Title: Chlorzoxazone
CAS Registry Number: 95-25-0
CAS Name: 5-Chloro-2(3H)-benzoxazolone
Additional Names: 5-chloro-2-benzoxazolol; 5-chloro-2-hydroxybenzoxazole; 2-hydroxy-5-chlorobenzoxazole; 5-chlorbenzoxazolin-2-one; 5-chlorobenzoxazolidone
Trademarks: Paraflex (McNeil); Biomioran; Solaxin
Molecular Formula: C7H4ClNO2
Molecular Weight: 169.57
Percent Composition: C 49.58%, H 2.38%, Cl 20.91%, N 8.26%, O 18.87%
Literature References: Prepn from 2-amino-5-chlorobenzoxazole: Marsh, US 2895877 (1959 to McNeil Labs.). LC determn in plasma: I. L. Honigsberg et al., J. Pharm. Sci. 68, 253 (1979). Fluorometric determn in tablets and biological fluids: J. T. Stewart, C. W. Chan, ibid. 910. Clinical studies: R. Herman, Curr. Ther. Res. 9, 537 (1967); J. J. Scheiner, ibid. 19, 51 (1976). Metabolism study: R. Twele, G. Spiteller, Arzneim.-Forsch. 32, 759 (1982). Toxicity: G. Hofrichter et al., Arzneim.-Forsch. 17, 242 (1967). Review of clinical studies: J. K. Elenbaas, Am. J. Hosp. Pharm. 37, 1313 (1980). Comprehensive description: J. T. Stewart, C. A. Janicki, Anal. Profiles Drug Subs. 16, 119-144 (1987).
Properties: Crystals from acetone, mp 191-191.5°. Sparingly sol in water; sol in methanol, ethanol, isopropanol. Freely sol in aq solns of alkali hydroxides and ammonia. LD50 in mice (mg/kg): 3650 orally, 380 i.p. (suspensions); 440 orally, 183 i.p. (solns of Na salt) (Hofrichter).
Melting point: mp 191-191.5°
Toxicity data: LD50 in mice (mg/kg): 3650 orally, 380 i.p. (suspensions); 440 orally, 183 i.p. (solns of Na salt) (Hofrichter)
Therap-Cat: Muscle relaxant (skeletal).
Keywords: Muscle Relaxant (Skeletal).
Cholane Cholanic Acid Cholanthrene Cholecystokinin Cholestanol

Systematic (IUPAC) name
Clinical data
Trade names Parafonforte
AHFS/ monograph
MedlinePlus a682577
Legal status  ?
Routes oral
Pharmacokinetic data
Bioavailability well absorbed
Protein binding 13–18%
Metabolism hepatic
Half-life 1.1 hr
Excretion urine (<1%)
CAS number 95-25-0 YesY
ATC code M03BB03
PubChem CID 2733
IUPHAR ligand 2322
DrugBank DB00356
ChemSpider 2632 YesY
KEGG D00771 YesY
Chemical data
Formula C7H4ClNO2 
Mol. mass 169.565 g/mol
 YesY (what is this?)  (verify)

Chlorzoxazone (Paraflex) is a centrally acting muscle relaxant used to treat muscle spasm and the resulting pain or discomfort. It acts on the spinal cord by depressing reflexes. It is sold as Muscol or Parafon Forte, a combination of chlorzoxazone and acetaminophen (Paracetamol). Possible side effects include dizziness, lightheadedness, malaise, nausea, vomiting, and liver dysfunction. Used with acetaminophen it has added risk of hepatoxicity,[medical citation needed] which is why the combination is not recommended. It can also be administered for acute pain in general and for tension headache (muscle contraction headache).