Chlorzoxazone

(BAN, rINN)
Synonyms: Chlorobenzoxazolinone; Chlorzoxazonum; Clorzoxazona; Klooritsoksatsoni; Klorzoksazon; Klorzoxazon. 5-Chlorobenzoxazol-2(3H)-one.
Cyrillic synonym: Хлорзоксазон.

💊 Chemical information

Chemical formula: C7H4ClNO2 = 169.6.
CAS — 95-25-0.
ATC — M03BB03.
ATC Vet — QM03BB03.

Pharmacopoeias.

In US.

USP 31

(Chlorzoxazone). A white or practically white, practically odourless, crystalline powder. Slightly soluble in water; sparingly soluble in alcohol, in isopropyl alcohol, and in methyl alcohol; soluble in solutions of alkali hydroxides and ammonia. Store in airtight containers.

💊 Adverse Effects and Treatment

The most common adverse effects of chlorzoxazone are drowsiness and dizziness. There may occasionally also be gastrointestinal irritation and gastrointestinal bleeding has been reported rarely. Other effects that have occurred are headache, overstimulation, and rarely sensitivity reactions including skin rashes, petechiae, ecchymoses, urticaria and pruritus; very rarely, angioedema or anaphylactoid reactions may occur. Some patients taking chlorzoxazone have developed jaundice and liver damage suspected to be caused by the drug. After overdosage there may be gastrointestinal disturbances, drowsiness, dizziness, headache, malaise, and sluggishness followed by marked loss of muscle tone, hypotension, and respiratory depression. Emptying the stomach by lavage should be considered, followed by activated charcoal and supportive therapy.

Effects on the liver.

Hepatotoxicity, sometimes fatal, has been associated with chlorzoxazone treatment.1
1. Powers BJ, et al. Chlorzoxazone hepatotoxic reactions: an analysis of 21 identified or presumed cases. Arch Intern Med 1986; 146: 1183–6.

Overdosage.

Overdosage and coma occurred on 2 occasions in a patient taking chlorzoxazone; on the second occasion, the patient responded to intravenous flumazenil.1
1. Roberge RJ, et al. Two chlorzoxazone (Parafon forte) overdoses and coma in one patient: reversal with flumazenil. Am J Emerg Med 1998; 16: 393–5.
To r t i c o l l i s . There has been a report of a patient with a spasmodic torticollis-like syndrome, consisting of tonic deviation of the head to the right, clenching of the teeth, and dysarthria, which developed repeatedly within 2 hours of ingesting chlorzoxazone for low back pain.1 Intravenous injection of benzatropine mesilate 1 mg gave rapid relief of symptoms.
1. Rosin MA. Chlorzoxazone-induced spasmodic torticollis. JAMA 1981; 246: 2575.

💊 Precautions

Chlorzoxazone should not be given to patients with impaired liver function and should be stopped if signs of liver toxicity appear. Patients should be advised to report to their doctor any signs or symptoms of possible liver toxicity such as fever, rash, jaundice, dark urine, anorexia, nausea, vomiting, or right upper quadrant pain. Chlorzoxazone may cause drowsiness; patients affected should not drive or operate machinery. The urine of patients taking chlorzoxazone may be coloured orange or reddish-purple by a phenolic metabolite.

Porphyria.

Chlorzoxazone has been associated with acute attacks of porphyria and is considered unsafe in porphyric patients.

💊 Interactions

The CNS effects of chlorzoxazone may be enhanced by alcohol and other CNS depressants.

Disulfiram.

A study1 of the efficacy of disulfiram as an inhibitor of the cytochrome P450 isoenzyme CYP2E1 (an enzyme involved in the metabolism of chlorzoxazone) found that a single 500-mg dose of disulfiram reduced plasma clearance of chlorzoxazone by 85%, resulting in a doubling of the latter’s peak plasma concentrations and prolongation of its elimination halflife from a mean of 0.92 to 5.1 hours.
1. Kharasch ED, et al. Single-dose disulfiram inhibition of chlorzoxazone metabolism: a clinical probe for P450 2E1. Clin Pharmacol Ther 1993; 53: 643–50.

Isoniazid.

Isoniazid inhibited the clearance of chlorzoxazone by 56% when given to 10 slow acetylator subjects resulting in an increase in sedation, headache, and nausea.1 Two days after stopping isoniazid there had been a rebound increase in the clearance of chlorzoxazone by 56% over the pre-isoniazid clearance value. Similar but less pronounced effects have also been reported2 in rapid acetylators with chlorzoxazone’s pharmacokinetic parameters returning to baseline values in 2 days.
1. Zand R, et al. Inhibition and induction of cytochrome P4502E1catalyzed oxidation by isoniazid in humans. Clin Pharmacol Ther 1993; 54: 142–9
2. O’Shea D, et al. Modulation of CYP2E1 activity by isoniazid in rapid and slow N-acetylators. Br J Clin Pharmacol 1997; 43: 99–103.

💊 Pharmacokinetics

Chlorzoxazone is reported to be completely absorbed after oral doses and peak plasma concentrations are achieved after 1 to 2 hours. It is rapidly metabolised in the liver via the cytochrome P450 isoenzyme CYP2E1, mainly to 6-hydroxychlorzoxazone, and excreted in the urine primarily as the glucuronide metabolite. The elimination half-life of chlorzoxazone is about 1 hour.

💊 Uses and Administration

Chlorzoxazone is a centrally acting skeletal muscle relaxant with sedative properties. It is claimed to inhibit muscle spasm by exerting an effect primarily at the level of the spinal cord and subcortical areas of the brain. Its effects begin within an hour of an oral dose and last for 3 to 4 hours. It is used as an adjunct in the symptomatic treatment of painful muscle spasm associated with musculoskeletal conditions. The usual initial oral dose is 500 mg three or four times daily; the dose can often be reduced subsequently to 250 mg three or four times daily, although doses of up to 750 mg three or four times daily may be given if necessary. Chlorzoxazone is also given with analgesics in compound preparations.

💊 Preparations

USP 31: Chlorzoxazone Tablets.

Proprietary Preparations

Chile: Fenarol-S†; Denm.: Paraflex; Hong Kong: Solaxin; Hung.: Myoflexin; India: Parafon DSC; Indon.: Solaxin; S.Afr.: Paraflex; Swed.: Paraflex; Thai.: Chlorzox†; Turk.: Paraflex; USA: Paraflex; Parafon Forte DSC; Remular-S. Multi-ingredient: Arg.: Ibupirac Flex; Paraflex AN; Paraflex Plus; Rucaten Forte; Austria: Parafon; Braz.: Paralon; Canad.: Acetazone Forte; Acetazone Forte C8; Back-Aid; Parafon Forte; Tylenol Aches & Strains; Chile: Beserol-S; Brevex; Desdol; Flectadol; Tonoflex; Winasorb Flex; Fin.: Paraflex comp†; Hong Kong: Relaxin-P†; India: Cip-Zox; Dolocide MR; Duodil; Fenaplus-MR; Flamar-MX; Flexon-MR; Myospaz; Myospaz Forte; New Panazox; Nicip MR; Osteoflam-MR; Pacizox; Parafon; Systaflam; Malaysia: Paras; Mex.: Parafon Forte; Reumophan; Tafirol Flex; Philipp.: Parafon; S.Afr.: Parafon; Swed.: Paraflex comp†; Thai.: Cezox; Myora; Myoserv†; Parafon; Turk.: Mepadol; Muskazon; Parafon; USA: Flexaphen.
Published October 18, 2018.