Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
METAXALONE vs ALFENTA
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.
μ-opioid receptor agonist that activates G-protein coupled receptors to inhibit adenylate cyclase, decreasing c AMP production, leading to reduced neuronal excitability and pain transmission.
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.
Induction and maintenance of anesthesia,Analgesic supplement during surgical procedures,Intravenous use for monitored anesthesia care (MAC)
800 mg orally 3 to 4 times daily
Intravenous: Initial dose 8-20 mcg/kg (0.5-1 min) then 0.5-3 mcg/kg/min or 3-5 mcg/kg q5-20min. For short procedures: 8-20 mcg/kg. For longer procedures: 50-75 mcg/kg followed by 0.5-3 mcg/kg/min.
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: 90–111 minutes (1.5–1.85 hours); prolonged in 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 via CYP3A4 to inactive metabolites; major metabolite is desmethylalfentanil (inactive).
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.
Primarily renal (urinary) elimination as metabolites; approximately 80% recovered in urine, 20% in feces.
Approximately 98% bound to plasma proteins, primarily albumin.
Approximately 92% bound, primarily to alpha-1 acid glycoprotein and albumin.
Approximately 0.3–0.5 L/kg, indicating moderate distribution into total body water and peripheral tissues.
0.5–1.0 L/kg; reflects moderate tissue distribution; higher Vd in neonates and elderly.
Oral bioavailability is high, estimated at >80% based on urinary recovery studies.
Intravenous: 100%; intramuscular: approximately 90%; intrathecal: approximately 10% (due to systemic absorption following spinal administration).
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 dose adjustment is recommended for renal impairment; however, alfentanil is primarily metabolized in the liver and its pharmacokinetics are not significantly altered in renal failure.
No specific dose adjustment guidelines available; use with caution in severe hepatic impairment (Child-Pugh class C) as metabolism may be reduced.
In hepatic impairment (Child-Pugh class A, B, C): Reduce dose by 50% and titrate carefully due to prolonged elimination half-life. Consider lower initial doses and extended dosing intervals.
Safety and efficacy not established; not recommended for use in children under 12 years of age.
Children (1-12 years): Induction of anesthesia: 10-20 mcg/kg IV; maintenance: 5-10 mcg/kg IV or infusion 0.5-1 mcg/kg/min. For neonates and infants: Dose individualization required; titrate to effect.
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.
Elderly patients (>65 years): Reduce initial dose by 30-50% and administer slowly. Due to decreased clearance and increased sensitivity, lower infusion rates (e.g., 0.3-0.5 mcg/kg/min) may be needed.
None.
Risk of respiratory depression, particularly in elderly or debilitated patients. Concomitant use with benzodiazepines or other CNS depressants may cause profound sedation, respiratory depression, coma, and death.
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; abuse potential; hypotension; bradycardia; muscle rigidity; serotonin syndrome with concurrent serotonergic drugs; adrenal insufficiency; risk of withdrawal with prolonged use.
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 alfentanil or any component; significant respiratory insufficiency; severe asthma; paralytic ileus; concurrent use of MAOIs (or within 14 days); acute or postoperative pain management in children (except for procedural sedation).
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.
No known interactions with food. However, grapefruit juice may increase alfentanil serum concentrations due to CYP3A4 inhibition; avoid concurrent consumption.
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.
Alfentanil, a short-acting opioid analgesic, is classified as FDA Pregnancy Category C. No well-controlled studies in pregnant women exist. In animal studies, no teratogenic effects were observed at clinically relevant doses; however, high doses caused embryotoxicity and increased fetal mortality. Trimester-specific risks: First trimester - potential for minor malformations based on limited human data; second trimester - possible risk if used chronically; third trimester - prolonged use may lead to neonatal respiratory depression, withdrawal syndrome, or opioid dependence. Use only if benefits outweigh risks.
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.
Alfentanil is excreted into human breast milk in low concentrations. The milk-to-plasma (M/P) ratio is approximately 0.3. Estimated infant dose is <1% of maternal weight-adjusted dose, which is considered clinically insignificant. However, due to potential for neonatal opioid effects, caution is advised; monitor infant for drowsiness, respiratory depression, and feeding difficulties. Consider alternative analgesics with established safety profiles, such as acetaminophen or ibuprofen, for lactation.
