Cyclobenzaprine (sold under the brand name Flexeril, among others) is a muscle relaxer used for muscle spasms from musculoskeletal conditions of sudden onset. It is not useful in cerebral palsy. It is taken by mouth. Use is not recommended for more than a few weeks. Common side effects include headache, feeling tired, dizziness, and dry mouth. Serious side effects may include an irregular heartbeat. There is no evidence of harm in pregnancy, but it has not been well studied in this population. It should not be used with an MAO inhibitor. How it works is unclear. Cyclobenzaprine was approved for medical use in the United States in 1977. It is available by prescription as a generic medication. In 2020, it was the 39th most commonly prescribed medication in the United States, with more than 16million prescriptions. It was not available in the United Kingdom as of 2012. Medical use Cyclobenzaprine is used, in conjunction with physical therapy, to treat muscle spasms that occur because of acute musculoskeletal conditions. After sustaining an injury, muscle spasms occur to stabilize the affected body part, which may increase pain to prevent further damage. Cyclobenzaprine is used 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 the best-studied muscle relaxer. It is not useful for spasticity due to neurologic conditions such as cerebral palsy. A 2004 review found benefit for fibromyalgia symptoms, with a reported number needed to treat of 4.8 (meaning that 1 person out of every 4.8 benefits from treatment) for pain reduction, but no change in fatigue or tender points. A 2009 Cochrane review found insufficient evidence to justify its use in myofascial pain syndrome. It may also be used along with other treatments for tetanus. Side effects Cyclobenzaprine results in increased rates of drowsiness (38%), dry mouth (24%), and dizziness (10%). 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. Some experts believe that cyclobenzaprine should be avoided in elderly patients because it can cause confusion, delirium, and cognitive impairment. In general,