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Pharmacologic Category
Dosing: Adult
Musculoskeletal conditions: Oral: 250 to 500 mg 3 or 4 times daily, may increase to 750 mg 3 or 4 times daily; for painful musculoskeletal conditions, start with 500 mg 3 or 4 times daily. Consider dose reductions as symptoms improve.
* See Dosage and Administration in AHFS Essentials for additional information.
Dosing: Geriatric
Avoid use (Beers Criteria [AGS 2019]).
Dosing: Renal Impairment: Adult
There are no dosage adjustments provided in the manufacturer’s labeling.
Dosing: Hepatic Impairment: Adult
There are no dosage adjustments provided in the manufacturer’s labeling.
Dosing: Pediatric
Muscle spasm: Limited data available, efficacy results variable (Losin 1966); Note: Chlorzoxazone not a routine treatment option for management of spasticity in pediatric patients (eg, cerebral palsy); use has been replaced by newer, more effective agents (Delgado 2010):
Children and Adolescents: 20 mg/kg/day in 3 to 4 divided doses; dosing based on generally recognized expert recommendations; published pediatric trial data is lacking; in reported experience, initial dose range: 125 to 250 mg 3 times daily (Berman 1964, Berman 1964a, Darienzo 1966, Kliegman 2007, Losin 1966); maximum dose: 750 mg/dose
Dosing: Renal Impairment: Pediatric
There are no dosage adjustments provided in the manufacturer’s labeling.
Dosing: Hepatic Impairment: Pediatric
There are no dosage adjustments provided in the manufacturer’s labeling.
Use: Labeled Indications
Musculoskeletal conditions: Adjunct to rest, physical therapy, and other measures for the relief of discomfort associated with acute, painful musculoskeletal conditions
* See Uses in AHFS Essentials for additional information.
Administration: Oral
Administer orally.
Administration: Pediatric
Oral: Administer with or without food
Storage/Stability
Store at 20°C to 25°C (68°F to 77° F).
Medication Patient Education with HCAHPS Considerations
What is this drug used for?
• It is used to relax muscles.
Frequently reported side effects of this drug
• Fatigue
• Dizziness
• Loss of strength and energy
• Anxiety
Other side effects of this drug: Talk with your doctor right away if you have any of these signs of:
• Liver problems like dark urine, fatigue, lack of appetite, nausea, abdominal pain, light-colored stools, vomiting, or yellow skin
• Signs of a significant reaction like wheezing; chest tightness; fever; itching; bad cough; blue skin color; seizures; or swelling of face, lips, tongue, or throat.
Note: This is not a comprehensive list of all side effects. Talk to your doctor if you have questions.
Consumer Information Use and Disclaimer: This information should not be used to decide whether or not to take this medicine or any other medicine. Only the healthcare provider has the knowledge and training to decide which medicines are right for a specific patient. This information does not endorse any medicine as safe, effective, or approved for treating any patient or health condition. This is only a brief summary of general information about this medicine. It does NOT include all information about the possible uses, directions, warnings, precautions, interactions, adverse effects, or risks that may apply to this medicine. This information is not specific medical advice and does not replace information you receive from the healthcare provider. You must talk with the healthcare provider for complete information about the risks and benefits of using this medicine.
Medication Safety Issues
Geriatric Patients: High-Risk Medication:
Contraindications
Hypersensitivity to chlorzoxazone or any component of the formulation
Warnings/Precautions
Concerns related to adverse effects:
• CNS depression: May cause CNS depression, which may impair physical or mental abilities; patients must be cautioned about performing tasks that require mental alertness (eg, operating machinery or driving).
• Hepatotoxicity: Rare, serious (including fatal) idiosyncratic and unpredictable hepatocellular toxicity has been reported with use. Discontinue immediately if early signs/symptoms of hepatic toxicity arise (eg, fever, rash, anorexia, nausea, vomiting, fatigue, right upper quadrant pain, dark urine, jaundice). Also discontinue if elevated liver enzymes (eg, AST, ALT, alkaline phosphatase, bilirubin) develop.