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 can alter pharmacokinetics of alfentanil. Increased plasma volume and distribution may require higher doses to achieve same effect, while decreased plasma protein binding may increase free fraction, potentiating effects. Alpha-1-acid glycoprotein levels change in pregnancy, affecting binding. In third trimester, clearance may be increased by up to 50% due to enhanced hepatic metabolism. Therefore, dose adjustments may be needed: consider starting at low dose and titrating to effect, with close monitoring. For intravenous administration, typical adult doses (5-20 μg/kg) may need adjustments; no standard pregnancy-specific dosing exists. Use the lowest effective dose for the shortest duration. In labor, avoid high doses prior to delivery due to risk of neonatal respiratory depression.
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.
Alfentanil is a potent, rapid-onset, short-acting opioid analgesic used primarily for induction and maintenance of anesthesia. Due to its high protein binding (90%) and rapid redistribution, it has a shorter duration of action than fentanyl, making it suitable for brief, painful procedures. It undergoes hepatic metabolism via CYP3A4, so concomitant use with CYP3A4 inhibitors like ketoconazole or erythromycin can prolong its effects. Use caution in elderly or hypovolemic patients due to increased risk of hypotension. Naloxone reverses respiratory depression. Alfentanil is 5-10 times less potent than fentanyl.
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.
This medication is given only by a healthcare professional in a hospital or surgical setting.,You may feel drowsy, dizzy, or nauseated after receiving this drug.,Report any difficulty breathing or slow heart rate to your healthcare provider immediately.,Avoid alcohol and sedatives for 24 hours after administration, as they can increase side effects.,Do not drive or operate machinery until the effects have fully worn off.
"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."
"Propantheline, an anticholinergic agent, can competitively antagonize muscarinic acetylcholine receptors, potentially reducing gastrointestinal motility and secretion. Alfentanil, a mu-opioid receptor agonist, also decreases gastrointestinal motility through central and peripheral opioid receptors. Concomitant use may synergistically inhibit peristalsis, leading to severe constipation, paralytic ileus, or delayed gastric emptying, which can increase the risk of aspiration and complicate anesthesia recovery."
"Alfentanil, a potent opioid analgesic, can cause significant hypotension and respiratory depression. When combined with furosemide, a loop diuretic that reduces blood volume and vascular resistance, there is a synergistic decrease in blood pressure, which may precipitate cardiovascular collapse, especially in patients with compromised circulatory reserves. Additionally, furosemide may enhance the sedative and respiratory depressant effects of alfentanil, leading to increased risk of respiratory acidosis and altered mental status."
"Alfentanil, a potent mu-opioid receptor agonist, can enhance the bradycardic effects of nebivolol, a beta-1 selective blocker with additional nitric oxide-mediated vasodilation. The combination may lead to excessive slowing of heart rate, reduced cardiac output, and potential hemodynamic instability, particularly in patients with underlying cardiac conduction abnormalities or hypovolemia."
Explore head-to-head clinical comparisons of other medications in the same therapeutic classes.
Common clinical questions about METAXALONE vs ALFENTA, 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.. ALFENTA is a Opioid Analgesic that works by μ-opioid receptor agonist that activates G-protein coupled receptors to inhibit adenylate cyclase, decreasing c AMP production, leading to reduced neuronal excitability and pain transmission.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between METAXALONE and ALFENTA 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 ALFENTA is: Intravenous: Initial dose 8-20 mcg/kg (0.5-1 min) then 0.5-3 mcg/kg/min or 3-5 mcg/kg q5-20min. For short procedures: 8-20 mcg/kg. For longer procedures: 50-75 mcg/kg followed by 0.5-3 mcg/kg/min.. Dosing should always be individualized based on indication, renal and hepatic function, age, and other patient factors.
A moderate-severity drug interaction has been identified when combining METAXALONE and ALFENTA. The risk or severity of adverse effects can be increased when Alfentanil is combined with Metaxalone. Consult your prescriber before combining these medications.
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. ALFENTA is classified as Category C. Alfentanil, a short-acting opioid analgesic, is classified as FDA Pregnancy Category C. No well-controlled studies in pregnant women exist. In animal studies, no teratogenic effect. Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.