Cyclobenzaprine, sold under the brand name Flexeril among others, is a medication 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 heart beat. 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 as a generic medication. In 2018, it was the 43rd most commonly prescribed medication in the United States, with more than 18 million prescriptions.