• Hypersensitivity reaction: Use caution in patients with known allergies or a history of allergic reactions to drugs; if sensitivity (itching, redness, urticaria) occurs, discontinue therapy.
Concurrent drug therapy issues:
• Drug-drug interactions: Potentially significant interactions may exist, requiring dose or frequency adjustment, additional monitoring, and/or selection of alternative therapy. Consult drug interactions database for more detailed information.
Dosage form specific issues:
• Benzyl alcohol and derivatives: Some dosage forms may contain sodium benzoate/benzoic acid; benzoic acid (benzoate) is a metabolite of benzyl alcohol; large amounts of benzyl alcohol (≥99 mg/kg/day) have been associated with a potentially fatal toxicity ("gasping syndrome") in neonates; the "gasping syndrome" consists of metabolic acidosis, respiratory distress, gasping respirations, CNS dysfunction (including convulsions, intracranial hemorrhage), hypotension and cardiovascular collapse (AAP 1997; CDC 1982); some data suggests that benzoate displaces bilirubin from protein binding sites (Ahlfors 2001); avoid or use dosage forms containing benzyl alcohol derivative with caution in neonates. See manufacturer's labeling.
* See Cautions in AHFS Essentials for additional information.
Geriatric Considerations
Because it can cause unpredictable, fatal hepatic toxicity, the use of chlorzoxazone should be avoided.
Pregnancy Considerations
Animal reproduction studies have not been conducted.
Briggs' Drugs in Pregnancy & Lactation
Adverse Reactions
Frequency not defined.
Central nervous system: Dizziness, drowsiness, malaise, paradoxical central nervous system stimulation
Genitourinary: Urine discoloration
<1%, postmarketing, and/or case reports: Allergic skin rash, anaphylaxis (very rare), angioedema (very rare), ecchymoses, gastrointestinal hemorrhage, hepatotoxicity, petechia
* See Cautions in AHFS Essentials for additional information.
Allergy and Idiosyncratic Reactions
Toxicology
Metabolism/Transport Effects
Substrate of CYP1A2 (minor), CYP2A6 (minor), CYP2D6 (minor), CYP2E1 (major), CYP3A4 (minor); Note: Assignment of Major/Minor substrate status based on clinically relevant drug interaction potential; Inhibits CYP3A4 (weak)
Drug Interactions Open Interactions
Alcohol (Ethyl): May enhance the CNS depressant effect of Chlorzoxazone. Alcohol (Ethyl) may decrease the serum concentration of Chlorzoxazone. Specifically, chronic alcohol ingestion may decrease serum concentrations of chlorzoxazone. Risk C: Monitor therapy
Alizapride: May enhance the CNS depressant effect of CNS Depressants. Risk C: Monitor therapy
ARIPiprazole: CYP3A4 Inhibitors (Weak) may increase the serum concentration of ARIPiprazole. Management: Monitor for increased aripiprazole pharmacologic effects. Aripiprazole dose adjustments may or may not be required based on concomitant therapy and/or indication. Consult full interaction monograph for specific recommendations. Risk C: Monitor therapy
Azelastine (Nasal): CNS Depressants may enhance the CNS depressant effect of Azelastine (Nasal). Risk X: Avoid combination
Blonanserin: CNS Depressants may enhance the CNS depressant effect of Blonanserin. Risk D: Consider therapy modification
Botulinum Toxin-Containing Products: Muscle Relaxants (Centrally Acting) may enhance the adverse/toxic effect of Botulinum Toxin-Containing Products. Specifically, the risk for increased muscle weakness may be enhanced. Risk C: Monitor therapy
Brexanolone: CNS Depressants may enhance the CNS depressant effect of Brexanolone. Risk C: Monitor therapy
Brimonidine (Topical): May enhance the CNS depressant effect of CNS Depressants. Risk C: Monitor therapy
Bromopride: May enhance the CNS depressant effect of CNS Depressants. Risk C: Monitor therapy
Bromperidol: May enhance the CNS depressant effect of CNS Depressants. Risk X: Avoid combination
