Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
AMRIX vs CYCLOBENZAPRINE HYDROCHLORIDE
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
Centrally acting muscle relaxant; it is the R-enantiomer of baclofen. Agonist at GABA-B receptors in the spinal cord, leading to inhibition of monosynaptic and polysynaptic spinal reflexes, thereby reducing muscle spasticity.
Cyclobenzaprine is a centrally acting muscle relaxant that reduces tonic somatic motor activity at the supraspinal level, primarily at the brainstem reticular formation and descending pathways. It is structurally related to tricyclic antidepressants and inhibits reuptake of norepinephrine and serotonin, but the direct relationship to its muscle relaxant effects is not fully established.
Treatment of spasticity due to multiple sclerosis, spinal cord injury, or other spinal cord disorders
Treatment of muscle spasm associated with acute, painful musculoskeletal conditions (FDA approved),Adjunct to rest and physical therapy for relief of muscle spasm (FDA approved)
15 mg orally once daily. May increase to 30 mg once daily if needed, after at least 1 week. Maximum 30 mg/day.
Adults: 5 mg orally three times daily; may increase to 10 mg three times daily based on response. Maximum 30 mg per day.
Terminal elimination half-life approximately 32 hours (range 28–40 hours); clinically relevant for once-daily dosing in chronic muscle spasm
Terminal half-life: 18–24 hours (range 8–37 hours). Clinical context: requires multiple doses to achieve steady state (5–6 days); active metabolite norcyclobenzaprine has half-life ~30 hours.
Hepatic via deamination; primarily metabolized by monoamine oxidase B (MAO-B) to inactive metabolites.
Hepatic metabolism primarily via CYP3A4, CYP1A2, and CYP2D6; also undergoes N-demethylation and glucuronidation. Active metabolites include norcyclobenzaprine.
Renal: approximately 40% as unchanged drug and metabolites; biliary/fecal: minimal; total clearance: 2.5 L/min
Renal: ~50% as unchanged drug and metabolites; Fecal: ~40% primarily as metabolites; Biliary: minimal.
40–45% bound to serum proteins, primarily albumin
~93% bound to albumin and alpha-1-acid glycoprotein.
5–8 L/kg; suggests extensive tissue distribution, including skeletal muscle
~5 L/kg (range 3–7 L/kg). Clinical meaning: extensive tissue distribution, including central nervous system.
Oral: 85–95% (extended-release formulation)
Oral: 33–55% due to first-pass metabolism; lower for immediate-release compared to extended-release (same extent but slower absorption).
No specific dose adjustment recommended; use with caution in severe renal impairment (Cr Cl < 30 m L/min).
No specific dosing adjustment recommended; use caution in severe renal impairment due to potential accumulation.
Contraindicated in Child-Pugh class C. For Child-Pugh class A or B: initiate at 15 mg once daily; do not increase dose. Use with caution.
Child-Pugh Class A or B: No adjustment. Child-Pugh Class C: Contraindicated due to risk of toxicity (minimal data). Use with caution in mild to moderate impairment; consider lower starting dose.
Safety and efficacy not established in pediatric patients under 12 years. For ages 12 and older, same as adult dosing.
Not recommended for children under 15 years; safety and efficacy not established. For adolescents ≥15 years: same as adult dosing.
Initiate at 15 mg once daily. Due to higher incidence of anticholinergic effects and falls, monitor closely; consider lower doses in frail elderly.
Start with 5 mg once daily; increase slowly to a maximum of 10 mg three times daily over 2 weeks. Increased sensitivity; monitor for anticholinergic effects and sedation.
None
None
Abrupt discontinuation may precipitate withdrawal syndrome including hallucinations, seizures, autonomic instability.,May cause sedation, dizziness, and muscle weakness; caution with activities requiring alertness.,Use with caution in patients with impaired renal function due to reduced clearance.,May exacerbate seizures in patients with epilepsy.,Avoid concomitant use with other CNS depressants.
Serotonin syndrome risk, especially with concomitant serotonergic drugs (e.g., SSRIs, SNRIs, MAOIs),Sedation and impairment of motor skills; caution with driving or operating machinery,Anticholinergic effects (e.g., urinary retention, angle-closure glaucoma, constipation),Cardiovascular effects: tachycardia, QT prolongation, arrhythmias (especially in elderly or with pre-existing heart disease),Hepatic impairment: use with caution; reduced clearance in mild impairment, avoid in severe impairment,Withdrawal symptoms after abrupt discontinuation: dysphoria, anxiety, insomnia,Elderly patients: increased risk of falls, confusion, anticholinergic toxicity
Hypersensitivity to amrix or baclofen.,Abrupt withdrawal is contraindicated; must be tapered.,Concomitant use with MAO inhibitors is contraindicated due to risk of hypertensive crisis.
