Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
NORGESIC vs CHLORZOXAZONE
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
NORGESIC is a combination of orphenadrine citrate, aspirin, and caffeine. Orphenadrine is a centrally acting muscle relaxant with anticholinergic properties; its exact mechanism is not fully understood, but it may act via central atropine-like effects and inhibition of reuptake of norepinephrine and serotonin. Aspirin inhibits cyclooxygenase (COX) enzymes, reducing prostaglandin synthesis, leading to analgesic, antipyretic, and anti-inflammatory effects. Caffeine is a central nervous system stimulant that may enhance analgesia via adenosine receptor antagonism.
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.
Adjunct to rest, physical therapy, and other measures for the relief of discomfort associated with acute, painful musculoskeletal conditions
Adjunct for relief of acute painful musculoskeletal conditions associated with muscle spasm
1-2 tablets orally 2-4 times daily. Each tablet contains orphenadrine citrate 100 mg and acetaminophen 325 mg.
250-500 mg orally 3-4 times daily, maximum 750 mg 4 times daily.
Terminal elimination half-life is 2–4 hours; clinical multiple dosing may require 4–6 hour intervals
Terminal elimination half-life approximately 1–2 hours; clinically relevant for muscle relaxant effect duration.
Orphenadrine is extensively metabolized in the liver via oxidative N-demethylation and hydroxylation; aspirin is hydrolyzed to salicylic acid, which is primarily metabolized in the liver by conjugation (glucuronidation and glycine conjugation) and oxidation; caffeine is metabolized in the liver via cytochrome P450 1A2 (CYP1A2) to paraxanthine, theobromine, and theophylline.
Hepatic, primarily via CYP2E1, also CYP1A2 and CYP3A4
Primarily renal (70% as unchanged drug and metabolites; 10% as unchanged) and biliary (30%)
Primarily hepatic metabolism followed by renal excretion of metabolites; <1% excreted unchanged in urine; minor biliary/fecal elimination.
~90% bound to albumin and alpha-1-acid glycoprotein
Approximately 90–95% bound, primarily to albumin.
1.5–2.0 L/kg; indicates extensive tissue distribution
0.46–0.64 L/kg; indicates distribution into total body water.
Oral: ~70% (first-pass metabolism reduces absolute bioavailability); Intramuscular: ~100%
Oral: nearly complete; rapidly absorbed with extensive first-pass metabolism; systemic bioavailability approximately 30–50% due to first-pass effect.
GFR 30-59 m L/min: reduce dose by 50% (maximum 1 tablet twice daily). GFR <30 m L/min: avoid use due to risk of accumulation.
No specific guidelines; use with caution in severe renal impairment (GFR <30 m L/min) due to potential accumulation of active metabolite.
Child-Pugh Class B: reduce dose by 50% (maximum 1 tablet twice daily). Child-Pugh Class C: contraindicated.
Contraindicated in hepatic impairment; avoid use in Child-Pugh class B or C due to risk of hepatotoxicity.
Not recommended for patients under 18 years of age due to lack of safety and efficacy data.
Not established; safety and efficacy not studied in pediatric patients.
Start with 1 tablet twice daily; titrate slowly due to increased risk of anticholinergic effects (confusion, urinary retention).
Initiate at lower end of dosing range (250 mg 3-4 times daily); monitor for CNS effects (dizziness, drowsiness) and liver function.
No FDA black box warning.
None
May impair mental and/or physical abilities required for driving or operating machinery due to anticholinergic effects (e.g., drowsiness, blurred vision),Use with caution in patients with glaucoma, prostatic hypertrophy, or gastrointestinal obstruction due to anticholinergic effects,Aspirin component increases risk of bleeding, particularly in patients with bleeding disorders or on anticoagulant therapy,Reye's syndrome risk in children and teenagers with viral infections,Hypersensitivity reactions including anaphylaxis may occur,Renal impairment may increase risk of salicylate toxicity
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.