Buprenorphine: CNS Depressants may enhance the CNS depressant effect of Buprenorphine. Management: Consider reduced doses of other CNS depressants, and avoiding such drugs in patients at high risk of buprenorphine overuse/self-injection. Initiate buprenorphine at lower doses in patients already receiving CNS depressants. Risk D: Consider therapy modification
Cannabidiol: May enhance the CNS depressant effect of CNS Depressants. Risk C: Monitor therapy
Cannabis: May enhance the CNS depressant effect of CNS Depressants. Risk C: Monitor therapy
Chlormethiazole: May enhance the CNS depressant effect of Chlorzoxazone. Chlormethiazole may increase the serum concentration of Chlorzoxazone. Management: Consider reduced doses of chlorzoxazone when combined with chlormethiazole. Monitor patients for increased chlorzoxazone effects/toxicities (ie, CNS depression, sedation) if these agents are combined. Risk D: Consider therapy modification
Chlorphenesin Carbamate: May enhance the adverse/toxic effect of CNS Depressants. Risk C: Monitor therapy
CNS Depressants: May enhance the adverse/toxic effect of other CNS Depressants. Risk C: Monitor therapy
CYP2E1 Inhibitors (Strong): May increase the serum concentration of Chlorzoxazone. Risk C: Monitor therapy
Dimethindene (Topical): May enhance the CNS depressant effect of CNS Depressants. Risk C: Monitor therapy
Dofetilide: CYP3A4 Inhibitors (Weak) may increase the serum concentration of Dofetilide. Risk C: Monitor therapy
Doxylamine: May enhance the CNS depressant effect of CNS Depressants. Management: The manufacturer of Diclegis (doxylamine/pyridoxine), intended for use in pregnancy, specifically states that use with other CNS depressants is not recommended. Risk C: Monitor therapy
Dronabinol: May enhance the CNS depressant effect of CNS Depressants. Risk C: Monitor therapy
Droperidol: May enhance the CNS depressant effect of CNS Depressants. Management: Consider dose reductions of droperidol or of other CNS agents (eg, opioids, barbiturates) with concomitant use. Exceptions to this monograph are discussed in further detail in separate drug interaction monographs. Risk D: Consider therapy modification
Esketamine: May enhance the CNS depressant effect of CNS Depressants. Risk C: Monitor therapy
Flibanserin: CYP3A4 Inhibitors (Weak) may increase the serum concentration of Flibanserin. Risk C: Monitor therapy
Flunitrazepam: CNS Depressants may enhance the CNS depressant effect of Flunitrazepam. Risk D: Consider therapy modification
HydrOXYzine: May enhance the CNS depressant effect of CNS Depressants. Risk C: Monitor therapy
Isoniazid: May increase the serum concentration of Chlorzoxazone. Isoniazid may decrease the serum concentration of Chlorzoxazone. Specifically, it may decrease chlorzoxazone concentrations below baseline after isoniazid discontinuation. Risk C: Monitor therapy
Kava Kava: May enhance the adverse/toxic effect of CNS Depressants. Risk C: Monitor therapy
Lemborexant: CYP3A4 Inhibitors (Weak) may increase the serum concentration of Lemborexant. Management: The maximum recommended dosage of lemborexant is 5 mg, no more than once per night, when coadministered with weak CYP3A4 inhibitors. Risk D: Consider therapy modification
Lemborexant: May enhance the CNS depressant effect of CNS Depressants. Management: Dosage adjustments of lemborexant and of concomitant CNS depressants may be necessary when administered together because of potentially additive CNS depressant effects. Close monitoring for CNS depressant effects is necessary. Risk D: Consider therapy modification
Lofexidine: May enhance the CNS depressant effect of CNS Depressants. Management: Drugs listed as exceptions to this monograph are discussed in further detail in separate drug interaction monographs. Risk C: Monitor therapy
