Cyclobenzaprine

Skeletal muscle relaxant (centrally-acting), Tricyclic-related compound

Cyclobenzaprine

Flexeril (IR; brand discontinued in US, generic widely available), Amrix (ER)

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Summary

Classes

Skeletal muscle relaxant (centrally-acting), Tricyclic-related compound

Common uses

Short-term relief of muscle spasm from acute musculoskeletal conditions (FDA; ≤2-3 weeks)▲0▼, Fibromyalgia (off-label, low-dose evening)▲0▼, Insomnia adjunct (off-label, low-dose)▲0▼

Pharmacy

Starting dose

IR 5 mg PO TID; titrate to 10 mg TID if needed. Amrix ER 15 mg PO once daily. Off-label insomnia / fibromyalgia: 5-10 mg PO at bedtime

Preparations

IR tablets 5, 7.5, 10 mg; Amrix ER capsules 15, 30 mg

US FDA Max

30 mg/day (IR or ER)

Pharmacology

Routes

Oral

Onset

~1 hour

Duration

12-24 hours (IR); 24 hours (Amrix ER)

Half-life

18 hours (IR); 32-33 hours (ER)

Bioavailability

~33-55% (IR); reduced by food

Pregnancy

Limited human data.^{[citation needed]}

Legal status

Rx-only in US. Not a controlled substance, distinguishing it from the carisoprodol alternative for muscle spasm

Purported mechanism

Structurally a tricyclic compound closely related to amitriptyline but FDA-approved as a muscle relaxant rather than an antidepressant. Centrally-acting via brainstem and supraspinal sites, reducing tonic somatic motor activity through noradrenergic descending pathways; the spinal cord is not the primary site of action. Substantial anticholinergic, H1 antihistaminergic, and α1-adrenergic antagonist activity accounts for sedation, dry mouth, and orthostatic adverse effects.▲0▼ Serotonin syndrome risk applies through the tricyclic-class identity when combined with SSRIs, SNRIs, MAOIs, tramadol, or other serotonergic agents. FDA labeling restricts to short-term use (≤2-3 weeks); evidence for prolonged use is limited.