Hypersensitivity to any component,Peptic ulcer or gastrointestinal bleeding,Severe renal impairment (creatinine clearance <10 m L/min),Severe hepatic impairment,Bleeding disorders (e.g., hemophilia),Concurrent use of anticoagulants or antiplatelet drugs (relative),Children and teenagers with varicella or influenza-like symptoms (due to Reye's syndrome risk),Glaucoma (angle-closure), prostatic hypertrophy, or gastrointestinal obstruction (relative, due to anticholinergic effects)
Hypersensitivity to chlorzoxazone or any component of the formulation; impaired hepatic function
Avoid alcohol. Take with food or milk to minimize GI irritation. No specific food interactions beyond alcohol and potential for increased gastric irritation with aspirin.
No significant food interactions. Take with or without food. Grapefruit juice may increase drug levels; avoid large quantities.
Norgesic contains orphenadrine citrate, aspirin, and caffeine. Aspirin: First trimester: possible increased risk of miscarriage and cardiac defects; third trimester: premature closure of ductus arteriosus and oligohydramnios; avoid in third trimester. Orphenadrine: limited human data; animal studies not sufficient; avoid in pregnancy unless benefit outweighs risk. Caffeine: considered low risk at moderate doses.
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.
Orphenadrine: excreted in breast milk; M/P ratio unknown; potential for anticholinergic effects in infant. Aspirin: excreted in breast milk; risk of Reye's syndrome if infant has viral illness; avoid breastfeeding while using aspirin. Caffeine: low levels in milk; generally considered safe in moderate amounts. Not recommended during breastfeeding.
Not recommended during breastfeeding due to potential for sedation in the infant. No M/P ratio data available.
No standard dosing adjustments established. Avoid use in pregnancy, especially third trimester, due to aspirin component. If necessary, use lowest effective dose for shortest duration. Increased renal clearance in pregnancy may affect caffeine elimination, but no specific dose adjustment recommended.
No dosage adjustment specific to pregnancy is required based on pharmacokinetic data; however, clinical response should be monitored.
Norgesic (orphenadrine/aspirin/caffeine) is used for musculoskeletal pain. Orphenadrine has anticholinergic properties; use cautiously in elderly, glaucoma, or prostatic hypertrophy. May cause drowsiness and impair motor skills. Avoid in myasthenia gravis. Monitor for GI bleeding due to aspirin.
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.
Take with food or milk to reduce GI upset.,Avoid alcohol and other CNS depressants.,Do not drive or operate machinery until you know how this medication affects you.,Report signs of bleeding (bruising, black stools) or anticholinergic effects (dry mouth, blurred vision, constipation).,Do not exceed recommended dose; orphenadrine can cause serious anticholinergic toxicity.
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.
No interactions on record
"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."
Explore head-to-head clinical comparisons of other medications in the same therapeutic classes.
Common clinical questions about NORGESIC vs CHLORZOXAZONE, answered by our medical review team.
NORGESIC is a Muscle Relaxant that works by NORGESIC is a combination of orphenadrine citrate, aspirin, and caffeine. Orphenadrine is a centrally acting muscle relaxant with anticholinergic properties; its exact mechanism is not fully understood, but it may act via central atropine-like effects and inhibition of reuptake of norepinephrine and serotonin. Aspirin inhibits cyclooxygenase (COX) enzymes, reducing prostaglandin synthesis, leading to analgesic, antipyretic, and anti-inflammatory effects. Caffeine is a central nervous system stimulant that may enhance analgesia via adenosine receptor antagonism.. 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.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between NORGESIC and CHLORZOXAZONE 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 NORGESIC is: 1-2 tablets orally 2-4 times daily. Each tablet contains orphenadrine citrate 100 mg and acetaminophen 325 mg.. The standard adult dose of CHLORZOXAZONE is: 250-500 mg orally 3-4 times daily, maximum 750 mg 4 times 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 NORGESIC and CHLORZOXAZONE 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. NORGESIC is classified as Category C. Norgesic contains orphenadrine citrate, aspirin, and caffeine. Aspirin: First trimester: possible increased risk of miscarriage and cardiac defects; third trimester: premature clos. 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. Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.