Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
METAXALONE vs ABSTRAL
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
Metaxalone is a centrally acting muscle relaxant whose exact mechanism is unknown. It is thought to produce skeletal muscle relaxation by depressing the central nervous system (CNS), possibly through general CNS depression or by blocking polysynaptic reflexes in the spinal cord.
Fentanyl is a potent mu-opioid receptor agonist, producing analgesia and sedation by activating G-protein coupled opioid receptors in the central nervous system.
FDA-approved: Adjunct to rest, physical therapy, and other measures for the relief of discomfort associated with acute, painful musculoskeletal conditions.,Off-label: Management of muscle spasms, spasticity.
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.
800 mg orally 3 to 4 times daily
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.
Terminal elimination half-life is approximately 0.5 to 1.5 hours, reflecting rapid clearance and supporting short-lived clinical effects.
Terminal elimination half-life: 6-10 hours (mean 8 hours); prolonged in elderly and hepatic impairment
Extensively metabolized in the liver via cytochrome P450 enzymes (CYP1A2, CYP2D6, CYP3A4, CYP2C19, and CYP2E1) to unidentified metabolites. Less than 1% excreted unchanged in urine.
Hepatic metabolism primarily via CYP3A4; major metabolites include norfentanyl (inactive) and other minor metabolites.
Primarily renal; approximately 90% of a dose is excreted in urine as glucuronide conjugates and unchanged drug, with less than 1% eliminated in feces via biliary excretion.
Renal: ~70% as metabolites (primarily fentanyl conjugates and norfentanyl), ~10% unchanged; Fecal: ~9%; Biliary: minimal
Approximately 98% bound to plasma proteins, primarily albumin.
80-85% bound primarily to albumin and alpha-1-acid glycoprotein
Approximately 0.3–0.5 L/kg, indicating moderate distribution into total body water and peripheral tissues.
4-6 L/kg; large Vd indicates extensive tissue distribution
Oral bioavailability is high, estimated at >80% based on urinary recovery studies.
Sublingual: 70-90% (mean 80%); buccal: 50-65%; oral: ~30% due to first-pass metabolism
No specific dose adjustment guidelines available; use with caution in severe renal impairment (Cr Cl <30 m L/min) due to potential for accumulation.
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.
No specific dose adjustment guidelines available; use with caution in severe hepatic impairment (Child-Pugh class C) as metabolism may be reduced.
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.
Safety and efficacy not established; not recommended for use in children under 12 years of age.
Not approved for pediatric patients <18 years; safety and efficacy not established.
Start at lower end of dosing range (e.g., 800 mg 3 times daily) due to increased sensitivity and risk of adverse effects; monitor closely.
Initiate at the lowest available dose (100 mcg) and titrate cautiously; elderly patients may have altered pharmacokinetics and increased sensitivity to fentanyl.
None.
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.
Serotonin syndrome risk when co-administered with serotonergic drugs (e.g., SSRIs, SNRIs, MAOIs).,Hepatic toxicity: rare reports of liver injury; use caution in patients with hepatic impairment.,CNS depressant effects: may impair mental and physical abilities; avoid concurrent alcohol or other CNS depressants.,Elderly may be more sensitive to sedative effects.
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.
Hypersensitivity to metaxalone or any component of the formulation.,Significant hepatic impairment (e.g., severe liver disease, cirrhosis).,History of drug-induced hemolytic anemia.,Concurrent use of MAOIs or within 14 days of MAOI therapy (potential for serotonin syndrome).
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.
Metaxalone may be taken with or without food. Grapefruit juice may increase metaxalone levels by inhibiting CYP1A2 and CYP3A4; avoid concurrent consumption. High-fat meals may slightly delay absorption but not clinically significant.
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.
FDA pregnancy category C. No adequate studies in pregnant women. Animal studies have shown adverse effects (fetal resorptions, decreased fetal weight) at doses 5-10 times the human dose. Risk cannot be ruled out. Use only if potential benefit justifies potential risk to fetus. First trimester: Avoid; data insufficient. Second/third trimester: Limited data; may cause maternal sedation and neonatal respiratory depression if used near term.
