Chlorzoxazone
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Chlorzoxazone
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Chlorzoxazone USP is a centrally acting skeletal muscle relaxant, available
as tablets of 500 mg for oral administration. Its chemical name is
5-Chloro-2-benzoxazolinone C7H4CINO2;
MW 169.57
Chlorzoxazone USP is a white or practically white, practically odorless,
crystalline powder. Chlorzoxazone is slightly soluble in water; sparingly
soluble in alcohol, in isopropyl alcohol, and in methanol; soluble in solutions
of alkali hydroxides and ammonia.
Chlorzoxazone tablets contain the inactive ingredients Docusate Sodium,
Lactose (hydrous), Magnesium Stearate, Microcrystalline Cellulose,
Pregelatinized Starch, Sodium Benzoate, and Sodium Starch Glycolate.
Chlorzoxazone is a centrally acting agent for painful
musculoskeletal conditions. Data available from animal experiments as well as
human study indicate that Chlorzoxazone acts primarily at the level of the
spinal cord and subcortical areas of the brain where it inhibits multisynaptic
reflex arcs involved in producing and maintaining skeletal muscle spasm of
varied etiology. The clinical result is a reduction of the skeletal muscle
spasm with relief of pain and increased mobility of the involved muscles. Blood
levels of Chlorzoxazone can be detected in people during the first 30 minutes
and peak levels may be reached, in the majority of the subjects, in about 1 to
2 hours after oral administration of Chlorzoxazone. Chlorzoxazone is rapidly
metabolized and is excreted in the urine, primarily in a conjugated form as the
glucuronide. Less than one percent of a dose of Chlorzoxazone is excreted
unchanged in the urine in 24 hours.
Chlorzoxazone is indicated as an adjunct to rest,
physical therapy, and other measures for the relief of discomfort associated
with acute, painful musculoskeletal conditions. The mode of action of this drug
has not been clearly identified, but may be related to its sedative properties.
Chlorzoxazone does not directly relax tense skeletal muscles in man.
Chlorzoxazone is contraindicated in patients with known
intolerance to the drug.
Serious (including fatal) hepatocellular toxicity has
been reported rarely in patients receiving Chlorzoxazone. The mechanism is
unknown but appears to be idiosyncratic and unpredictable. Factors predisposing
patients to this rare event are not known. Patients should be instructed to
report early signs and/or symptoms of hepatotoxicity such as fever, rash,
anorexia, nausea, vomiting, fatique, right upper quadrant pain, dark urine, or
jaundice. Chlorzoxazone should be discontinued immediately and a physician
consulted if any of these signs or symptoms develop. Chlorzoxazone use should
also be discontinued if a patient develops abnormal liver enzymes (e.g., AST,
ALT, alkaline phosphatase and bilirubin.)
The concomitant use of alcohol or other central nervous system depressants
may have an additive effect.
Usage in Pregnancy
The safe use of Chlorzoxazone has not been established with respect to the
possible adverse effects upon fetal development. Therefore, it should be used
in women of childbearing potential only when, in the judgement of the
physician, the potential benefits outweigh the possible risks.
Chlorzoxazone should be used with caution in patients
with known allergies or with a history of allergic reactions to drugs. If a
sensitivity reaction occurs such as urticaria, redness, or itching of the skin,
the drug should be stopped.
If any symptoms suggestive of liver dysfunction are observed, the drug
should be discontinued.
Chlorzoxazone containing products are usually well
tolerated. It is possible in rare instances that Chlorzoxazone may have been
associated with gastrointestinal bleeding. Drowsiness, dizziness,
lightheadedness, malaise, or over-stimulation may be noted by an occasional
patient. Rarely, allergic-type skin rashes, petechiae, or ecchymoses may
develop during treatment. Angioneurotic edema or anaphylactic reactions are
extremely rare. There is no evidence that the drug will cause renal damage.
Rarely, a patient may note discoloration of the urine resulting from a phenolic
metabolite of Chlorzoxazone. This finding is of no known clinical significance.
Symptoms
Initially, gastrointestinal disturbances such as nausea,
vomiting or diarrhea together with drowsiness, dizziness, lightheadedness or
headache may occur. Early in the course there may be malaise or sluggishness
followed by marked loss of muscle tone, making voluntary movement impossible.
The deep tendon reflexes may be decreased or absent. The sensorium remains
intact, and there is no peripheral loss of sensation. Respiratory depression
may occur with rapid, irregular respiration and intercostal and substernal
retraction. The blood pressure is lowered, but shock has not been observed.
Treatment
Gastric lavage or induction of emesis should be carried
out, followed by administration of activated charcoal. Thereafter, treatment is
entirely supportive. If respirations are depressed, oxygen and artificial
respiration should be employed and a patent airway assured by use of an
oropharyngeal airway or endotracheal tube. Hypotension may be counteracted by
use of dextran, plasma, concentrated albumin or a vasopressor agent such as
norepinephrine. Cholinergic drugs or analeptic drugs are of no value and should
not be used.
Usual Adult Dosage
One tablet three or four times
daily. If adequate response is not obtained with this dose, it may be increased
to one and one half tablets (750 mg) three or four times daily. As improvement
occurs dosage can usually be reduced.
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