Methylnaltrexone !
An opioid blocker that stays outside the brain: it reverses opioid constipation in the gut without taking away pain relief.
Updated 2026-11-01
Also called: Methylnaltrexone bromide
The figure shows naloxone, which occupies the same binding site, in the inactive human μ-opioid receptor; the structure also holds a nanobody, which is hidden.
Mechanism of action
Methylnaltrexone is a μ-opioid receptor antagonist with a quaternary amine. The permanent charge keeps it from crossing the blood-brain barrier easily, so it blocks opioid receptors in the gut wall but not in the brain. The opioid-induced slowing of the bowel and the extra water absorption are reversed while central analgesia remains.
- Quaternary amine: barely crosses the blood-brain barrier
- Blocks μ-opioid receptors in the gut wall
- Reverses opioid slowing of the bowel; analgesia stays
Pharmacokinetics
- Distribution
- Poor entry into the central nervous system because of its quaternary charge
- Onset and duration
- A bowel movement often follows within minutes to a few hours of an injection
Indications
- Opioid-induced constipation in adults with chronic non-cancer pain
- Opioid-induced constipation in advanced illness or active cancer pain (palliative care)
Adverse effects
Common
- Abdominal pain
- Flatulence
- Nausea and vomiting
- Diarrhoea
- Dizziness and sweating
Contraindications
- Known or suspected gastrointestinal obstruction
Key interactions
- Opioid analgesics
- It blocks their gut effects without reversing central analgesia; it is stopped when the opioid is stopped
- Other laxatives
- Usually stopped when methylnaltrexone is started, and resumed if it does not work
What is monitored
- Bowel movements and diarrhoea
- Abdominal pain (possible perforation)
- Signs of opioid withdrawal
- Pain control
Administration and safety
- Injected subcutaneously into the abdomen, thigh or upper arm; sites are rotated and tender, bruised, scarred or hard skin is avoided.
- Prefilled syringes and vials are single use and go into a puncture-resistant container.
- An oral tablet form is also available in some countries.
Nursing care
- Ask about bowel obstruction, peptic ulcer, diverticulitis, Crohn's disease or gastrointestinal cancer before the first dose.
- Make sure the patient is close to a toilet after the injection.
- Report severe or persistent abdominal pain or diarrhoea at once.
- Check that pain remains controlled and watch for withdrawal symptoms.
Pregnancy and breastfeeding
Use in pregnancy may cause opioid withdrawal in the fetus or newborn; breastfeeding is avoided during treatment.
Test yourself
Think of the answer, then tap the question to see it.
Why does methylnaltrexone not reverse opioid analgesia?
Its quaternary amine barely crosses the blood-brain barrier, so it blocks μ receptors only in the gut.
What serious complication can occur in patients with underlying bowel disease?
Gastrointestinal perforation.
What is its main contraindication?
Known or suspected gastrointestinal obstruction.
Related drugs
← Back to the class: Laxatives
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: Methylnaltrexone Injection: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2018); Methylnaltrexone: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2018); Farmacología básica y clínica (Katzung), 16.ª ed., cap. 62 (McGraw Hill, 2025); RCSB PDB - 9PXU: Inactive-state naloxone-mu opioid receptor nanobody6 complex (RCSB Protein Data Bank, 2026).
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Representative model, for educational purposes only.