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(me-thil-nal-trex-one) ,


(trade name)


Therapeutic: laxatives
Pharmacologic: opioid antagonists
Pregnancy Category: B


Treatment of constipation caused by opioid use in patients being treated palliatively, when laxative therapy has failed.


Acts peripherally as mu-opioid receptor antagonist, blocking opioid effects on the GI tract.

Therapeutic effects

Blocks constipating effects of opioids on the GI tract without loss of analgesia.


Absorption: Rapidly absorbed after subcutaneous administration.
Distribution: Moderate tissue distribution, does not cross the blood-brain barrier.
Metabolism and Excretion: Some metabolism; 85% excreted unchanged in urine.
Half-life: 8 hr.

Time/action profile

Subcutrapid0.5 hr24–48 hr


Contraindicated in: Known/suspected mechanical GI obstruction.
Use Cautiously in: Known/suspected lesions of GI tract (↑ risk for GI perforation); Obstetric / Lactation: Use in pregnancy only if clearly needed; use cautiously during lactation; Pediatric: Safety and efficacy not established.

Adverse Reactions/Side Effects

Central nervous system

  • dizziness


  • GI perforation (life-threatening)
  • abdominal pain (most frequent)
  • flatulence (most frequent)
  • nausea (most frequent)
  • diarrhea


  • hyperhidrosis


Drug-Drug interaction

None noted.


Subcutaneous (Adults) 38–<62 kg—8 mg every other day, not to exceed every 24 hr; 62–114 kg—12 mg every other day, not to exceed every 24 hr; other weights—0.15 mg/kg every other day, not to exceed every 24 hr.

Renal Impairment

(Adults) CCr <30 mL/min—use 50% of recommended dose based on weight.


Solution for subcutaneous injection (single use vial): 12 mg/0.6 mL Cost: $500.22 / 7 vials
Solution for subcutaneous injection (prefilled syringes): 8 mg/0.4 mL, 12 mg/0.6 mL Cost: All strengths $71.46 / syringe

Nursing implications

Nursing assessment

  • Assess bowel sounds and frequency, quantity, and consistency of stools periodically during therapy.
  • Monitor pain intensity during therapy. Methylnaltrexone does not affect pain or effects of opioid analgesics on pain control.

Potential Nursing Diagnoses

Constipation (Indications)
Diarrhea (Adverse Reactions)


  • Subcutaneous: Pinch skin and administer in upper arm, abdomen, or thigh at a 45° angle using a 1-mL syringe with a 27-gauge needle inserted the full length of the needle. Do not rub the injection site. Solution is clear and colorless to pale yellow. Do not administer solutions that are discolored or contain a precipitate. Solution is stable for 24 hr at room temperature. Protect vials from light. Do not freeze. Do not use single-use vials for more than 1 dose.

Patient/Family Teaching

  • Instruct patient on administration of methylnaltrexone and disposal of supplies. Usual schedule is one dose every other day, as needed, but no more than one dose in a 24-hr period. Advise patient to read the Patient Information prior to starting therapy and with each Rx refill.
  • Advise patient that laxation may occur within 30 min, so toilet facilities should be available following administration.
  • Advise patient to continue taking other medications for constipation unless told not to by health care professional.
  • May cause dizziness. Caution patient to avoid driving and other activities requiring alertness until response to medication is known.
  • Advise patient to notify health care professional and discontinue therapy if severe or persistent diarrhea occurs or if abdominal pain, nausea, or vomiting persists or worsens.
  • Instruct patient to stop taking methylnaltrexone if they stop taking opioid medications.
  • Advise patient to consult health care professional prior to taking other Rx, OTC, or herbal products.
  • Advise female patients to notify health care professional if pregnancy is planned or suspected or if breast feeding.

Evaluation/Desired Outcomes

  • Laxation and relief of opioid-induced constipation.


A proprietary quaternary amine and mu opioid receptor antagonist, which reverses opioid-induced constipation, without impacting on central opioid analgesia.

Patients in palliative care for whom usual laxatives have failed.

Adverse effect
Abdominal pain, flatulence.
References in periodicals archive ?
Oral methylnaltrexone is efficacious and well tolerated for the treatment of opioid-induced constipation in patients with chronic noncancer pain taking concomitant methadone.
The peripherally acting [mu]-opioid receptor antagonists methylnaltrexone and naloxegol and the locally acting chloride channel activator lubiprostone have been shown to be effective for many--but not all--patients with acceptable safety, and are FDA-approved for opioid-induced constipation.
14] in which they examined participants WTP in Methylnaltrexone in the Treatment of Opioid-Induced Constipation in Cancer Patients Receiving Palliative Care.
The researchers also found that methylnaltrexone (Relistor)--developed to treat opiate-induced constipation--reduced the proliferation of cancer cells in normal mice by 90 percent, suggesting a potential new use for the drug.
The discovery may mean that in future cancer patients will be given methylnaltrexone (MNTX), which was developed in the 1980s to prevent morphine causing constipation.
Moss's palliative-care patients were taking methylnaltrexone (MNTX), developed in the 1980s for opiate-induced constipation by the late University of Chicago pharmacologist Leon Goldberg.
Chicago University's Dr Patrick Singleton found another drug, methylnaltrexone, can be used to block the bad effects while allowing morphine to fight pain.
The drug developer and its partner Wyeth said their constipation drug candidate, known as methylnaltrexone, failed a late-stage clinical trial.
The company, in collaboration with Wyeth, is developing methylnaltrexone for the treatment of opioid-induced side effects, including constipation (oral and subcutaneous formulations) and post-operative ileus (intravenous formulation).
DALLAS -- Backed by two positive phase III randomized trials, methylnaltrexone is now under Food and Drug Administration review for treatment of opiold-induced constipation in patients with advanced illness.
and Wyeth Pharmaceuticals, a division of Wyeth, today announced the initiation of the first of two phase 3 clinical trials to evaluate the safety and efficacy of intravenous methylnaltrexone for the treatment of post-operative ileus (POI).
NASDAQ:SLXP) today announced that five presentations describing the investigation of methylnaltrexone bromide as a potential treatment for opioid-induced constipation in adult patients with chronic, non-cancer pain will occur during The National Conference on Pain for Frontline Practitioners, or "PAINWeek".