Peripheral μ-opioid receptor antagonist Pregnancy: There are no adequate data on the use of methylnaltrexone bromide in pregnant women; animal studies have shown reproductive toxicity at high doses and the potential risk for humans is unknown. It should NOT be used during pregnancy unless clearly necessary. It is unknown whether it is excreted in human breast milk (animal studies have shown excretion in breast milk) — a decision to continue or discontinue breast-feeding or therapy should take account of the benefit of breast-feeding to the child and of therapy to the woman. US labelling adds that use during pregnancy may precipitate opioid withdrawal in a fetus due to the immature fetal blood-brain barrier.

Methylnaltrexone bromide

Brand names: Relistor

Methylnaltrexone is a peripherally acting mu-opioid receptor antagonist used to treat opioid-induced constipation when laxatives have given an inadequate response.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: Opioid-induced constipation in adult patients with chronic pain (except palliative care patients with advanced illness): 12 mg (0.6 mL of solution) subcutaneously, as needed

Route: Subcutaneous injection — into the upper legs, abdomen or upper arms; rotate injection sites; may be injected without regard to food. Do not inject into skin that is tender, bruised, red or hard, or into areas with scars or stretch marks

Frequency: As needed, given as at least 4 doses weekly up to once daily (7 doses weekly)

Max: Once daily (7 doses weekly) in adults with chronic pain. In advanced illness the usual schedule is one single dose every other day; two consecutive doses 24 hours apart may be given ONLY when there has been no response (bowel movement) to the dose on the preceding day

SECOND INDICATION — opioid-induced constipation in adult patients with ADVANCED ILLNESS (palliative care): 8 mg (0.4 mL of solution) for patients weighing 38-61 kg, or 12 mg (0.6 mL of solution) for patients weighing 62-114 kg; the usual administration schedule is one single dose every other day, and doses may be given at longer intervals as per clinical need. Patients weighing less than 38 kg or more than 114 kg should use Relistor VIALS, not the pre-filled syringe, because the recommended mg/kg dose cannot be accurately delivered with the pre-filled syringe (the fetched UK SPC does not state that mg/kg figure; US labelling gives 0.15 mg/kg subcutaneously for adults with advanced illness weighing less than 38 kg or more than 114 kg — confirm from the Relistor vial SPC). LAXATIVES: in chronic-pain patients, usual laxative treatment should be STOPPED when commencing methylnaltrexone; in palliative care patients methylnaltrexone is ADDED to usual laxative treatment. Treatment in advanced illness has not been studied beyond 4 months and should only be used for a limited period. Elderly: no dose adjustment recommended based on age. Hepatic impairment: no dose adjustment in mild to moderate; NOT recommended in severe hepatic impairment (Child-Pugh Class C) as no data are available. Should not be used for constipation not related to opioid use. Bowel movement may occur rapidly (within 30 to 60 minutes on average). Fetched product: 'Relistor 12 mg Solution for injection in pre-filled syringes'. US labelling (openFDA) additionally describes ORAL Relistor tablets 450 mg once daily in the morning, taken with water on an empty stomach at least 30 minutes before the first meal, for OIC in adults with chronic non-cancer pain (150 mg once daily in moderate/severe renal impairment or moderate/severe hepatic impairment) — the oral tablet is NOT covered by the fetched UK SPC; verify UK availability and licensing before publishing an oral dose.

Dose adjustments

Renal

Severe renal impairment (creatinine clearance less than 30 mL/min): the dose should be reduced from 12 mg to 8 mg (0.4 mL of solution) for patients weighing 62 to 114 kg. Patients with severe renal impairment whose weight falls outside the 62 to 114 kg range need to reduce their mg/kg dose by 50% and should use Relistor vials, not the pre-filled syringe. There are no data in patients with end-stage renal impairment on dialysis and methylnaltrexone bromide is NOT recommended in these patients. (US labelling uses a different threshold — creatinine clearance less than 60 mL/min — and gives 6 mg subcutaneously once daily for chronic non-cancer pain; verify against the UK SPC.)

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

Hypersensitivity to the active substance or to any of the excipients
Known or suspected mechanical gastrointestinal obstruction, patients at increased risk for recurrent obstruction, or patients with acute surgical abdomen — due to the potential for gastrointestinal perforation

Side effects

Abdominal pain, nausea, diarrhoea, flatulence (very common) — generally mild or moderate
Vomiting and dizziness (common)
Opioid-withdrawal-like symptoms (common) — chills, tremor, rhinorrhoea, piloerection, hot flush, palpitation, hyperhidrosis, vomiting, abdominal pain
Injection site reactions (common) — stinging, burning, pain, redness, oedema
Gastrointestinal perforation (frequency not known) — reported post-authorisation in patients with conditions reducing gastrointestinal wall integrity; monitor for severe, persistent or worsening abdominal pain and discontinue if it occurs

Interactions

Other opioid antagonists — avoid concomitant use because of the potential for additive opioid receptor antagonism and increased risk of opioid withdrawal (US labelling §7.1)
Cytochrome P450 substrates — a subcutaneous dose of 0.3 mg/kg did not significantly affect the metabolism of dextromethorphan, a CYP2D6 substrate, in healthy subjects (US labelling §7.2)
NOTE: the UK SPC §4.5 interactions section was not captured in this bundle; the entries above are from the US labelling and should be confirmed against the UK SPC

Clinical monograph

How it works

As a quaternary amine it does not readily cross the blood-brain barrier, so it antagonises mu-opioid receptors in the gut wall to reverse opioid effects on bowel motility without compromising central analgesia.

Prescribing in practice

Contraindicated where mechanical gastrointestinal obstruction is known or suspected, owing to the risk of perforation.
It is given by subcutaneous injection in many indications, with oral and intravenous formulations available depending on the product.
Discontinue if severe or persistent diarrhoea occurs, and review if opioid therapy is stopped.

Monitoring

Monitor for bowel response, abdominal pain and the development of severe diarrhoea, and remain alert for signs of gastrointestinal perforation.

Counselling the patient

Tell patients the medicine often produces a bowel movement quite quickly and to stay near a toilet after a dose.
Advise patients to stop the medicine and seek urgent help if they develop severe, persistent abdominal pain.

Evidence & guidelines

Use of peripherally acting mu-opioid receptor antagonists for opioid-induced constipation is supported by NICE guidance and the relevant SPCs.

Reference: NICE TA651; SmPC; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

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