What is cyclobenzaprine?
Cyclobenzaprine is a prescription-only oral medication used to treat muscle spasms, usually taken for two or three weeks. It is often used in combination with rest and physiotherapy to relieve pain and discomfort caused by muscle injuries and strains.
Cyclobenzaprine is classed as a muscle relaxer, or relaxant, that specifically works on skeletal muscle (not cardiac muscle or smooth muscle).
A starting dose often consists of 5mg regular-release tablets, taken three times a day. Dosage may be increased to three 10mg tablets per day if the starting dose is ineffective. Alternatively, extended-release formulas (such as those sold under the brand name Amrix) are available as 15mg or 30mg capsules taken once per day.
Cyclobenzaprine is available both as a generic medication and under the brand name Flexeril (discontinued in the US), as well as Amrix, Fexmid, and others.
What is cyclobenzaprine hydrochloride (HCl)?
Cyclobenzaprine/cyclobenzaprine hydrochloride/cyclobenzaprine HCl all refer to the same drug.
The word ‘hydrochloride’ (HCl) simply gives more information about its chemical makeup but is often dropped from the name for convenience.
Cyclobenzaprine, like many medications, is made into its salt form to improve its solubility in water in order to increase absorption and provide the maximum therapeutic effect. Hydrochloride is the salt of cyclobenzaprine.
Other common salts found in medicine include:
- Sodium
- Sulfate
- Acetate
- Potassium
What is cyclobenzaprine used for?
The FDA recommends cyclobenzaprine, in addition to rest and physiotherapy, for relief of muscle spasms associated with acute, painful musculoskeletal conditions.
Cyclobenzaprine is not approved as a treatment for long-term (chronic) musculoskeletal conditions, such as fibromyalgia, although it is commonly prescribed for such conditions (particularly to help with sleep).
The FDA states that cyclobenzaprine should only be used for two to three weeks. Longer treatment is not normally needed for the type of conditions cyclobenzaprine is used for and evidence regarding the effectiveness and safety of long-term use is lacking.
Cyclobenzaprine has not been found effective in the treatment of spasticity associated with cerebral or spinal cord disease, or in children with cerebral palsy.
Is cyclobenzaprine available in the UK and Europe?
Availability of cyclobenzaprine varies around the globe. While it is widely available in the US, Canada, and North America, as well as Australia and New Zealand, it is not licensed in the UK nor throughout Europe.
How does cyclobenzaprine work?
The way in which cyclobenzaprine works is not fully understood. However, it is believed to act upon the central nervous system – specifically in the brain stem – and block nerve impulses that cause pain, helping muscles to relax. It is therefore classed as a centrally-acting skeletal muscle relaxer (SMR).
It is believed to act in a similar way to other centrally-acting SMRs, such as metaxalone.
Cyclobenzaprine is structurally similar to tricyclic antidepressants (TCAs) such as amitriptyline (cyclobenzaprine was itself first studied as an antidepressant). Like TCAs, cyclobenzaprine is associated with ‘anticholinergic’ side effects, caused by the blocking of a neurotransmitter called acetylcholine.
Anticholinergic side effects include drowsiness, dizziness, and dry mouth. You can read more about anticholinergic side effects and the other side effects of cyclobenzaprine in the following question: What are the side effects of cyclobenzaprine?
What are the side effects of cyclobenzaprine?
As mentioned above, anticholinergic side effects are common when taking cyclobenzaprine. They include:
- Drowsiness or sedation
- Dizziness
- Dry mouth
- Blurriness
Other common side effects include:
- Headache
- Stomach pain
- Lack of appetite
- Nausea
- Constipation
- Gas
- Muscle weakness
These side effects do not usually need medical attention and will often ease as your body gets used to the drug. However, if they are severe, persistent, get worse, or interfere with your daily life, you should speak to your doctor.
More serious side effects include:
- Extreme confusion
- Increased heart rate
- Hallucinations
- Urinary retention
- Fainting
- Depression
- Convulsions
- Dry, hot, flushed skin
- Change in body temperature
If you experience any of these side effects while taking cyclobenzaprine, you should seek immediate medical attention. They may also be signs of an overdose.
Allergic reactions to cyclobenzaprine are rare but can occur. Signs of a serious allergic reaction should be treated as a medical emergency. They include:
- Skin rash – for example itchy, red, or swollen skin
- Wheezing
- Tightness in the chest or throat
- Trouble breathing or talking
- Swollen mouth, face, lips, tongue, or throat
This is not an extensive list of possible side effects of cyclobenzaprine. For more information about side effects, please read the information leaflet that comes with the medication or speak to your doctor or pharmacist.
What warnings and precautions are there for taking cyclobenzaprine?
You should avoid driving or performing any hazardous activities when you first start taking cyclobenzaprine, as some of the potential side effects can severely impair your ability to do so.
Medical history
Because of the way cyclobenzaprine works, the FDA says it should be used with caution in patients with a history of urinary retention, angle-closure glaucoma, increased intraocular pressure, and in people taking other anticholinergic medications.
Cyclobenzaprine is not usually prescribed to people who have had a recent heart attack, live with heart disease, or who have an overactive thyroid.
Similarly, the sedating effects of cyclobenzaprine may be worse in people living with liver disease (hepatic impairment). Depending on the severity of the condition, cyclobenzaprine may be unsuitable or may need to be prescribed at a lower dosage.
