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# 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

- Class: [Laxatives](https://durgical.com/en/pharmacology/classes/laxatives/)
- Receptor: μ-opioid receptors of the gut (peripheral)
- ATC code: A06AH01

Also called: Methylnaltrexone bromide

◎**μ-opioid receptors of the gut (peripheral)**Methylnaltrexone

ⓘ 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.

**Target in 3D:** μ-opioid receptors of the gut (peripheral). PDB structure 9PXU (RCSB PDB, public domain). [View the structure at RCSB PDB](https://www.rcsb.org/structure/9PXU) · Drag to rotate · pinch to zoom

## 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

### Serious (rare, important)

- Gastrointestinal perforation in patients with underlying bowel disease
- Severe diarrhoea or severe abdominal pain
- Opioid withdrawal symptoms (sweating, chills, anxiety, yawning, tremor)

## 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.

## Memory aid

METHYL = «Methyl stays in the hall»: the charged molecule guards the gut door but never enters the brain.

## 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.

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## Related drugs

- [Naloxone](https://durgical.com/en/pharmacology/naloxone/)
- [Morphine](https://durgical.com/en/pharmacology/morphine/)
- [Lubiprostone](https://durgical.com/en/pharmacology/lubiprostone/)

[← Back to the class: Laxatives](https://durgical.com/en/pharmacology/classes/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](https://medlineplus.gov/druginfo/meds/a608052.html) (MedlinePlus (U.S. National Library of Medicine), 2018); [Methylnaltrexone: MedlinePlus Drug Information](https://medlineplus.gov/druginfo/meds/a616047.html) (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](https://www.rcsb.org/structure/9PXU) (RCSB Protein Data Bank, 2026).

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Representative model, for educational purposes only.

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