Cyclobenzaprine

Cyclobenzaprine, sold under several brand names including, historically, Flexeril, 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.

Not to be confused with cyclosporine.

Cyclobenzaprine
Clinical data
Trade namesFlexeril, Amrix, others
AHFS/Drugs.comMonograph
MedlinePlusa682514
License data
Routes of
administration		By mouth
ATC code
Legal status
Legal status
  • US: ℞-only
  • In general: ℞ (Prescription only)
Pharmacokinetic data
Bioavailability33–55%
Protein binding93%
Metabolismmajor: CYP3A4, CYP1A2; minor: CYP2D6, N-demethylation
MetabolitesNorcyclobenzaprine
Elimination half-life32 hours (extended-release, range 8–37 hours), 18 hours (immediate release, range 8–37 hours)
ExcretionKidney
Identifiers
  • 3-(5H-dibenzo[a,d]cyclohepten-5-ylidene)-N,N-dimethyl-propan-1-amine
CAS Number
PubChem CID
IUPHAR/BPS
DrugBank
ChemSpider
UNII
KEGG
ChEBI
ChEMBL
CompTox Dashboard (EPA)
ECHA InfoCard100.005.588
Chemical and physical data
FormulaC20H21N
Molar mass275.395 g·mol−1
3D model (JSmol)
  • c3cc\2c(\C=C/c1c(cccc1)C/2=C/CCN(C)C)cc3
  • InChI=1S/C20H21N/c1-21(2)15-7-12-20-18-10-5-3-8-16(18)13-14-17-9-4-6-11-19(17)20/h3-6,8-14H,7,15H2,1-2H3 Y
  • Key:JURKNVYFZMSNLP-UHFFFAOYSA-N Y
  (verify)

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 must 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 2021, it was the 45th most commonly prescribed medication in the United States, with more than 15 million prescriptions. It was not available in the United Kingdom as of 2012.

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