Hypersensitivity to cyclobenzaprine or any component of the formulation,Concomitant use or within 14 days of MAO inhibitors (hypertensive crisis risk),Acute recovery phase after myocardial infarction,Arrhythmias, heart block, or conduction disturbances,Hyperthyroidism,Severe hepatic impairment
Avoid grapefruit and grapefruit juice during treatment as they may increase cyclobenzaprine levels. Taking AMRIX with or without food does not significantly affect absorption. Alcohol should be strictly avoided as it potentiates CNS depression.
Alcohol should be avoided due to additive CNS depression. Grapefruit juice may increase cyclobenzaprine levels (though data is limited, caution is advised). High-fat meals may delay absorption but not clinically significant. No specific dietary restrictions are required.
Cyclobenzaprine (AMRIX) is classified as FDA Pregnancy Category B. Animal studies have not demonstrated fetal risk, but adequate well-controlled studies in pregnant women are lacking. Use only if clearly needed. First trimester: no specific teratogenic effects documented; second and third trimesters: avoid near term due to potential neonatal effects (e.g., sedation, withdrawal).
Cyclobenzaprine is classified as FDA Pregnancy Category B. Animal reproduction studies have not shown fetal risk, and there are no adequate and well-controlled studies in pregnant women. Risk cannot be ruled out. First trimester: Limited human data, but no structural anomalies reported. Second trimester: No specific adverse effects documented. Third trimester: Potential for neonatal withdrawal symptoms (e.g., jitteriness, respiratory depression) if used near term.
Cyclobenzaprine is excreted into human milk in small amounts. M/P ratio: not established. Use with caution in nursing mothers; monitor infant for sedation, poor feeding, or hypotonia.
Cyclobenzaprine is excreted into breast milk in low amounts; the M/P ratio is unknown. Due to its anticholinergic effects, there is potential for adverse effects in the nursing infant (e.g., sedation, constipation). The American Academy of Pediatrics considers it compatible with breastfeeding, but caution is advised; alternatives may be preferred.
No specific dose adjustments are recommended based on pharmacokinetic changes in pregnancy; however, due to potential for increased clearance, lowest effective dose should be used. Avoid use during labor and delivery due to potential neonatal depression.
No specific dose adjustments are recommended during pregnancy. Pharmacokinetic parameters (e.g., clearance) are not significantly altered by pregnancy. Use the lowest effective dose for the shortest duration due to lack of safety data.
AMRIX (cyclobenzaprine extended-release) should not be used longer than 2-3 weeks due to lack of evidence for efficacy in muscle spasm beyond that period. It has significant anticholinergic effects; avoid in patients with glaucoma, urinary retention, or those taking MAOIs. Do not crush or chew capsules; administer once daily at same time. Onset of action is delayed compared to immediate-release cyclobenzaprine.
Cyclobenzaprine is structurally related to tricyclic antidepressants and shares similar anticholinergic and sedative properties. Onset of action for muscle relaxation is typically 1 hour, but maximal effect may take several days. Avoid use in patients with hyperthyroidism, cardiac disease, or those on MAOIs. Not recommended for use longer than 2-3 weeks due to lack of evidence for chronic use. Caution in elderly due to anticholinergic effects and fall risk.
Take AMRIX exactly once daily at the same time each day; do not crush, chew, or open the capsule.,Avoid alcohol and other CNS depressants (e.g., benzodiazepines, opioids) as they increase the risk of severe drowsiness and dizziness.,Do not drive or operate heavy machinery until you know how AMRIX affects you; it may cause drowsiness, dizziness, or blurred vision.,Contact your healthcare provider if you experience symptoms of serotonin syndrome (e.g., agitation, hallucinations, rapid heart rate, fever, muscle stiffness, nausea, diarrhea).,Do not use AMRIX for longer than 2-3 weeks unless specifically directed by your doctor; prolonged use is not recommended.,Inform your doctor if you have a history of urinary retention, glaucoma, thyroid disorders, heart problems, or liver disease.,If you miss a dose, take it as soon as you remember unless it is almost time for your next dose; do not double the dose.
