Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
ABSTRAL vs RELISTOR
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
Fentanyl is a potent mu-opioid receptor agonist, producing analgesia and sedation by activating G-protein coupled opioid receptors in the central nervous system.
Peripherally acting mu-opioid receptor antagonist that blocks opioid-induced constipation without affecting central analgesia.
Management of breakthrough pain in cancer patients aged 18 and older who are already receiving and tolerant to around-the-clock opioid therapy for their underlying persistent cancer pain.
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
For breakthrough pain in opioid-tolerant patients: initial dose 100 mcg sublingual tablet, titrate across strengths (100, 200, 300, 400, 600, 800 mcg) as needed; maximum 2 doses per episode, minimum 2 hours between episodes.
0.15 mg/kg subcutaneously once daily, maximum 16 mg per dose; for opioid-induced constipation, 8 mg subcutaneously once daily.
Terminal elimination half-life: 6-10 hours (mean 8 hours); prolonged in elderly and hepatic impairment
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).
Hepatic metabolism primarily via CYP3A4; major metabolites include norfentanyl (inactive) and other minor metabolites.
Primarily hepatic via CYP3A4 and CYP2D6 isoenzymes; also undergoes gut wall metabolism.
Renal: ~70% as metabolites (primarily fentanyl conjugates and norfentanyl), ~10% unchanged; Fecal: ~9%; Biliary: minimal
Renal excretion of unchanged drug accounts for approximately 16% of the dose; biliary/fecal excretion is the major route (approximately 54% recovered in feces).
80-85% bound primarily to albumin and alpha-1-acid glycoprotein
Approximately 11-15% bound to plasma proteins (primarily albumin).
4-6 L/kg; large Vd indicates extensive tissue distribution
Approximately 1.1 L/kg (central volume ~0.3 L/kg); indicates extensive extravascular distribution.
Sublingual: 70-90% (mean 80%); buccal: 50-65%; oral: ~30% due to first-pass metabolism
Subcutaneous: approximately 82-100% (mean ~97%); oral: approximately 6% (low due to first-pass metabolism).
No specific GFR-based dose adjustment recommended; use caution in severe renal impairment (Cr Cl <30 m L/min) due to potential accumulation of fentanyl.
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.
For Child-Pugh Class A or B: no adjustment required; for Child-Pugh Class C: reduce dose and monitor closely for toxicity due to reduced clearance.
No dose adjustment required for mild to moderate hepatic impairment (Child-Pugh A or B); not studied in severe impairment (Child-Pugh C).
Not approved for pediatric patients <18 years; safety and efficacy not established.
Safety and efficacy not established in pediatric patients.
Initiate at the lowest available dose (100 mcg) and titrate cautiously; elderly patients may have altered pharmacokinetics and increased sensitivity to fentanyl.
No specific dose adjustment recommended; use caution due to potential for renal impairment, monitor renal function.
Risk of respiratory depression, addiction, abuse, and misuse; risk of accidental ingestion; risk of medication errors resulting in fatal overdose; life-threatening respiratory depression in opioid-non-tolerant patients; risk of opioid analgesic drug interactions with CNS depressants; risk of neonatal opioid withdrawal syndrome with prolonged use during pregnancy.
Gastrointestinal perforation: Cases of gastrointestinal perforation have been reported in patients with conditions that may result in impaired structural integrity of the gastrointestinal tract.
Respiratory depression, QT prolongation, serotonin syndrome, adrenal insufficiency, severe hypotension, seizures, biliary tract disease, gastrointestinal obstruction, withdrawal syndrome, and risk of overdose with alcohol or other CNS depressants.
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
Hypersensitivity to fentanyl or any components; opioid-non-tolerant patients; acute or severe bronchial asthma; known or suspected gastrointestinal obstruction; concurrent use of MAOIs or within 14 days of discontinuation.
Known or suspected mechanical gastrointestinal obstruction,Known hypersensitivity to methylnaltrexone or any component of the formulation
Avoid grapefruit and grapefruit juice during treatment as they inhibit CYP3A4, increasing fentanyl exposure. No other significant food interactions; however, avoid alcohol due to additive CNS depressant effects. Maintain consistent meal timing relative to dosing to minimize variability.
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.
FDA Pregnancy Category C. First trimester: Inadequate human data; opioid analgesics are not associated with major malformations but may cause neural tube defects at high doses in animal studies. Second trimester: No specific malformation risk. Third trimester: Prolonged use can cause neonatal opioid withdrawal syndrome (NOWS) and respiratory depression at birth.
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.
Minimal excretion into breast milk; M/P ratio not reported. Fentanyl is poorly absorbed orally, making significant infant exposure unlikely. Monitor infant for sedation, respiratory depression, and poor feeding. Avoid use in breastfeeding mothers with opioid dependence or high doses.
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.
Pregnancy increases clearance and volume of distribution, potentially reducing drug levels. Dose adjustments may be needed: initiate with lower doses and titrate to effect; consider increasing frequency or using breakthrough doses. Monitor for inadequate analgesia. Avoid abrupt discontinuation; taper if stopping.
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.
ABSTRAL (fentanyl sublingual spray) is a transmucosal immediate-release fentanyl (TIRF) formulation indicated for breakthrough pain in opioid-tolerant patients. Due to high bioavailability (~70%) and rapid onset (peak plasma concentration at 15-30 minutes), initial titration must start with 100 mcg, with dose escalation based on efficacy and tolerability. Weight-based conversion from other fentanyl products is not valid; utilize the provided conversion table. Patients must have a rescue agent (e.g., naloxone) available. Concomitant use with CYP3A4 inhibitors (e.g., ketoconazole, ritonavir) or inducers (e.g., rifampin, carbamazepine) requires dose adjustment. Avoid use in opioid-naïve patients due to risk of respiratory depression.
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.
Use only for breakthrough cancer pain while on around-the-clock opioid therapy.,Do not switch from other fentanyl products based on dose; follow specific conversion instructions.,Spray entire dose into mouth; do not swallow or rinse for at least 10 minutes.,Store at room temperature, away from children and pets.,Dispose of unused units via drug take-back program or by flushing down toilet per FDA guidelines.,Never share this medication with others; death may occur.,Seek emergency if severe drowsiness, confusion, or slow breathing occurs.
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.
No interactions on record
No interactions on record
Explore head-to-head clinical comparisons of other medications in the same therapeutic classes.
Common clinical questions about ABSTRAL vs RELISTOR, answered by our medical review team.
ABSTRAL is a Opioid Analgesic that works by Fentanyl is a potent mu-opioid receptor agonist, producing analgesia and sedation by activating G-protein coupled opioid receptors in the central nervous system.. RELISTOR is a Peripheral Opioid Antagonist that works by Peripherally acting mu-opioid receptor antagonist that blocks opioid-induced constipation without affecting central analgesia.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between ABSTRAL and RELISTOR depend on the specific clinical indication. These are agents from distinct pharmacological classes and are not directly interchangeable by dose. A physician or clinical pharmacist should guide any therapeutic switching decisions.
The standard adult dose of ABSTRAL is: For breakthrough pain in opioid-tolerant patients: initial dose 100 mcg sublingual tablet, titrate across strengths (100, 200, 300, 400, 600, 800 mcg) as needed; maximum 2 doses per episode, minimum 2 hours between episodes.. 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.. 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 ABSTRAL and RELISTOR 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. ABSTRAL is classified as Category C. FDA Pregnancy Category C. First trimester: Inadequate human data; opioid analgesics are not associated with major malformations but may cause neural tube defects at high doses in a. 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. Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.