Lomitapide: CYP3A4 Inhibitors (Weak) may increase the serum concentration of Lomitapide. Management: Patients on lomitapide 5 mg/day may continue that dose. Patients taking lomitapide 10 mg/day or more should decrease the lomitapide dose by half. The lomitapide dose may then be titrated up to a max adult dose of 30 mg/day. Risk D: Consider therapy modification
Magnesium Sulfate: May enhance the CNS depressant effect of CNS Depressants. Risk C: Monitor therapy
Methotrimeprazine: CNS Depressants may enhance the CNS depressant effect of Methotrimeprazine. Methotrimeprazine may enhance the CNS depressant effect of CNS Depressants. Management: Reduce adult dose of CNS depressant agents by 50% with initiation of concomitant methotrimeprazine therapy. Further CNS depressant dosage adjustments should be initiated only after clinically effective methotrimeprazine dose is established. Risk D: Consider therapy modification
MetyroSINE: CNS Depressants may enhance the sedative effect of MetyroSINE. Risk C: Monitor therapy
Minocycline (Systemic): May enhance the CNS depressant effect of CNS Depressants. Risk C: Monitor therapy
Nabilone: May enhance the CNS depressant effect of CNS Depressants. Risk C: Monitor therapy
NiMODipine: CYP3A4 Inhibitors (Weak) may increase the serum concentration of NiMODipine. Risk C: Monitor therapy
Opioid Agonists: CNS Depressants may enhance the CNS depressant effect of Opioid Agonists. Management: Avoid concomitant use of opioid agonists and benzodiazepines or other CNS depressants when possible. These agents should only be combined if alternative treatment options are inadequate. If combined, limit the dosages and duration of each drug. Risk D: Consider therapy modification
Orphenadrine: CNS Depressants may enhance the CNS depressant effect of Orphenadrine. Risk X: Avoid combination
Oxomemazine: May enhance the CNS depressant effect of CNS Depressants. Risk X: Avoid combination
OxyCODONE: CNS Depressants may enhance the CNS depressant effect of OxyCODONE. Management: Avoid concomitant use of oxycodone and benzodiazepines or other CNS depressants when possible. These agents should only be combined if alternative treatment options are inadequate. If combined, limit the dosages and duration of each drug. Risk D: Consider therapy modification
Paraldehyde: CNS Depressants may enhance the CNS depressant effect of Paraldehyde. Risk X: Avoid combination
Perampanel: May enhance the CNS depressant effect of CNS Depressants. Management: Patients taking perampanel with any other drug that has CNS depressant activities should avoid complex and high-risk activities, particularly those such as driving that require alertness and coordination, until they have experience using the combination. Risk D: Consider therapy modification
Pimozide: CYP3A4 Inhibitors (Weak) may increase the serum concentration of Pimozide. Risk X: Avoid combination
Piribedil: CNS Depressants may enhance the CNS depressant effect of Piribedil. Risk C: Monitor therapy
Pramipexole: CNS Depressants may enhance the sedative effect of Pramipexole. Risk C: Monitor therapy
ROPINIRole: CNS Depressants may enhance the sedative effect of ROPINIRole. Risk C: Monitor therapy
Rotigotine: CNS Depressants may enhance the sedative effect of Rotigotine. Risk C: Monitor therapy
Rufinamide: May enhance the adverse/toxic effect of CNS Depressants. Specifically, sleepiness and dizziness may be enhanced. Risk C: Monitor therapy
Selective Serotonin Reuptake Inhibitors: CNS Depressants may enhance the adverse/toxic effect of Selective Serotonin Reuptake Inhibitors. Specifically, the risk of psychomotor impairment may be enhanced. Risk C: Monitor therapy
Sodium Oxybate: May enhance the CNS depressant effect of CNS Depressants. Management: Consider alternatives to combined use. When combined use is needed, consider minimizing doses of one or more drugs. Use of sodium oxybate with alcohol or sedative hypnotics is contraindicated. Risk D: Consider therapy modification
