A muscle relaxant for muscle spasm
Chlorzoxazone
A muscle relaxant taken by mouth for short-term relief of painful muscle spasm, with a rare but serious risk of liver injury.
What is Chlorzoxazone?
Chlorzoxazone is a muscle relaxant taken by mouth for short-term relief of painful muscle spasm, usually alongside rest and physiotherapy. It works on the nervous system to ease muscle spasm and tends to cause drowsiness. The most important safety point is that, although rare, it can cause serious liver injury, so any signs of liver trouble such as yellowing of the skin, dark urine, severe tiredness or tummy pain should be reported at once. It is meant for short-term use only. It is largely a United States medicine and is not generally used in the UK.
Education and reference only. This is a plain-language guide to Chlorzoxazone — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Chlorzoxazone is a muscle relaxant used for the short-term relief of discomfort from painful muscle spasm, normally as part of a plan that also includes rest, pain relief and physiotherapy. It is taken by mouth and works on the nervous system rather than directly on the muscle. It is intended for short courses only. It is largely a United States medicine and is not in routine use in the UK, where muscle pain is usually managed in other ways. A harmless but startling effect is that it can turn the urine an orange or reddish colour.
How it works
Chlorzoxazone acts on the spinal cord and parts of the nervous system to reduce the reflexes that drive muscle spasm, which is felt as muscle relaxation and relief of the associated pain. It also has a calming, drowsy-making effect. Because it is processed by the liver, in rare cases it can trigger serious liver inflammation, which is why liver-related symptoms must be watched for. Its effect is short-lived, so it is taken a few times a day during a short course while the underlying muscle problem settles with rest and physiotherapy.
Company & origin
Originated / developed by: Various manufacturers.
A muscle relaxant used for short-term relief of muscle spasm and discomfort, mainly in the United States; it is not generally used in the UK.
Practical use
How to take Chlorzoxazone
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Take it by mouth exactly as prescribed, for a short course only.
- Avoid driving or operating machinery until you know how drowsy it makes you.
- Avoid alcohol and other sedatives, which add to its drowsiness.
- Stop it and seek advice straight away if you notice yellowing of the skin, dark urine or severe tiredness.
- Use it alongside rest and physiotherapy rather than on its own long-term.
Weighing it up
Advantages & disadvantages of Chlorzoxazone
Advantages
- Can give short-term relief from painful muscle spasm alongside rest and physiotherapy.
- Taken by mouth as a simple short course.
- May help people get moving again when muscle spasm is limiting them.
Disadvantages
- Rarely, can cause serious liver injury, which needs prompt attention.
- Sedating, so it affects driving, concentration and daily activities.
- For short-term use only, and largely not used in the UK.
Practical use
Good to know
The most important thing to know about chlorzoxazone is that, although it is rare, it can cause serious liver injury that can occur unpredictably; any signs such as yellowing of the skin or eyes, dark urine, pale stools, severe tiredness, nausea or upper-tummy pain should be reported straight away and the medicine stopped. It is sedating, so it can affect driving and concentration, and combining it with alcohol or other sedatives increases drowsiness. It can also turn the urine orange or reddish, which is harmless. It is for short-term use only and works best as part of a plan with rest and physiotherapy. It is largely a United States product and is not generally used in the UK, where simple pain relief and movement are usually preferred. Tell your prescriber about liver problems and all other medicines before starting.
Who should not take it / use with caution
- People who have had a serious allergic reaction to chlorzoxazone should not take it.
- It is used with great caution, or avoided, in people with liver problems.
- It is used with caution in older people, who are more sensitive to its drowsy effects.
- It should not be combined with alcohol or other sedatives unless advised.
Monitoring
- Watching for signs of liver problems, such as yellowing of the skin, dark urine or severe tiredness.
- Reviewing whether it is still needed, keeping use short.
- Watching for excessive drowsiness, especially in older people.
Side effects
- Drowsiness, dizziness or feeling unsteady.
- A harmless orange or reddish colouring of the urine.
- Nausea or stomach upset in some people.
- Rarely but seriously, liver injury, with yellowing of the skin, dark urine and severe tiredness needing urgent attention.
Key interactions
- Alcohol and other sedatives add to its drowsiness.
- It is used with care alongside other medicines that can affect the liver.
- Tell your prescriber about all medicines you take, as some add to its sedating effect.
Available as: Tablets taken by mouth.
Answers
Chlorzoxazone: frequently asked questions
What is chlorzoxazone used for?
It is a muscle relaxant used for short-term relief of painful muscle spasm, usually alongside rest and physiotherapy.
What is the main safety concern?
Although rare, it can cause serious liver injury, so report any yellowing of the skin, dark urine, severe tiredness or tummy pain straight away and stop the medicine.
Why has my urine turned orange?
Chlorzoxazone can give the urine an orange or reddish colour, which is harmless and goes away after stopping the medicine.
Can I drink alcohol with it?
Alcohol adds to its drowsiness and can also stress the liver, so it is best avoided while taking chlorzoxazone.
Is it used in the UK?
It is mainly a United States medicine and is not generally used in the UK, where muscle pain is usually managed with simple pain relief, movement and physiotherapy.
The wider class
About Muscle relaxant (short-term use)
Chlorzoxazone belongs to the muscle relaxant (short-term use) class. For how the class as a whole works, its shared safety principles and monitoring, see the full guide.
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Authoritative sources
- BNF
- NICE CKS
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