Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
METAXALONE vs ACEPHEN
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.
ACEPHEN (acetaminophen) is a para-aminophenol derivative with analgesic and antipyretic activity. Its mechanism involves inhibition of cyclooxygenase (COX) enzymes in the central nervous system, particularly COX-2, reducing prostaglandin synthesis. It has weak peripheral COX inhibition and minimal anti-inflammatory effect.
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.
Mild to moderate pain,Fever
800 mg orally 3 to 4 times daily
325-650 mg orally every 4-6 hours as needed; maximum 4 g/day.
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: 1.0-1.5 hours in adults with normal renal function. Prolonged to 2-5 hours in hepatic impairment or elderly; requires dose adjustment in severe hepatic disease.
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.
Acetaminophen is primarily metabolized in the liver via glucuronidation (UGT1A1, UGT1A6, UGT1A9) and sulfation (SULT1A1, SULT1A3). A minor fraction is oxidized by cytochrome P450 enzymes (CYP2E1, CYP1A2, CYP3A4) to a reactive toxic metabolite (NAPQI), which is normally detoxified by conjugation with glutathione.
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: 90-95% as unchanged drug; tubular secretion and glomerular filtration. Biliary/fecal: <5%.
Approximately 98% bound to plasma proteins, primarily albumin.
Approximately 10-20% bound to serum albumin; extensive tissue binding.
Approximately 0.3–0.5 L/kg, indicating moderate distribution into total body water and peripheral tissues.
Apparent Vd: 0.5-0.7 L/kg (30-40 L in a 70 kg adult). Distributions into CSF and breast milk.
Oral bioavailability is high, estimated at >80% based on urinary recovery studies.
Oral: 85-90% (first-pass metabolism minimal). Rectal: approximately 70-80% of oral bioavailability.
No specific dose adjustment guidelines available; use with caution in severe renal impairment (Cr Cl <30 m L/min) due to potential for accumulation.
GFR 10-50 m L/min: 650 mg every 6 hours; GFR <10 m L/min: 650 mg every 8 hours.
No specific dose adjustment guidelines available; use with caution in severe hepatic impairment (Child-Pugh class C) as metabolism may be reduced.
Child-Pugh Class A: no adjustment; Child-Pugh Class B: maximum 2 g/day; Child-Pugh Class C: maximum 1 g/day.
Safety and efficacy not established; not recommended for use in children under 12 years of age.
10-15 mg/kg/dose orally every 4-6 hours; maximum 75 mg/kg/day or 4 g/day, whichever is less.
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.
Start at lowest effective dose (325 mg every 6 hours); avoid exceeding 3 g/day unless closely monitored.
None.
Acetaminophen has been associated with cases of acute liver failure, at times resulting in liver transplant and death. Most of the cases of liver injury are associated with the use of acetaminophen at doses that exceed 4,000 milligrams per day, and often involve more than one acetaminophen-containing product.
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.
Risk of severe liver injury with doses >4000 mg/day; use caution with hepatic impairment, chronic alcoholism, malnutrition, or concomitant hepatotoxic drugs; avoid exceeding recommended dose; limit use to 10 days for pain or 3 days for fever unless directed by physician; serious skin reactions (Stevens-Johnson syndrome, toxic epidermal necrolysis) have occurred.
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 acetaminophen or any component of the formulation; severe hepatic impairment or active liver disease.
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.
Alcohol: increased risk of hepatotoxicity. Avoid concurrent use. Food: no significant interaction, but taking with food may reduce minor gastrointestinal irritation.
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.
Pregnancy Category C. First trimester: potential risk of neural tube defects and orofacial clefts (limited human data, animal studies show embryotoxicity). Second and third trimesters: NSAID exposure associated with oligohydramnios, premature ductus arteriosus constriction, and fetal renal impairment. Avoid in third trimester.
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.
Excreted into breast milk in low concentrations (M/P ratio approximately 0.10). Considered compatible with breastfeeding; however, use lowest effective dose for shortest duration given potential for neonatal adverse effects (e.g., thrombocytopenia, renal dysfunction).
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.
No standard dose adjustments recommended; however, due to increased plasma volume and metabolism in pregnancy, higher doses may be required to achieve therapeutic effect. Avoid near term.
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.
ACEPHEN (acetaminophen) is commonly used for mild to moderate pain and fever. Avoid exceeding 4 g/day in adults to prevent hepatotoxicity. In patients with hepatic impairment, reduce maximum daily dose to 2 g. Consider acetylcysteine for overdose. Onset of action is 15-30 minutes orally.
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.
Do not exceed 4000 mg (4 grams) in 24 hours.,Avoid drinking alcohol while taking this medication.,Do not combine with other products containing acetaminophen.,Take with food if stomach upset occurs.,Seek immediate medical help if you experience symptoms of liver damage: yellowing of skin/eyes, dark urine, severe abdominal pain.
"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 ACEPHEN, 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.. ACEPHEN is a Non-Opioid Analgesic that works by ACEPHEN (acetaminophen) is a para-aminophenol derivative with analgesic and antipyretic activity. Its mechanism involves inhibition of cyclooxygenase (COX) enzymes in the central nervous system, particularly COX-2, reducing prostaglandin synthesis. It has weak peripheral COX inhibition and minimal anti-inflammatory effect.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between METAXALONE and ACEPHEN 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 ACEPHEN is: 325-650 mg orally every 4-6 hours as needed; maximum 4 g/day.. 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 ACEPHEN 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. ACEPHEN is classified as Category C. Pregnancy Category C. First trimester: potential risk of neural tube defects and orofacial clefts (limited human data, animal studies show embryotoxicity). Second and third trimest. Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.