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, fatigue, 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
The concomitant use of alcohol or other central nervous system depressants may have an additive
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 judgment 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.