Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
CHLORZOXAZONE vs ACTRON
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
Chlorzoxazone acts centrally on the spinal cord and subcortical areas of the brain to inhibit multisynaptic reflex arcs involved in producing and maintaining muscle spasm. It may also have some sedative effects.
Acetaminophen (paracetamol) is a non-opioid analgesic and antipyretic. Its mechanism is not fully understood but involves inhibition of cyclooxygenase (COX) enzymes in the central nervous system, reducing prostaglandin synthesis. It also modulates the endocannabinoid system and serotonergic pathways.
Adjunct for relief of acute painful musculoskeletal conditions associated with muscle spasm
Mild to moderate pain,Fever
250-500 mg orally 3-4 times daily, maximum 750 mg 4 times daily.
Oral: 400 mg every 4-6 hours as needed for pain; maximum 1200 mg/day.
Terminal elimination half-life approximately 1–2 hours; clinically relevant for muscle relaxant effect duration.
Terminal elimination half-life 2-4 hours; prolonged to 6-12 hours in elderly or renal impairment (Cr Cl <30 m L/min).
Hepatic, primarily via CYP2E1, also CYP1A2 and CYP3A4
Primarily metabolized in the liver via glucuronidation (UGT1A1, UGT1A6, UGT1A9), sulfation (SULT1A1, SULT1A3), and oxidation (CYP2E1, CYP3A4) to form the toxic metabolite N-acetyl-p-benzoquinone imine (NAPQI), which is detoxified by glutathione.
Primarily hepatic metabolism followed by renal excretion of metabolites; <1% excreted unchanged in urine; minor biliary/fecal elimination.
Renal: 90% as unchanged drug; biliary/fecal: 10% as metabolites.
Approximately 90–95% bound, primarily to albumin.
>99% bound to albumin.
0.46–0.64 L/kg; indicates distribution into total body water.
0.1-0.2 L/kg; indicates limited extravascular distribution.
Oral: nearly complete; rapidly absorbed with extensive first-pass metabolism; systemic bioavailability approximately 30–50% due to first-pass effect.
Oral: 70-90% (first-pass metabolism minimal); IV: 100%.
No specific guidelines; use with caution in severe renal impairment (GFR <30 m L/min) due to potential accumulation of active metabolite.
GFR <30 m L/min: Avoid use. GFR 30-50 m L/min: Reduce dose to 50% of normal, maximum 600 mg/day.
Contraindicated in hepatic impairment; avoid use in Child-Pugh class B or C due to risk of hepatotoxicity.
Child-Pugh Class B: Reduce dose by 50%; maximum 600 mg/day. Child-Pugh Class C: Contraindicated.
Not established; safety and efficacy not studied in pediatric patients.
Children ≥12 years: 400 mg orally every 6-8 hours as needed; maximum 1200 mg/day. Children <12 years: Not recommended.
Initiate at lower end of dosing range (250 mg 3-4 times daily); monitor for CNS effects (dizziness, drowsiness) and liver function.
Initiate at 200 mg every 6-8 hours; maximum 600 mg/day due to increased risk of gastrointestinal bleeding and renal impairment.
None
Acetaminophen has been associated with cases of acute liver failure, sometimes resulting in liver transplant and death. Most cases involve use of acetaminophen at doses exceeding 4000 mg per day, often involving more than one acetaminophen-containing product.
May cause drowsiness, dizziness, or impaired coordination. Caution in patients with hepatic impairment. Discontinue if hypersensitivity reactions occur. Avoid concurrent use with alcohol or other CNS depressants.
Hepatotoxicity: risk increased with chronic alcohol use, liver disease, or use of other acetaminophen-containing products. Avoid exceeding 4000 mg/day. Severe skin reactions: Stevens-Johnson syndrome, toxic epidermal necrolysis, acute generalized exanthematous pustulosis. Hypersensitivity reactions: anaphylaxis.
Hypersensitivity to chlorzoxazone or any component of the formulation; impaired hepatic function
Severe hepatic impairment or active liver disease. Known hypersensitivity to acetaminophen or any component of the formulation.
No significant food interactions. Take with or without food. Grapefruit juice may increase drug levels; avoid large quantities.
Avoid alcohol; may increase risk of GI bleeding. No specific food restrictions, but taking with food can reduce gastrointestinal irritation. Maintain adequate hydration to prevent renal impairment.
Teratogenic risk in humans is not well-studied. No major teratogenic effects have been reported in animal studies. However, as with all medications, use during pregnancy only if clearly needed and after weighing risks vs. benefits. Avoid during first trimester unless necessary.
First trimester: Based on animal studies and limited human data, possible increased risk of cardiovascular and neural tube defects. Second/third trimester: Risk of premature closure of ductus arteriosus and oligohydramnios with prolonged use. Avoid after 30 weeks gestation.
Not recommended during breastfeeding due to potential for sedation in the infant. No M/P ratio data available.
Excreted in breast milk; M/P ratio 0.15. Low oral bioavailability to infant; considered compatible with breastfeeding. Monitor infant for sedation or feeding problems.
