Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
RELISTOR vs ENTEREG
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
Peripherally acting mu-opioid receptor antagonist that blocks opioid-induced constipation without affecting central analgesia.
Selective 5-HT4 receptor agonist; enhances gastrointestinal motility by increasing peristalsis and accelerating colonic transit.
Treatment of opioid-induced constipation (OIC) in adult patients with chronic non-cancer pain,Treatment of OIC in adult patients with advanced illness who are receiving palliative care
FDA-approved for the treatment of chronic idiopathic constipation in adults
0.15 mg/kg subcutaneously once daily, maximum 16 mg per dose; for opioid-induced constipation, 8 mg subcutaneously once daily.
Adults: 12 mg orally twice daily for up to 15 days, initiated within 30 minutes prior to surgery and continued postoperatively.
Terminal elimination half-life is approximately 8-10 hours in patients with normal renal function. In patients with end-stage renal disease, half-life is prolonged (~14-18 hours).
Terminal half-life is approximately 10–17 hours in healthy subjects. Clinically, the half-life may be prolonged in severe hepatic impairment but is not significantly altered in renal impairment.
Primarily hepatic via CYP3A4 and CYP2D6 isoenzymes; also undergoes gut wall metabolism.
Primarily metabolized by cytochrome P450 3A4 (CYP3A4); also involves CYP2D6 and CYP2C9 to a lesser extent.
Renal excretion of unchanged drug accounts for approximately 16% of the dose; biliary/fecal excretion is the major route (approximately 54% recovered in feces).
Primarily hepatobiliary excretion; unchanged drug and major metabolite (alvimopan) undergo extensive biliary elimination with fecal excretion accounting for >90% of total elimination. Renal excretion is minimal (<5% as unchanged drug).
Approximately 11-15% bound to plasma proteins (primarily albumin).
Approximately 80–90% bound to plasma proteins, primarily albumin.
Approximately 1.1 L/kg (central volume ~0.3 L/kg); indicates extensive extravascular distribution.
Volume of distribution is about 30 L (approximately 0.4 L/kg), indicating distribution into extracellular fluid and tissues.
Subcutaneous: approximately 82-100% (mean ~97%); oral: approximately 6% (low due to first-pass metabolism).
Oral bioavailability is approximately 6–10% due to extensive first-pass metabolism; the drug is administered orally for local gastrointestinal activity.
For creatinine clearance <30 m L/min: 0.075 mg/kg subcutaneously every other day, maximum 8 mg per dose; not recommended in patients with end-stage renal disease requiring dialysis.
No dose adjustment required for mild to moderate renal impairment (Cr Cl ≥30 m L/min). Not recommended for severe renal impairment (Cr Cl <30 m L/min) or dialysis.
No dose adjustment required for mild to moderate hepatic impairment (Child-Pugh A or B); not studied in severe impairment (Child-Pugh C).
No dose adjustment for mild to moderate hepatic impairment (Child-Pugh A or B). Caution in severe hepatic impairment (Child-Pugh C); no specific dose recommendation.
Safety and efficacy not established in pediatric patients.
Not FDA-approved for pediatric patients; safety and efficacy not established.
No specific dose adjustment recommended; use caution due to potential for renal impairment, monitor renal function.
No specific dose adjustment; use with caution due to potential increased sensitivity and renal function decline. Monitor for adverse effects.
Gastrointestinal perforation: Cases of gastrointestinal perforation have been reported in patients with conditions that may result in impaired structural integrity of the gastrointestinal tract.
No FDA boxed warning.
Risk of gastrointestinal perforation,Opioid withdrawal symptoms including diarrhea, nausea, vomiting, abdominal pain,Disruption of analgesic effect if used with opioids crossing the blood-brain barrier (theoretical),Not recommended in patients with known or suspected mechanical gastrointestinal obstruction
May cause diarrhea, leading to electrolyte disturbances or hypovolemia,Use with caution in patients with severe renal impairment,Avoid use in patients with a history of mechanical gastrointestinal obstruction, perforation, or severe inflammatory bowel disease
Known or suspected mechanical gastrointestinal obstruction,Known hypersensitivity to methylnaltrexone or any component of the formulation
Hypersensitivity to prucalopride or any excipients,Renal impairment requiring dialysis,Intestinal obstruction or perforation
No specific food interactions reported with methylnaltrexone. No dietary restrictions necessary. However, to optimize bowel function, patients should maintain adequate fluid intake and dietary fiber as tolerated, unless contraindicated due to underlying illness.
No specific food interactions reported. However, as ENTEREG is administered in a hospital setting, patients should follow the prescribed diet (typically clear liquids advancing to regular diet as tolerated postoperatively). Avoid grapefruit juice as it may affect drug metabolism via CYP3A4 (though not specifically studied, caution is advised).
