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Clinical data
Trade namesParafonforte
Routes of
ATC code
Pharmacokinetic data
Bioavailabilitywell absorbed
Protein binding13–18%
Elimination half-life1.1 hr Duration of action 3-4 hrs.
Excretionurine (<1%)
CAS Number
PubChem CID
CompTox Dashboard (EPA)
ECHA InfoCard100.002.186 Edit this at Wikidata
Chemical and physical data
Molar mass169.56 g·mol−1
3D model (JSmol)

Chlorzoxazone (INN) 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 under the trade names "'Lorzone'", Paraflex and Muscol and in combination form as 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]. It can also be administered for acute pain in general and for tension headache (muscle contraction headache).

Like metaxalone, its mechanism of action is still in question. It is believed that metaxalone works by altering serotonin levels and acting as a mild MAO inhibitor.[medical citation needed] The mechanism of action of chlorzoaxazone is thought[by whom?] to act on Gaba-A & B receptors and voltage-gated calcium channels to a degree.[medical citation needed] General central nervous system depression is the only currently accepted aspect to its medical benefits.[medical citation needed] Elucidation of the exact mechanism of action is ongoing but there is limited study due to the existence of more effective, safe muscle relaxants (ex. diazepam, cyclobenzaprine, tizanidine), greatly limiting the potential benefit of identifying novel compounds which share chlorzoxazone's mechanism of action.

See also[edit]



  • D.F. Marsh, U.S. Patent 2,895,877 (1959).
  • Dong DL, Luan Y, Feng TM, Fan CL, Yue P, Sun ZJ, Gu RM, Yang BF (2006). "Chlorzoxazone inhibit contraction of rat thoracic aorta". Eur J Pharmacol. 545 (2–3): 161–6. doi:10.1016/j.ejphar.2006.06.063. PMID 16859676.
  • Park J, Kim K, Park P, Ha J (2006). "Effect of high-dose aspirin on CYP2E1 activity in healthy subjects measured using chlorzoxazone as a probe". J Clin Pharmacol. 46 (1): 109–14. doi:10.1177/0091270005282635. PMID 16397290.
  • Wan J, Ernstgård L, Song B, Shoaf S (2006). "Chlorzoxazone metabolism is increased in fasted Sprague-Dawley rats". J Pharm Pharmacol. 58 (1): 51–61. doi:10.1211/jpp.58.1.0007. PMC 1388188. PMID 16393464.
  • PARAFON DSC (chlorzoxazone) tablet, Daily Med, U.S. National Library of Medicine