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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.
Drug Guide, © 2015 Farlex and Partners


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.
Segen's Medical Dictionary. © 2012 Farlex, Inc. All rights reserved.
References in periodicals archive ?
RELISTOR (methylnaltrexone bromide) tablets, for oral use and RELISTOR (methylnaltrexone bromide) injection, for subcutaneous use
District Court of New Jersey upheld the validity and determined Actavis' infringement of a patent protecting RELISTOR[R] (methylnaltrexone bromide) tablets, expiring March 2031.
Methylnaltrexone is positively charged and typically does not cross the blood-brain barrier into the CNS.
(31) Opioid antagonists modified to prevent entry into the CNS (eg methylnaltrexone, naloxegol) are second-line remedies.
Although at least five randomized, controlled trials have evaluated methylnaltrexone, the evidence was low quality and therefore the guideline only conditionally recommends prescribing this PAMORA over no treatment.
This mechanism involves the MOP ([mu]) receptor, since the administration of methylnaltrexone, which is a peripheral antagonist of the MOP receptor, reduces angiogenesis.
Methylnaltrexone (MNTX) is one of the latest agents of peripherally acting [mu]-opioid antagonists which cannot cross the hematoencephalic barrier and was considered to counteract the side effects of opioids:
Pharmaceutical company Valeant Pharmaceuticals International Inc (NYSE:VRX)(TSX:VRX) reported on Monday an increase in its sales force by nearly 40% to capture untapped primary care OIC market for its key products, Xifaxan (rifaximin) and RELISTOR (methylnaltrexone bromide) with immediate effect.
Drugs that block mu receptors such as methylnaltrexone can be used to block the emetic effects of opiates in humans and pica in rats [54].
Relistor (methylnaltrexone bromide) is indicated for treatment of opioid-induced constipation when response to laxative therapy has not been sufficient in adult patients, aged 18 years and older
Methylnaltrexone ([mu]-opioid antagonist) is another medicinal agent approved in the USA and indicated for the treatment of opioid-induced constipation, with approximately 60% of patients having reported beneficial intestinal effects [28, 128].