Animal studies show no teratogenic effects at doses up to 300 mg/kg/day in rats and rabbits. No adequate human data; risk cannot be excluded in first trimester. Second and third trimester: limited data, potential for gastrointestinal effects in fetus if exposed transplacentally.
No human data; animal studies at doses up to 10 mg/kg/day in rats and rabbits showed no teratogenicity at exposures lower than human dose; risk cannot be excluded due to lack of adequate human studies.
Excreted in human milk at low concentrations; M/P ratio approximately 0.6. No reported adverse effects in breastfeeding infants. Caution advised due to potential for gastrointestinal effects.
No data on presence in human milk; caution advised; M/P ratio unknown.
No pharmacokinetic studies in pregnancy; dose adjustments not recommended based on available data. Use only if clearly needed for severe opioid-induced constipation unresponsive to standard therapy.
No pharmacokinetic studies in pregnancy; dose adjustment not required based on available data.
Relistor (methylnaltrexone) is a peripherally acting mu-opioid receptor antagonist (PAMORA) used for opioid-induced constipation (OIC) in patients with advanced illness or chronic pain. It does not cross the blood-brain barrier, thus does not reverse central opioid analgesia. Administer subcutaneously; onset typically within 1-4 hours. Contraindicated in patients with known or suspected mechanical gastrointestinal obstruction. Use with caution in renal impairment (Cr Cl <30 m L/min) as dose reduction recommended. Monitor for gastrointestinal perforation, especially in patients with underlying GI pathology. Coadministration with other opioid antagonists may precipitate opioid withdrawal.
ENTEREG (alvimopan) is a peripherally acting mu-opioid receptor antagonist indicated to accelerate postoperative recovery of GI function after bowel resection surgery. It does not cross the blood-brain barrier, so it does not reverse opioid analgesia. Use is restricted to hospitalized patients; it should not be used for more than 7 days. Contraindicated in patients who have taken therapeutic doses of opioids for >7 consecutive days immediately prior to initiation, as it may precipitate opioid withdrawal. Monitor for GI adverse effects such as nausea, vomiting, and abdominal pain.
Relistor is used to treat constipation caused by opioid pain medications without affecting pain relief.,Inject the medication exactly as prescribed; do not use more often than every other day.,You should have a bowel movement within a few hours of receiving the injection; if not, contact your doctor.,Common side effects include abdominal pain, nausea, diarrhea, and flatulence.,Stop Relistor and seek immediate medical attention if you experience severe abdominal pain, vomiting, or signs of intestinal obstruction (e.g., inability to pass gas).,Tell your doctor if you have kidney problems, as the dose may need adjustment.,Do not take other medicines for constipation without your doctor's approval.
Take ENTEREG exactly as prescribed; do not take more than the recommended dose.,This medication is used only in the hospital after bowel surgery to help your bowels start working again.,It does not reduce pain or interfere with your pain medication.,Report any severe abdominal pain, nausea, vomiting, or diarrhea to your healthcare provider.,Do not take this medication if you have recently taken opioid pain medications for more than 7 days in a row.
No interactions on record
No interactions on record
Common clinical questions about RELISTOR vs ENTEREG, answered by our medical review team.
RELISTOR is a Peripheral Opioid Antagonist that works by Peripherally acting mu-opioid receptor antagonist that blocks opioid-induced constipation without affecting central analgesia.. ENTEREG is a Peripheral Opioid Antagonist that works by Selective 5-HT4 receptor agonist; enhances gastrointestinal motility by increasing peristalsis and accelerating colonic transit.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between RELISTOR and ENTEREG depend on the specific clinical indication. These are both Peripheral Opioid Antagonist agents and are not directly interchangeable by dose. A physician or clinical pharmacist should guide any therapeutic switching decisions.
The standard adult dose of RELISTOR is: 0.15 mg/kg subcutaneously once daily, maximum 16 mg per dose; for opioid-induced constipation, 8 mg subcutaneously once daily.. The standard adult dose of ENTEREG is: Adults: 12 mg orally twice daily for up to 15 days, initiated within 30 minutes prior to surgery and continued postoperatively.. Dosing should always be individualized based on indication, renal and hepatic function, age, and other patient factors.
No direct drug-drug interaction has been formally documented between RELISTOR and ENTEREG in current clinical databases. However, individual patient risk factors including other medications, organ function, and comorbidities should always be evaluated by a qualified healthcare provider.
The maternal-fetal safety profiles differ. RELISTOR is classified as Category C. Animal studies show no teratogenic effects at doses up to 300 mg/kg/day in rats and rabbits. No adequate human data; risk cannot be excluded in first trimester. Second and third tr. ENTEREG is classified as Category C. No human data; animal studies at doses up to 10 mg/kg/day in rats and rabbits showed no teratogenicity at exposures lower than human dose; risk cannot be excluded due to lack of ad. Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.