease); Monitor for development of severe, persistent or worsening abdominal pain; discontinue therapy if this occurs. Use is contraindicated in patients with known or suspected GI obstruction or in patients at increased risk of recurrent GI obstruction.

• Opioid withdrawal: May precipitate symptoms of opioid withdrawal (eg, abdominal pain, anxiety, chills, diarrhea, hyperhidrosis, and yawning). Use with caution in patients with disruptions to the blood-brain barrier; may increase the risk for withdrawal and/or reduced analgesia. Monitor for symptoms of opioid withdrawal in such patients.

Disease-related concerns:

• Hepatic impairment: Use with caution in patients with hepatic impairment; dosage adjustments recommended for moderate to severe hepatic impairment.

• Renal impairment: Use with caution in patients with renal impairment; dosage adjustment recommended for moderate to severe renal impairment (CrCl <60 mL/minute).

Other warnings/precautions:

• Appropriate use: Use of injection beyond 4 months has not been studied. Discontinue methylnaltrexone if opioids are discontinued.

• Opioid-induced constipation with chronic non-cancer pain: Appropriate use: Efficacy has been established in patients who have taken opioids for ≥4 weeks; sustained exposure to opioids prior to initiation of methylnaltrexone may increase sensitivity to effects. All laxative maintenance therapy should be discontinued prior to initiation of therapy; laxative therapy may be added if a suboptimal response to therapy is noted after 3 days. When the opioid regimen has been changed, the patient should be re-evaluated for the need to continue methylnaltrexone therapy.

Monitoring Parameters

Severe, persistent, or worsening abdominal pain; symptoms of opioid withdrawal; adequate analgesia.

Pregnancy Considerations

Adverse events have not been observed in animal reproduction studies. Maternal use of methylnaltrexone during pregnancy may precipitate opioid withdrawal effects in newborn.

Patient Education

What is this drug used for?

• It is used to treat constipation caused by some pain drugs.

All drugs may cause side effects. However, many people have no side effects or only have minor side effects. Call your doctor or get medical help if any of these side effects or any other side effects bother you or do not go away:

• Sensation of fullness

• Headache

• Muscle spasm

• Runny nose

• Anxiety

• Dizziness

• Sweating a lot

• Flushing

• Passing gas

WARNING/CAUTION: Even though it may be rare, some people may have very bad and sometimes deadly side effects when taking a drug. Tell your doctor or get medical help right away if you have any of the following signs or symptoms that may be related to a very bad side effect:

• Severe diarrhea

• Persistent diarrhea

• Chills

• Severe abdominal pain

• Severe abdominal swelling

• Tremors

• Vomiting blood

• Nausea

• Vomiting

• Black, tarry, or bloody stools

• Signs of an allergic reaction, like rash; hives; itching; red, swollen, blistered, or peeling skin with or without fever; wheezing; tightness in the chest or throat; trouble breathing, swallowing, or talking; unusual hoarseness; or swelling of the mouth, face, lips, tongue, or throat.

Note: This is not a comprehensive list of all side effects. Talk to your doctor if you have questions.

Consumer Information Use and Disclaimer: This information should not be used to decide whether or not to take this medicine or any other medicine. Only the healthcare provider has the knowledge and training to decide which medicines are right for a specific patient. This information does not endorse any medicine as safe, effective, or approved for treating any patient or health condition. This is only a limited summary of general information about the medicine's uses from the patient education leaflet and is not intended to be comprehensive. This limited summary does NOT include all information available about the possible uses, directions, warnings, precautions, interactions, adverse effects, or risks that may apply to this medicine. This information is not intended to provide medical advice, diagnosis or treatment and does not replace information you receive from the healthcare provider. For a more detailed summary of information about the risks and benefits of using this medicine, please speak with your healthcare provider and review the entire patient education leaflet.

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.