Dosing and uses of Flexeril, Amrix (cyclobenzaprine)
Adult dosage forms and strengths
tablet
- 5mg
- 7.5mg
- 10mg
capsule, extended-release
- 15mg
- 30mg
Muscle Spasm
Immediate-release tablet
- 5 mg PO q8hr; may increase dose to 7.5-10 mg PO q8hr PRN
Extended-release capsule
- 15 mg PO qDay; some patients may require up to 30 mg PO qDay
Geriatric Dosing
Muscle Spasm
- Immediate-release tablet: 5 mg/day PO initially; titrate slowly upward and consider less frequent dosing
- Extended-release capsule not recommended in elderly, because of increased plasma levels (40%) and prolonged half-life (56%) compared with young adults
Dosing Modifications
Hepatic impairment
- Immediate-release tablet: 5 mg/day PO initially; titrate slowly and consider less frequent dosing
- Extended-release capsule: Not recommended in mild-to-severe hepatic impairment
Renal Impairment
- Not studied
Pediatric dosage forms and strengths
tablet
- 5mg
- 7.5mg
- 10mg
Muscle Spasm
Immediate-release tablet
- <15 years: Safety and efficacy not established
- >15 years: 5 mg PO q8hr; may increase dose to 7.5-10 mg PO q8hr PRN
Extended-release capsule
- <18 years: Safety and efficacy not established
- >18 years: 15 mg PO qDay; some patients may require up to 30 mg PO qDay
Flexeril, Amrix (cyclobenzaprine) adverse (side) effects
>10%
Drowsiness (up to 39% immediate-release; 100% extended-release)
Dry mouth (21-32%)
Dizziness (3-11%)
1-10%
Pharyngitis (1-3%)
Headache (1-5%)
Fatigue (6%)
Palpitations (6%)
Bad taste in mouth (1-6%)
Indigestion (4%)
Blurred vision (3%)
Constipation (1-3%)
Asthenia (1-3%)
Confusion (1-3%)
Nausea (1-3%)
Nervousness (1-3%)
<1%
Arrhythmia
Hypotension
Palpitation
Syncope
Tachycardia
Vasodilation
Cardiac dysrhythmia (rare)
Cholestasis (rare)
Hepatitis
Jaundice
Anaphylaxis (rare)
Immune hypersensitivity reaction
Warnings
Contraindications
Hypersensitivity to drug or formulation components
Hyperthyroidism
During the acute recovery phase of myocardial infarction and in patients with arrhythmia, heart block or conduction disturbances, or congestive heart failure
Concomitant use or within 14 days of discontinuing MAO inhibitors
Hyperpyretic crisis seizures and deaths have occurred in patients receiving cyclobenzaprine (or structurally similar tricyclic antidepressants) concomitantly with MAO inhibitors
Cautions
Use only for short periods (2-3 wk)
Use caution in urinary retention, narrow-angle glaucoma or IOP, or concomitant use of other anticholinergic drugs
May cause drowsiness/dizziness; do not ingest alcohol or other CNS depressants; may impair ability to operate heavy machinery
May take with food to avoid stomach upset
Serotonin syndrome reported when coadministered with other drugs that increase serotonin (eg, SSRIs, SNRIs, TCAs, tramadol, bupropion, meperidine, verapamil, or MAO inhibitors [see also Contraindications])
Not effective for treatment of spasticity associated with cerebral/spinal cord disease or for pediatric cerebral palsy
Elderly patients may be more prone to adverse effects and require dose/frequency reduction
Use immediate release with caution in hepatic impairment; extended-release form not recommended with hepatic impairment
Pregnancy and lactation
Pregnancy category: B
Lactation: Excretion in milk unknown; use with caution
Pregnancy categories
A: Generally acceptable. Controlled studies in pregnant women show no evidence of fetal risk.
B: May be acceptable. Either animal studies show no risk but human studies not available or animal studies showed minor risks and human studies done and showed no risk.
C: Use with caution if benefits outweigh risks. Animal studies show risk and human studies not available or neither animal nor human studies done.
D: Use in LIFE-THREATENING emergencies when no safer drug available. Positive evidence of human fetal risk.
X: Do not use in pregnancy. Risks involved outweigh potential benefits. Safer alternatives exist.
NA: Information not available.
Pharmacology of Flexeril, Amrix (cyclobenzaprine)
Mechanism of action
Relieves local skeletal muscle spasm; clinical response similar to diazepam
Structurally related to cyclic antidepressants, and pharmacologic effects are similar, including reserpine antagonism, norepinephrine potentiation, potent peripheral and central anticholinergic effects, and sedation; reduces tonic somatic motor activity influencing alpha and gamma motor neurons
Absorption
Onset: 1 hr
Duration: 12-24 hr
Bioavailability: 33-55%
Peak plasma time: 7-8 hr
Peak plasma concentration: 15-25 ng/mL
Distribution
Protein bound: 93%
Metabolism
Hepatic via CYP3A4, 1A2, and 2D6; may undergo enterohepatic recirculation
Elimination
Half-life: 8-37 hours (immediate release); 32-33 hr (extended release)
Excretion: Urine, feces