No dosage adjustment specific to pregnancy is required based on pharmacokinetic data; however, clinical response should be monitored.
Dose adjustment not typically required; however, due to increased renal clearance and volume of distribution in pregnancy, higher doses may be needed to achieve therapeutic effect. Use lowest effective dose for shortest duration.
Chlorzoxazone is a centrally acting muscle relaxant used for acute musculoskeletal pain. Onset of action is within 1 hour; peak effect at 1-2 hours. Monitor for hepatotoxicity, especially with prolonged use or high doses. Can cause drowsiness and impair motor skills; avoid concurrent use with alcohol or other CNS depressants. Tablets may be crushed for patients with swallowing difficulties.
ACTRON (ketorolac tromethamine) is a nonsteroidal anti-inflammatory drug (NSAID) for short-term management of moderate to severe acute pain, typically not exceeding 5 days due to risk of GI bleeding, renal impairment, and cardiovascular events. Avoid in patients with active peptic ulcer disease, bleeding diathesis, or advanced renal disease. Monitor renal function and signs of bleeding. Use lowest effective dose for shortest duration. May cause bronchospasm in aspirin-sensitive asthma.
Take exactly as prescribed; do not increase dose or frequency.,May cause drowsiness or dizziness; avoid driving or operating machinery until you know how it affects you.,Avoid alcohol and other CNS depressants while taking this medication.,Report signs of liver problems: dark urine, yellowing of eyes/skin, persistent nausea, abdominal pain.,Do not suddenly stop taking if used long-term; taper under medical supervision to avoid withdrawal.
Take with food or milk to reduce stomach upset.,Do not take for more than 5 days as prescribed; longer use increases risk of serious side effects.,Avoid alcohol while taking this medication to lower risk of stomach bleeding.,Report any signs of bleeding (e.g., black stools, vomiting blood), unusual bruising, or decreased urination.,Do not take with other NSAIDs (e.g., ibuprofen, naproxen) or aspirin without consulting your doctor.,Inform your doctor about all medications, especially blood thinners (e.g., warfarin) and diuretics.,If you have asthma, be aware of potential bronchospasm; seek immediate help if you have breathing trouble.,Not recommended during pregnancy, especially in the third trimester.
"Lumacaftor is a strong inducer of cytochrome P450 (CYP) 3A4 and other drug-metabolizing enzymes, including CYP2E1. Chlorzoxazone is primarily metabolized by CYP2E1 to its inactive metabolite. Concomitant use increases CYP2E1 activity, leading to accelerated chlorzoxazone clearance and reduced systemic exposure, potentially diminishing its therapeutic effect as a muscle relaxant."
"Chlorzoxazone, a centrally acting muscle relaxant, inhibits the metabolism of diltiazem, a calcium channel blocker, via competitive inhibition of CYP3A4. This leads to increased plasma concentrations of diltiazem, potentially causing enhanced negative chronotropic and vasodilatory effects, resulting in bradycardia, hypotension, or atrioventricular block. Patients may experience dizziness, syncope, or exacerbate heart failure symptoms."
"Butalbital, a barbiturate, induces hepatic cytochrome P450 enzymes (particularly CYP2E1), accelerating the metabolism of chlorzoxazone, a centrally acting muscle relaxant primarily metabolized by CYP2E1. This results in reduced plasma concentrations of chlorzoxazone, leading to diminished therapeutic efficacy and potential loss of symptom control. Clinically, patients may experience inadequate muscle relaxation, requiring dose adjustments or alternative therapy."
No interactions on record
Explore head-to-head clinical comparisons of other medications in the same therapeutic classes.
Common clinical questions about CHLORZOXAZONE vs ACTRON, answered by our medical review team.
CHLORZOXAZONE is a Skeletal Muscle Relaxant that works by Chlorzoxazone acts centrally on the spinal cord and subcortical areas of the brain to inhibit multisynaptic reflex arcs involved in producing and maintaining muscle spasm. It may also have some sedative effects.. ACTRON is a NSAID that works by Acetaminophen (paracetamol) is a non-opioid analgesic and antipyretic. Its mechanism is not fully understood but involves inhibition of cyclooxygenase (COX) enzymes in the central nervous system, reducing prostaglandin synthesis. It also modulates the endocannabinoid system and serotonergic pathways.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between CHLORZOXAZONE and ACTRON 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 CHLORZOXAZONE is: 250-500 mg orally 3-4 times daily, maximum 750 mg 4 times daily.. The standard adult dose of ACTRON is: Oral: 400 mg every 4-6 hours as needed for pain; maximum 1200 mg/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 CHLORZOXAZONE and ACTRON 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. CHLORZOXAZONE is classified as Category C. Teratogenic risk in humans is not well-studied. No major teratogenic effects have been reported in animal studies. However, as with all medications, use during pregnancy only if cl. ACTRON is classified as Category C. First trimester: Based on animal studies and limited human data, possible increased risk of cardiovascular and neural tube defects. Second/third trimester: Risk of premature closur. Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.