Suvorexant: CNS Depressants may enhance the CNS depressant effect of Suvorexant. Management: Dose reduction of suvorexant and/or any other CNS depressant may be necessary. Use of suvorexant with alcohol is not recommended, and the use of suvorexant with any other drug to treat insomnia is not recommended. Risk D: Consider therapy modification
Tetrahydrocannabinol: May enhance the CNS depressant effect of CNS Depressants. Risk C: Monitor therapy
Tetrahydrocannabinol and Cannabidiol: May enhance the CNS depressant effect of CNS Depressants. Risk C: Monitor therapy
Thalidomide: CNS Depressants may enhance the CNS depressant effect of Thalidomide. Risk X: Avoid combination
Tolperisone: May enhance the adverse/toxic effect of Muscle Relaxants (Centrally Acting). Management: Monitor for increased sedation or CNS effects if tolperisone is combined with other centrally acting muscle relaxants. Consider decreasing the tolperisone dose if these agents are combined. Risk D: Consider therapy modification
Triazolam: CYP3A4 Inhibitors (Weak) may increase the serum concentration of Triazolam. Management: Consider triazolam dose reduction in patients receiving concomitant weak CYP3A4 inhibitors. Risk D: Consider therapy modification
Trimeprazine: May enhance the CNS depressant effect of CNS Depressants. Risk C: Monitor therapy
Ubrogepant: CYP3A4 Inhibitors (Weak) may increase the serum concentration of Ubrogepant. Management: In patients taking weak CYP3A4 inhibitors, the initial and second dose (if needed) of ubrogepant should be limited to 50 mg. Risk D: Consider therapy modification
Zolpidem: CNS Depressants may enhance the CNS depressant effect of Zolpidem. Management: Reduce the Intermezzo brand sublingual zolpidem adult dose to 1.75 mg for men who are also receiving other CNS depressants. No such dose change is recommended for women. Avoid use with other CNS depressants at bedtime; avoid use with alcohol. Risk D: Consider therapy modification
Genes of Interest
Monitoring Parameters
Liver functions tests (periodic); signs/symptoms of hepatotoxicity
Advanced Practitioners Physical Assessment/Monitoring
Do not discontinue abruptly if patient using chronically; taper dosage slowly.
Nursing Physical Assessment/Monitoring
Monitor for effectiveness and CNS sedation.
Dosage Forms: US
Excipient information presented when available (limited, particularly for generics); consult specific product labeling. [DSC] = Discontinued product
Tablet, Oral:
Lorzone: 375 mg [contains sodium benzoate]
Lorzone: 750 mg [scored; contains sodium benzoate]
Parafon Forte DSC: 500 mg [DSC] [scored; contains brilliant blue fcf (fd&c blue #1), fd&c yellow #10 (quinoline yellow), sodium benzoate]
Generic: 250 mg, 375 mg, 500 mg, 750 mg
Anatomic Therapeutic Chemical (ATC) Classification
- M03BB03
Generic Available (US)
Yes
Pricing: US
Tablets (Chlorzoxazone Oral)
250 mg (per each): $24.88
375 mg (per each): $8.93
500 mg (per each): $1.08
750 mg (per each): $9.99
Tablets (Lorzone Oral)
375 mg (per each): $10.91
750 mg (per each): $12.20
Disclaimer: A representative AWP (Average Wholesale Price) price or price range is provided as reference price only. A range is provided when more than one manufacturer's AWP price is available and uses the low and high price reported by the manufacturers to determine the range. The pricing data should be used for benchmarking purposes only, and as such should not be used alone to set or adjudicate any prices for reimbursement or purchasing functions or considered to be an exact price for a single product and/or manufacturer. Medi-Span expressly disclaims all warranties of any kind or nature, whether express or implied, and assumes no liability with respect to accuracy of price or price range data published in its solutions. In no event shall Medi-Span be liable for special, indirect, incidental, or consequential damages arising from use of price or price range data. Pricing data is updated monthly.
