Cyclobenzaprine is a muscle relaxant that works in the central nervous system by altering the signals from the brain that cause muscles to tighten. It is marketed as Apo-Cyclobenzaprine (10 mg tablets) Flexeril (5 and 10 mg tablets), and also as Fexmid (7.5 mg tablet). Both Flexeril and Fexmid are available in generic form.

Once-a-day extended-release formulation Amrix was approved by the U.S. Food and Drug Administration (FDA) in 2007 and is available in 15- and 30-mg capsules. Cyclobenzaprine is structurally related to the first-generation tricyclic antidepressants such as imipramine and amitriptyline. The exact mechanism of action for cyclobenzaprine is unknown.

Current research appears to indicate that cyclobenzaprine has effects on the locus ceruleus, where it — like many tricyclic antidepressants — acts to inhibit the uptake of norepinephrine, resulting in increased transynaptic norepinephrine concentration. Tricyclic compounds with norepinephrine reuptake-inhibiting properties (e.g., amitriptyline) have been shown to exert analgesic effects in chronic nerve and muscle pain. Cyclobenzaprine also causes inhibition of descending serotonergic systems in the spinal cord by blocking 5-HT2A and 5-HT2C receptors.

This action is thought to have an inhibitory effect on the alpha motor neurons in the ventral horn of the spinal cord, thereby resulting in decreased firing of alpha-motor neurons and a reduction in spinal mono- and polysynaptic reflexes.

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