Section 19 · Monograph
Methylnaltrexone
Reviewed Nov 2026FDA label
RelistorPeripheral μ-opioid antagonist
- Forms
- SC: 8mg/0.4mL, 12mg/0.6mL | Tab: 150mg
- Dose
- Opioid-induced constipation: SC 8mg (38 to under 62kg) or 12mg (62-114kg) every other day; tablets 450mg once daily in chronic non-cancer pain
- MOA
- Blocks gut μ-opioid receptors without reversing central analgesia
Indications
- Opioid-induced constipation in adults with advanced illness receiving palliative care when response to laxatives is inadequate (SC)FDA-labeled
- Opioid-induced constipation in adults with chronic non-cancer pain (SC and tablets)FDA-labeled
Mechanism of Action
- Peripherally acting mu-opioid receptor antagonist that does not cross the blood-brain barrier, restoring gut motility without reversing central analgesia
Full Dosing
Adult / usual
- Advanced illness (SC): 8mg for 38 to under 62kg or 12mg for 62-114kg every other day as needed; 0.15mg/kg for other weights
- Chronic non-cancer pain: SC 12mg once daily; tablets 450mg (three 150mg tablets) once daily in the morning
Renal
- Halve the dose when CrCl is under 60mL/min (SC) or reduce per label for severe renal impairment; stop other maintenance laxatives before starting and add as needed
Contraindications
- Known or suspected mechanical gastrointestinal obstruction
- Hypersensitivity
Drug Interactions
- Other opioid antagonists: avoid
- Strong CYP3A inhibitors/inducers affect tablet exposure (see label)
Adverse Effects
- Abdominal pain, flatulence, nausea
- Diarrhea
- Gastrointestinal perforation (rare)
- Opioid withdrawal symptoms (rare)
Monitoring Parameters
- Bowel response and abdominal pain
- Signs of GI perforation or obstruction
- Renal function
Pregnancy & Lactation
- Limited data; may precipitate opioid withdrawal in the fetus
- Unknown whether present in milk; use caution