Chlorzoxazone
PARAFON FORTE
What it isSpinal-reflex inhibitor acting as a central muscle relaxant; acute musculoskeletal spasm.
Why this oneIts CYP2E1-dependent clearance makes it the standard research probe for that enzyme, a distinction with little clinical value.
What limits itIdiosyncratic hepatotoxicity can be fatal, so stop for liver symptoms or test abnormalities. Orange urine is harmless.
FDA-approved for
- painful musculoskeletal spasm
Off-label
- CYP2E1 phenotyping probe in research settings
FDA label
Read the full label on DailyMed ↗
| FDA dosing | Population | Start | Target | Max |
|---|---|---|---|---|
| Musculoskeletal conditions (muscle spasm) | Adults | 500 mg TID–QID | 500 mg TID–QID | 750 mg TID–QID |
Renal Undefined
Hepatic Undefined
Instructions
- Increase to 750 mg TID–QID if response inadequate; reduce as improvement occurs
Cautions
- Rare serious (sometimes fatal) hepatocellular toxicity; discontinue if signs of liver dysfunction
- Possible GI bleeding
- Drowsiness, dizziness — caution with driving
Adverse reactions
- Drowsiness, dizziness, lightheadedness, malaise, overstimulation
- GI bleeding (rare); allergic skin rashes, petechiae, ecchymoses
- Angioneurotic edema/anaphylaxis (extremely rare)
- Harmless urine discoloration (phenolic metabolite)
Pregnancy Safe use not established; use only if potential benefit outweighs risk
Lactation Undefined
NO BOXED WARNINGS
Principal riskIdiosyncratic hepatotoxicity, occasionally fatal
Cost, est. cash$10–$300/month
Generic entry1982
Classes and tags
Clinical profile
Legacy pregnancy categoryUnknown / historical label unavailable
Defining liabilityGenerally tolerated at therapeutic doses; clinically important risks are agent-specific and increase with interactions or organ impairment.
Mechanism of action: Chlorzoxazone ballicule
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