You should, therefore, discuss your medical history with your doctor before taking cyclobenzaprine.
Pregnancy and breastfeeding
Based on animal studies, there is no evidence that cyclobenzaprine impairs fertility or harms a developing fetus. However, there is also a lack of evidence that it is safe in humans. It is, therefore, recommended that cyclobenzaprine should only be used when clearly needed during pregnancy. You should inform your doctor if you are pregnant or trying to conceive, so an alternative medication with a more defined safety profile may be prescribed.
Similarly, it is not known whether cyclobenzaprine passes into breast milk. Given its similarity to tricyclic antidepressants (TCAs) (see question 5: How does cyclobenzaprine work?), some of which are known to pass into breast milk, cyclobenzaprine may not be ideal for nursing mothers. Again, this must be discussed with your doctor.
Pediatric and elderly patients
Cyclobenzaprine is not recommended for patients under 15 years old, as its safety and effectiveness have not been established.
Cyclobenzaprine is only recommended in elderly patients when clearly required. In such cases, it may be prescribed at a low dosage and slowly titrated upwards (increased). This is because plasma concentration – the amount of the medication in a sample of plasma from the blood – may be increased in elderly patients.
Elderly patients may also be more at risk for adverse events relating to the central nervous system, such as hallucinations and confusion, cardiac events resulting in falls, drug-drug, and drug-disease interactions (see below: What are possible interactions with cyclobenzaprine?).
What are possible interactions with cyclobenzaprine?
Cyclobenzaprine interacts with a class of antidepressant drugs called monoamine oxidase inhibitors (MAOIs), including:
- Rasagiline (Azilect)
- Selegiline (Eldepryl)
- Isocarboxazid (Marplan)
- Phenelzine (Nardil)
- Tranylcypromine (Parnate)
Together, cyclobenzaprine and MAOIs can cause serotonin syndrome, which can be life-threatening. You should not take cyclobenzaprine if you have taken an MAOI in the past 14 days.
Other medications that are known to interact with cyclobenzaprine include:
- Tricyclic antidepressants (including amitriptyline, doxepin, and protriptyline, among others)
- SSRI antidepressants (including fluoxetine and citalopram, among others
- Antihistamines (including loratadine and diphenhydramine, among others)
- Any drugs for sleep or anxiety
- Other muscle relaxers
The interaction between cyclobenzaprine and the above drugs can cause severe drowsiness, and – like MAOIs – lead to serotonin syndrome.
This is only a small number of possible drug interactions with cyclobenzaprine. You should discuss all medications and supplements you take with your doctor before taking cyclobenzaprine. Similarly, you should tell your doctor if you take recreational drugs. Some, such as marijuana and MDMA (ecstasy), can lead to potentially serious interactions.
Aside from alcohol (see below: Can I drink alcohol while taking cyclobenzaprine?, no food or drinks are known to interact with cyclobenzaprine.
Can I drink alcohol while taking cyclobenzaprine?
You should avoid drinking alcohol with cyclobenzaprine.
Both substances depress the central nervous system, which is why both can cause drowsiness, dizziness, slowed mental processing, and decreased motor skills. Combining the two can greatly exacerbate these effects.
Should I take cyclobenzaprine with food and water?
Cyclobenzaprine, whether taken three times per day or once per day (for extended relief capsules), may be taken with or without food. Ideally, you should take cyclobenzaprine at the same time each day, so coinciding it with meals may help you do so.
It is usually recommended that you take cyclobenzaprine with a glass of water, without breaking, crushing, or chewing the tablet or capsule.
How long does it take for cyclobenzaprine to work?
Muscle relaxers, including cyclobenzaprine, tend to start working after roughly 30 minutes. This can vary depending on your individual physiology.
How long does cyclobenzaprine stay in your system?
The half-life of a drug refers to the time it takes for 50% of the drug to be eliminated from the body. With each subsequent half-life, a further 50% of the remaining drug is eliminated.
Regular-release cyclobenzaprine has an average half-life of 18 hours. It takes approximately 5.5 x the half-life of a drug for it to be cleared from your system, which would mean it takes about four days for cyclobenzaprine.
However, this is just the average. Your physiology, metabolic rate, age, health, weight, and other aspects can increase or decrease the half-life of cyclobenzaprine. At the upper end of the scale, the half-life can be as long as 37 hours, meaning it would take eight to nine days to be eliminated from your system.
If you take the extended-release form of cyclobenzaprine, this has an average half-life of 32-33 hours, meaning it takes just over a week to be cleared.
What should I do if I miss a dose of cyclobenzaprine?
If you miss a dose, take it as soon as you realize. If it is almost time for your next dose, skip the missed dose and stick to your usual schedule. Do not take a double dose to make up for the one you’ve missed.
What class of drug is cyclobenzaprine?
Cyclobenzaprine is a centrally-acting skeletal muscle relaxer (SMR), as discussed in question 5: How does cyclobenzaprine work?
It can further be classed as an ‘antispasmodic,’ referring to the fact it prevents muscle spasms.
Other muscle relaxers, such as baclofen, are ‘antispastics,’ meaning they reduce spasticity rather than spasms. These may be used in treatment for conditions such as multiple sclerosis (MS).
The content on this page is provided for informational purposes only. If you have any questions or concerns about your treatment, you should talk to your doctor, pharmacist, or healthcare professional. This is particularly important if you are taking multiple medications or have any existing medical conditions.