Mechanism of Action
Centrally acting agent; acts on the spinal cord and subcortical areas of the brain to inhibit polysynaptic reflex arcs involved in causing and maintaining skeletal muscle spasms
Pharmacodynamics/Kinetics
Onset of action: Within 1 hour (Desiraju 1983)
Duration: Up to 6 hours (Desiraju 1983)
Absorption: Rapid (Desiraju 1983)
Metabolism: Extensively hepatic via glucuronidation
Half-life elimination: ~1 hour (Desiraju 1983)
Time to peak: ~1 to 2 hours
Excretion: Urine (predominately as conjugates; <1% as unchanged drug)
Dental Use
Treatment of muscle spasm and pain associated with acute temporomandibular joint pain (TMJ)
* See Uses in AHFS Essentials for additional information.
Local Anesthetic/Vasoconstrictor Precautions
No information available to require special precautions
Effects on Dental Treatment
No significant effects or complications reported
Effects on Bleeding
No information available to require special precautions
Dental Usual Dosing
Treatment of muscle spasm and pain associated with acute TMJ pain: Oral:
Adults: Oral: 500 mg 3-4 times daily, may increase up to 750 mg 3-4 times daily. May consider dose reductions as symptoms improve.
Related Information
Index Terms
Parafon Forte
FDA Approval Date
August 15, 1958
References
2019 American Geriatrics Society Beers Criteria Update Expert Panel. American Geriatrics Society 2019 updated AGS Beers Criteria for Potentially Inappropriate Medication Use in Older Adults. J Am Geriatr Soc. 2019;67(4):674-694. doi: 10.1111/jgs.15767.[PubMed 30693946]
Ahlfors CE. Benzyl alcohol, kernicterus, and unbound bilirubin. J Pediatr. 2001;139(2):317-319.[PubMed 11487763]
Centers for Disease Control (CDC). Neonatal deaths associated with use of benzyl alcohol—United States. MMWR Morb Mortal Wkly Rep. 1982;31(22):290-291. Available at: http://www.cdc.gov/mmwr/preview/mmwrhtml/00001109.htm.[PubMed 6810084]
Chlorzoxazone [prescribing information]. Madisonville, LA: Solubiomix LLC; August 2017.
Desiraju RK, Renzi NL Jr, Nayak RK, et al, “Pharmacokinetics of Chlorzoxazone in Humans,” J Pharm Sci, 1983, 72(9):991-4.[PubMed 6631711]
"Inactive" ingredients in pharmaceutical products: update (subject review). American Academy of Pediatrics Committee on Drugs. Pediatrics. 1997;99(2):268-278.[PubMed 9024461]
Kliegman RM, Behrman RE, Jenson HB, et al, eds. Nelson Textbook of Pediatrics. 18th ed. Philadelphia, PA: Saunders Elsevier; 2007.
Lorzone (chlorzoxazone) [prescribing information]. Bridgewater, NJ: Vertical Pharmaceuticals, LLC; January 2017.
Parafon Forte DSC (chlorzoxazone) [prescribing information]. Titusville, NJ: Janssen Pharmaceuticals; March 2012.
Pharmacy Quality Alliance. Use of high-risk medications in the elderly (2017 update) (HRM-2017). https://www.pqaalliance.org/medication-safety. Published 2017. Accessed March 21, 2019.
Brand Names: International
Chlorzox (TH); Escoflex (CH); Klorzoxazon (DK); Matalmin (TW); Muscol (TW); Myoflex (HR); Myoflexin (HN, HU); Paraflex (HR, SE, TR, ZA); Parafon DSC (IN); Parafon Forte (MX, TH); Prolax (TW); Reumophan (MX); Reumophan Alka (MX); Reumophan Vit (MX); Salalin (TW); Sola (TW); Solaxin (HK, ID, JP, SA); Tafirol Flex (MX)
Last Updated 3/13/20
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