This medication may cause drowsiness, dizziness, or blurred vision; avoid driving or operating machinery until you know how it affects you.,Do not drink alcohol or use other CNS depressants (e.g., benzodiazepines, opioids) while taking this medication, as it may increase sedation.,Take this medication exactly as prescribed, usually 3 times a day. Do not take more or less than directed.,This medication is intended for short-term use (up to 2-3 weeks) for muscle spasm. Do not use it for longer without consulting your doctor.,If you experience dry mouth, try sucking on sugar-free candy or ice chips. If you have difficulty urinating or vision changes, contact your doctor.,Do not stop taking this medication abruptly without consulting your doctor, although withdrawal is uncommon with short-term use.
No interactions on record
"The combination of cyclobenzaprine and carbinoxamine results in additive central nervous system depression due to their shared anticholinergic and sedative properties. This can lead to excessive sedation, impaired cognitive and motor function, and increased risk of falls or accidents. Severe cases may result in respiratory depression, especially in elderly patients or those with preexisting conditions."
"Cyclobenzaprine, a centrally acting muscle relaxant with tricyclic antidepressant (TCA)-like structure, and Dezocine, an opioid partial agonist analgesic with mu-opioid receptor activity, both depress the central nervous system (CNS) and have additive serotonergic effects. Concomitant use increases the risk of excessive CNS depression, manifesting as sedation, respiratory depression, and impaired psychomotor function, as well as potential serotonin syndrome due to combined serotonergic activity. Clinically, patients may experience profound drowsiness, confusion, respiratory compromise, and in severe cases, coma or death from respiratory failure."
"Lumacaftor, a potent inducer of cytochrome P450 (CYP) 3A4, significantly reduces the systemic exposure of cyclobenzaprine, a CYP3A4 substrate. This results in decreased plasma concentrations of cyclobenzaprine, potentially leading to reduced therapeutic efficacy for muscle spasm relief. Patients may require dose adjustments or alternative therapies to maintain clinical benefit."
Explore head-to-head clinical comparisons of other medications in the same therapeutic classes.
Common clinical questions about AMRIX vs CYCLOBENZAPRINE HYDROCHLORIDE, answered by our medical review team.
AMRIX is a Muscle Relaxant that works by Centrally acting muscle relaxant; it is the R-enantiomer of baclofen. Agonist at GABA-B receptors in the spinal cord, leading to inhibition of monosynaptic and polysynaptic spinal reflexes, thereby reducing muscle spasticity.. CYCLOBENZAPRINE HYDROCHLORIDE is a Skeletal Muscle Relaxant that works by Cyclobenzaprine is a centrally acting muscle relaxant that reduces tonic somatic motor activity at the supraspinal level, primarily at the brainstem reticular formation and descending pathways. It is structurally related to tricyclic antidepressants and inhibits reuptake of norepinephrine and serotonin, but the direct relationship to its muscle relaxant effects is not fully established.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between AMRIX and CYCLOBENZAPRINE HYDROCHLORIDE depend on the specific clinical indication. These are agents from distinct pharmacological classes and are not directly interchangeable by dose. A physician or clinical pharmacist should guide any therapeutic switching decisions.
The standard adult dose of AMRIX is: 15 mg orally once daily. May increase to 30 mg once daily if needed, after at least 1 week. Maximum 30 mg/day.. The standard adult dose of CYCLOBENZAPRINE HYDROCHLORIDE is: Adults: 5 mg orally three times daily; may increase to 10 mg three times daily based on response. Maximum 30 mg per day.. Dosing should always be individualized based on indication, renal and hepatic function, age, and other patient factors.
No direct drug-drug interaction has been formally documented between AMRIX and CYCLOBENZAPRINE HYDROCHLORIDE in current clinical databases. However, individual patient risk factors including other medications, organ function, and comorbidities should always be evaluated by a qualified healthcare provider.
The maternal-fetal safety profiles differ. AMRIX is classified as Category C. Cyclobenzaprine (AMRIX) is classified as FDA Pregnancy Category B. Animal studies have not demonstrated fetal risk, but adequate well-controlled studies in pregnant women are lacki. CYCLOBENZAPRINE HYDROCHLORIDE is classified as Category A/B. Cyclobenzaprine is classified as FDA Pregnancy Category B. Animal reproduction studies have not shown fetal risk, and there are no adequate and well-controlled studies in pregnant . Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.