After sustaining an injury, muscle spasms may occur to stabilize the affected body part and prevent further damage. They also generate pain. Cyclobenzaprine is FDA-approved to treat such muscle spasms associated with acute, painful musculoskeletal conditions. It decreases pain in the first two weeks, peaking in the first few days, but has no proven benefit after two weeks. Since no benefit is proven beyond that, therapy should not be continued long-term. It is not useful for spasticity due to neurologic conditions such as cerebral palsy.
Cyclobenzaprine has also shown effectiveness in the treatment of fibromyalgia symptoms, with a report of 4.8 patients needing treatment for each (1) patient reporting pain reduction (but no change in fatigue or tender points).
Some experts believe that cyclobenzaprine should be avoided in elderly patients, because it can cause confusion, delirium, and cognitive impairment.
Meta-analysis studies have found significantly increased rates of drowsiness (38% of patients), dry mouth (24%), dizziness (10%), and adverse events of any kind in patients taking cyclobenzaprine versus placebo. Drowsiness and dry mouth appear to intensify with increasing dose.
The sedative effects of cyclobenzaprine are likely due to its antagonistic effect on histamine, serotonin, and muscarinic receptors. Agitation is a common side effect observed especially in the elderly. In general, the National Committee for Quality Assurance recommends avoiding the use of cyclobenzaprine in the elderly because of the potential for more severe side effects. Treatment protocols and support should follow the same as for any structurally related tricyclic, such as tricyclic antidepressants.[dead link]
The most common effects of overdose are drowsiness and tachycardia. Rare but potentially critical complications are cardiac arrest, cardiac dysrhythmias, severe hypotension, seizures, and neuroleptic malignant syndrome. Life-threatening overdose is rare, however, as the median lethal dose is about 338 mg/kg in mice and 425 mg/kg in rats. The potential harm is increased when central nervous system depressants and antidepressants are also used; deliberate overdose often includes alcohol among other drugs.
Cyclobenzaprine has major contraindications with monoamine oxidase inhibitors (MAOIs). At least one study also found increased risk of serotonin syndrome when cyclobenzaprine was taken with the serotonergic drugs duloxetine or phenelzine.
These substances may interact with cyclobenzaprine:
- Central nervous system depressants (e.g. opioids, benzodiazepines, nonbenzodiazepines, phenothiazines, carbamates, barbiturates)
- Tricyclic antidepressants may increase the chance of side effects.
- Monoamine oxidase inhibitors taken within two weeks of cyclobenzaprine may result in serious, life-threatening side effects.
If co-administered with opioids, the dose of one or both medicines should be reduced accordingly. The patient should be monitored for excessive sedation.