Cyclobenzaprine
Also known as: Flexeril, Amrix, cyclobenzaprine hydrochloride, muscle relaxant
Cyclobenzaprine (Flexeril) is a muscle relaxant prescribed for short-term relief of muscle spasms and associated pain — typically from acute musculoskeletal conditions like back injuries, neck pain, or muscle strains. It works in the brain and spinal cord to reduce muscle spasm activity. It's intended for short-term use only (2–3 weeks) and is notably sedating — many people take it at bedtime. It has structural similarity to tricyclic antidepressants and shares some of their interactions and side effects.
What it is
Cyclobenzaprine is a centrally-acting skeletal muscle relaxant structurally related to the tricyclic antidepressants (TCAs). It acts primarily in the brain stem and spinal cord to reduce tonic somatic motor activity, influencing both gamma and alpha motor neurons. Despite structural similarity to TCAs, it does not inhibit serotonin or norepinephrine reuptake to a meaningful degree at therapeutic doses, and it is not used as an antidepressant. Its predominant effects are sedation and muscle relaxation, mediated through antihistamine and anticholinergic mechanisms.1
FDA-approved indications
Cyclobenzaprine is FDA-approved as an adjunct to rest and physical therapy for relief of muscle spasm associated with acute, painful musculoskeletal conditions. It is indicated for short-term use only — typically 2–3 weeks. Long-term use has not been adequately studied.1
Typical dosing
Immediate-release: 5 mg three times daily; may increase to 10 mg three times daily. Maximum 30 mg per day. Use for no longer than 2–3 weeks.
Extended-release (Amrix): 15 mg once daily; may increase to 30 mg once daily.1
Route of administration
Oral tablets or extended-release capsules.
Storage & handling
Store at controlled room temperature, away from heat and moisture.1
Common considerations
Key considerations from the FDA label:1
- Serotonin syndrome: Cyclobenzaprine, due to its TCA-like structure, can contribute to serotonin syndrome when combined with MAOIs, SSRIs, SNRIs, triptans, tramadol, or other serotonergic drugs. The FDA label specifically warns against use with MAOIs and within 14 days of MAOI discontinuation.
- CNS depression: Causes significant sedation. Additive CNS depression with alcohol, benzodiazepines, opioids, and other sedatives. Avoid driving or operating machinery.
- Anticholinergic effects: Dry mouth, blurred vision, urinary retention, constipation — common at typical doses. Use with caution in patients with glaucoma, urinary retention, or enlarged prostate.
- Cardiac effects: TCA-like cardiac effects at high doses — QT prolongation and arrhythmia risk. Use with caution in patients with cardiac conduction abnormalities.
- Short-term use only: Not appropriate for long-term muscle spasm management or fibromyalgia. Evidence does not support use beyond 2–3 weeks.
- Older adults: Cyclobenzaprine is on the Beers Criteria list of medications potentially inappropriate for older adults due to anticholinergic effects and sedation increasing fall and cognitive impairment risk.
- Hyperthyroidism: Contraindicated in patients with hyperthyroidism due to TCA-like cardiac risks.
References
The information on this page is provided for general educational purposes only. It is not medical advice and is not a substitute for consultation with a qualified healthcare provider. Individual needs, contraindications, and responses to supplementation vary, and decisions about starting, stopping, or modifying any supplement or medication should be made in consultation with a physician, pharmacist, or other appropriate professional. References are provided to authoritative sources; STACK Tracker does not endorse any specific product or brand.