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.
Unknown if excreted in human milk. M/P ratio not established. Caution advised due to potential for sedation in the infant. Monitor for drowsiness, poor feeding, or weight loss. Consider alternative agents with more safety data.
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.
No specific dose adjustments recommended due to lack of pharmacokinetic data in pregnancy. However, increased hepatic clearance during pregnancy may reduce drug levels; monitor clinical effect and adjust dose as needed. Use lowest effective dose for shortest duration.
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.
Metaxalone is a centrally acting muscle relaxant with a unique chemical structure (oxazolidinone derivative). It is metabolized primarily by CYP1A2 and CYP2D6; caution with inhibitors or inducers of these enzymes. Onset of action is 1-2 hours; peak effect at 3-4 hours. Due to sedative properties, avoid concurrent use with alcohol or other CNS depressants. Use with caution in elderly due to anticholinergic effects and fall risk. Metaxalone is not recommended for patients with significant hepatic impairment (Child-Pugh Class C). It has no direct effect on skeletal muscle contraction but acts on CNS polysynaptic pathways.
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.
Take metaxalone exactly as prescribed; do not increase dose or frequency without consulting your doctor.,May cause drowsiness or dizziness; do not drive or operate heavy machinery until you know how this medication affects you.,Avoid alcohol or other sedatives while taking metaxalone as they may worsen drowsiness.,If you miss a dose, skip the missed dose and continue with your regular schedule; do not double the dose.,Contact your healthcare provider if you experience signs of an allergic reaction (rash, hives, difficulty breathing) or jaundice (yellowing of skin/eyes).,Store at room temperature, away from moisture and heat.,Do not stop abruptly; gradual dose reduction may be recommended to prevent withdrawal symptoms.
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.
"The combination of Normethadone, a μ-opioid receptor agonist with respiratory depressant effects, and Metaxalone, a centrally acting muscle relaxant that also depresses the central nervous system (CNS), results in synergistic CNS depression and respiratory depression. This can lead to profound sedation, coma, or fatal respiratory failure, particularly in patients with pre-existing respiratory compromise or those taking other CNS depressants. Concomitant use increases the risk of hypotension and bradycardia due to combined cardiorespiratory depressant effects."
"The coadministration of Metaxalone and Tiagabine may lead to increased central nervous system (CNS) depression due to additive pharmacodynamic effects. Metaxalone, a centrally acting muscle relaxant, and Tiagabine, a selective GABA reuptake inhibitor, both potentiate GABAergic activity and depress neuronal excitability. This synergism can result in enhanced sedation, dizziness, ataxia, and cognitive impairment, increasing the risk of falls and respiratory depression, especially in susceptible patients."
"Concurrent use of fluticasone propionate, a corticosteroid with immunosuppressive and anti-inflammatory properties, and metaxalone, a centrally acting muscle relaxant with sedative effects, may result in additive immunosuppression and central nervous system (CNS) depression. Corticosteroids can mask signs of infection or exacerbate existing infections, while metaxalone contributes to sedation and dizziness. This combination may increase the risk of adverse effects such as heightened sedation, impaired cognitive function, and increased susceptibility to infections, particularly in elderly or debilitated patients."
No interactions on record
Explore head-to-head clinical comparisons of other medications in the same therapeutic classes.
Common clinical questions about METAXALONE vs ABSTRAL, answered by our medical review team.
METAXALONE is a Skeletal Muscle Relaxant that works by Metaxalone is a centrally acting muscle relaxant whose exact mechanism is unknown. It is thought to produce skeletal muscle relaxation by depressing the central nervous system (CNS), possibly through general CNS depression or by blocking polysynaptic reflexes in the spinal cord.. 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.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between METAXALONE and ABSTRAL 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 METAXALONE is: 800 mg orally 3 to 4 times daily. 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.. 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 METAXALONE and ABSTRAL 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. METAXALONE is classified as Category A/B. FDA pregnancy category C. No adequate studies in pregnant women. Animal studies have shown adverse effects (fetal resorptions, decreased fetal weight) at doses 5-10 times the human. 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. Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.