Cyclobenzaprine

Cyclobenzaprine is a skeletal muscle relaxant. It is marketed as Flexeril (5 and 10 mg tablets), and also as Fexmid (7.5 mg tablet). Both the 5 and 10 milligram tablets are available generically. Fexmid is not available generically and few pharmacies carry it for this reason. Once-a-day extended release formulation, Amrix, has been approved by the American FDA in 2007 and is available in 15 and 30 mg capsules.^[1]

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Mechanism of action

The exact mechanism of action for cyclobenzaprine is unknown. Current research appears to indicate that cyclobenzaprine acts on the locus coeruleus where it results in increased norepinephrine release, potentially through the gamma fibers which innervate and inhibit the alpha motor neurons in the ventral horn of the spinal cord.[1] Decreased firing of the alpha motor neuron results in decreased muscular tone. Cyclobenzaprine is a muscle relaxant acting primarily on the central nervous system. It is structurally similar to Amitriptyline, differing by only one double bond. Cyclobenzaprine is a weak inhibitor of presynaptic norepinephrine and serotonin. Skeletal muscle relaxant activity is due to brainstem mediated inhibition of gamma motor neurons[2]

Indications

Cyclobenzaprine is typically prescribed to relieve pain and muscle spasms. Typically, muscle spasms occur in an injury to stabilize the affected body part and prevent further damage. The spasm of the muscles can increase the pain level. It is believed that by decreasing muscular spasm, pain is diminished. A common application would be that of a whiplash injury in a car accident. Muscle relaxants such as Cyclobenzaprine (Flexeril) and Orphenadrine Citrate (Norflex) have also been studied in the treatment of fibromyalgia. In a study of 120 fibromyalgia patients, those receiving Cyclobenzaprine (10 to 40 mg) over a 12 week period had significantly improved quality of sleep and pain score. Interestingly, there was also a reduction in the total number of tender points and muscle tightness.

It is also prescribed off-label as a sleep-aid.

To avoid possible stomach sickness, take with food and a full glass of water.

Side effects

Common side effects include drowsiness, depression, headaches, dizziness, and blurred vision. Other side effects are respiratory depression and decreased functionality in various muscles. Long term use has been associated with vision damage. Another side effect is dryness of the mouth [3]. Agitation is a common side effect observed especially in the elderly[4]

Interactions

When you are taking cyclobenzaprine, it is especially important that your health care professional know if you are taking any of the following:

• Alcohol
• Central nervous system (CNS) depressants (medicines that cause drowsiness) or
• Tricyclic antidepressants (amitriptyline [e.g., Elavil], amoxapine [e.g., Asendin], clomipramine [e.g., Anafranil], desipramine [e.g., Pertofrane], doxepin [e.g., Sinequan], imipramine [e.g., Tofranil], nortriptyline [e.g., Aventyl], protriptyline [e.g., Vivactil], trimipramine [e.g., Surmontil])—The chance of side effects may be increased
• Monoamine oxidase (MAO) inhibitors (furazolidone [e.g., Furoxone], phenelzine [e.g., Nardil], procarbazine [e.g., Matulane], selegiline (e.g., Eldepryl), tranylcypromine [e.g., Parnate])—Taking cyclobenzaprine while you are taking or within 2 weeks of taking MAO inhibitors may increase the chance of side effects

Legality

Cyclobenzaprine is regulated in the U.S. for prescription use only. Cyclobenzaprine does not fall within most governmental guidelines as a controlled substance, however possession without a valid / current prescription may be illegal depending upon various state and local laws.

Abuse

Cyclobenzaprine doesn't seem to be particularly popular in recreational drug-using communities, despite having an arguably high potential for abuse.

When used for illicit purposes, the drug is often referred to as "cyclone" or "mellow yellow" with recreational doses ranging from 20 to 80 mg. At these dosages, users report mild to moderate drowsiness and relaxation as the primary effects. Compared with other commonly abused CNS depressants, cyclobenzaprine's effects are considered to be mild, limiting its popularity as a recreational drug.

Cyclobenzaprine, on the other hand, can induce moderate to severe anticholinergic effects at higher doses, as well as benzodiazepine-like sedation and often pleasurable muscle-relaxation. At even higher doses, cyclobenzaprine may cause severe ataxia, and due to excessive muscle-relaxation, and possibly disorienting side effects such as a floating sensation or other imagined movements (usually experienced when at rest.)

It is important to note that cyclobenzaprine is not the only muscle relaxant with increased intensity of abuse. Soma, or carisoprodol, is a muscle relaxant that carries increased abuse potential. A handful of states such as Georgia have classified the drug as a Schedule IV controlled substance. This classification includes the majority of benzodiazepines, non-benzodiazapine sleep agents, and dextro-propoxyphene (a mild narcotic analgesic).

Overdose

Although reportedly unpleasant, cyclobenzaprine is relatively benign in case of overdose. However, depending on the amount taken and on the many different factors unique each individual, harmful overdose effects can occur. Note that the susceptibility to these potentially damaging effects is greatly increased when cyclobenzaprine is used in conjunction with other drugs, particularly central nervous system depressants and antidepressants. Use of cyclobenzaprine with an MAOI (monoamine oxidase inhibitor) will very possibly result in fatality. A case of rhabdomyolysis (muscle breakdown) associated with its overdose has been reported in the scientific literature. This is a rare though potentially fatal complication. Treatment protocols and support should follow the same as for any tricyclic anti depressant.[5]

References

1. ^ Amrix website