Methylnaltrexone
Relistor
Peripherally-acting μ-opioid receptor antagonist (PAMORA)
Quaternary derivative of naltrexone — does NOT cross the blood-brain barrier. Reverses peripheral opioid effects (constipation, urinary retention, pruritus) without antagonizing central analgesia.
Indications
- •Opioid-induced constipation refractory to laxatives, in advanced illness or chronic non-cancer pain
- •Persistent opioid-induced pruritus (off-label)
Dosing
| Context | Adult | Pediatric |
|---|---|---|
| OIC subcut | 0.15 mg/kg SC every other day (≈ 8–12 mg per dose for typical adult) | — |
| OIC oral | 450 mg PO daily | — |
Pharmacokinetics
SC onset 30 min for laxation. Half-life 8 h. Renal elimination.
Hemodynamic effects
None directly.
Respiratory effects
None — no central effect.
Side effects
- !Abdominal pain, cramping, flatulence (laxation effect)
- !GI perforation — black-box warning in patients with bowel-wall pathology (cancer, peritoneal metastases, diverticular disease, ulceration)
- !Diarrhea (excessive response)
Contraindications
- ×Known or suspected GI obstruction
- ×GI perforation risk (mechanical, anatomical, peritoneal cancer)
- ×Severe hepatic impairment
Clinical pearls
- ★ONSET LAXATION: 30–60 min SC — fast enough to be useful in inpatient setting where laxatives aren't working.
- ★CENTRAL SAFETY: does NOT precipitate analgesic loss or withdrawal — give it without worrying about pain return.
- ★RENAL DOSING: CrCl < 60 → halve the SC dose.
- ★PERIOPERATIVE NICHE: rarely used in OR but worth knowing for ICU / palliative-care consults handed to anesthesia.
📊 Related teaching panels
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Other drugs in Narcotic Reversals
- Naloxone
Competitive antagonist at mu (μ), kappa, and delta opioid receptors with highest affinity for μ. Reverses respiratory depression, sedation, analgesia, miosis, pruritus, GI hypomotility from exogenous opioids.
- Naltrexone
Competitive antagonist at μ, κ, δ opioid receptors. Long half-life makes it useful for opioid use disorder maintenance — but poses major perioperative challenges if patients with chronic naltrexone need opioid analgesia.
Browse all classes: /reference/drugs