Chlorzoxazone
PARAFON FORTE
Pronunciation: klor-ZOX-a-zoen
“Cooler socks of Paraffin”
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.
Chlorzoxazone prescription history
Animated chart of estimated U.S. prescriptions per year, Muscle relaxants (1950–2026). Chlorzoxazone enters in 1958 (estimated).
FDA-approved for
- painful musculoskeletal spasm
Off-label
- CYP2E1 phenotyping probe in research settings
Principal riskIdiosyncratic hepatotoxicity, occasionally fatal
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.
FDA label
Read the full label on DailyMed ↗
NO BOXED WARNINGS
| 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
Available strengths
- tab 250, 375, 500, 750 mg
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
Cost
Generic entry1982
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Memory aids: interaction avatars
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Self-check
- Which enzyme is chlorzoxazone used to probe?
CYP2E1. - Does chlorzoxazone-associated orange urine imply harm?
No; harmless.