Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
PARAFLEX vs CHLORZOXAZONE
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
Centrally acting muscle relaxant; inhibits polysynaptic reflexes at the spinal cord level, possibly by depressing the central nervous system.
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
250-500 mg orally once daily, may increase to 500 mg twice daily if needed. Maximum 500 mg/day.
250-500 mg orally 3-4 times daily, maximum 750 mg 4 times daily.
Terminal elimination half-life is approximately 2–3 hours, allowing for multiple daily dosing.
Terminal elimination half-life approximately 1–2 hours; clinically relevant for muscle relaxant effect duration.
Hepatic via hydrolysis to chlorzoxazone and subsequent glucuronidation and sulfation.
Hepatic, primarily via CYP2E1, also CYP1A2 and CYP3A4
Renal excretion of unchanged drug and metabolites accounts for approximately 50% of an oral dose; fecal excretion accounts for about 20%.
Primarily hepatic metabolism followed by renal excretion of metabolites; <1% excreted unchanged in urine; minor biliary/fecal elimination.
Approximately 95% bound to plasma proteins, primarily albumin.
Approximately 90–95% bound, primarily to albumin.
Volume of distribution is approximately 1.5 L/kg, indicating extensive tissue distribution.
0.46–0.64 L/kg; indicates distribution into total body water.
Oral bioavailability is approximately 80% due to first-pass metabolism.
Oral: nearly complete; rapidly absorbed with extensive first-pass metabolism; systemic bioavailability approximately 30–50% due to first-pass effect.
GFR 30-50 m L/min: 250 mg once daily. GFR 15-29 m L/min: 250 mg every other day. GFR <15 m L/min: not recommended.
No specific guidelines; use with caution in severe renal impairment (GFR <30 m L/min) due to potential accumulation of active metabolite.
Child-Pugh A: no adjustment. Child-Pugh B: 250 mg once daily. Child-Pugh C: not recommended.
Contraindicated in hepatic impairment; avoid use in Child-Pugh class B or C due to risk of hepatotoxicity.
Not recommended for pediatric use due to lack of safety and efficacy data.
Not established; safety and efficacy not studied in pediatric patients.
Start at 250 mg once daily; increase cautiously. Monitor for renal function and adverse effects.
Initiate at lower end of dosing range (250 mg 3-4 times daily); monitor for CNS effects (dizziness, drowsiness) and liver function.
Paraflex is not known to have a black box warning.
None
May cause drowsiness, dizziness, or impaired mental/physical abilities,Use caution when driving or operating machinery,Potential for hepatotoxicity with chronic high-dose use,May be habit-forming with prolonged use
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 chlorzoxazone or any component,Severe hepatic impairment
Hypersensitivity to chlorzoxazone or any component of the formulation; impaired hepatic function
Avoid alcohol consumption due to increased risk of hepatotoxicity and CNS depression. No specific food interactions; take with or without food. However, taking with food may reduce gastrointestinal upset.
No significant food interactions. Take with or without food. Grapefruit juice may increase drug levels; avoid large quantities.
Paraflex (chlorzoxazone) is classified as FDA pregnancy category C. Animal studies have shown adverse effects on fetal development, but no adequate human studies exist. First trimester: Potential risk of teratogenicity; use only if clearly needed. Second and third trimesters: No known specific risks, but avoid unnecessary use. Near term: Theoretical risk of neonatal muscle weakness or CNS depression.
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.
Chlorzoxazone is excreted into breast milk in small amounts. M/P ratio is not established. Use caution in nursing mothers; consider risk of infant sedation and potential adverse effects. Monitor infant for drowsiness or feeding difficulties.
Not recommended during breastfeeding due to potential for sedation in the infant. No M/P ratio data available.
No established dose adjustments for pregnancy. Pharmacokinetic changes (increased volume of distribution, altered hepatic metabolism) may require individual titration to avoid toxicity. Start at lowest effective dose; monitor for excessive sedation or lack of efficacy.
No dosage adjustment specific to pregnancy is required based on pharmacokinetic data; however, clinical response should be monitored.
Para Flex (chlorzoxazone) is a centrally acting muscle relaxant used for acute musculoskeletal pain. It should not be used for longer than 2-3 weeks due to lack of evidence for chronic use and potential hepatotoxicity. Monitor liver enzymes in patients with pre-existing hepatic impairment. Onset of action is within 1 hour; peak effect at 1-2 hours. Combine with rest and physical therapy for optimal outcome.
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 this medication exactly as prescribed; do not exceed recommended dose or duration.,May cause drowsiness or dizziness; avoid driving or operating heavy machinery until you know how it affects you.,Avoid alcohol consumption as it may increase drowsiness and risk of liver damage.,Report symptoms of liver toxicity (e.g., yellowing of skin/eyes, dark urine, abdominal pain) immediately.,Do not discontinue abruptly; taper under medical advice if used for more than a few weeks.
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 PARAFLEX vs CHLORZOXAZONE, answered by our medical review team.
PARAFLEX is a Skeletal Muscle Relaxant that works by Centrally acting muscle relaxant; inhibits polysynaptic reflexes at the spinal cord level, possibly by depressing the central nervous system.. 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 PARAFLEX and CHLORZOXAZONE depend on the specific clinical indication. These are both Skeletal Muscle Relaxant agents and are not directly interchangeable by dose. A physician or clinical pharmacist should guide any therapeutic switching decisions.
The standard adult dose of PARAFLEX is: 250-500 mg orally once daily, may increase to 500 mg twice daily if needed. Maximum 500 mg/day.. 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 PARAFLEX 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. PARAFLEX is classified as Category C. Paraflex (chlorzoxazone) is classified as FDA pregnancy category C. Animal studies have shown adverse effects on fetal development, but no adequate human studies exist. First trime. 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.