AMRIX
Clinical safety rating: caution
Comprehensive clinical and safety monograph for AMRIX (AMRIX).
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.
| Metabolism | Hepatic via deamination; primarily metabolized by monoamine oxidase B (MAO-B) to inactive metabolites. |
| Excretion | Renal: approximately 40% as unchanged drug and metabolites; biliary/fecal: minimal; total clearance: 2.5 L/min |
| Half-life | Terminal elimination half-life approximately 32 hours (range 28–40 hours); clinically relevant for once-daily dosing in chronic muscle spasm |
| Protein binding | 40–45% bound to serum proteins, primarily albumin |
| Volume of Distribution | 5–8 L/kg; suggests extensive tissue distribution, including skeletal muscle |
| Bioavailability | Oral: 85–95% (extended-release formulation) |
| Onset of Action | Oral: 1–2 hours; maximal effect by 3–4 hours |
| Duration of Action | 24 hours with once-daily dosing; sustained muscle relaxant effect for 24 hours |
15 mg orally once daily. May increase to 30 mg once daily if needed, after at least 1 week. Maximum 30 mg/day.
| Dosage form | CAPSULE, EXTENDED RELEASE |
| Renal impairment | No specific dose adjustment recommended; use with caution in severe renal impairment (CrCl < 30 mL/min). |
| Liver impairment | 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. |
| Pediatric use | Safety and efficacy not established in pediatric patients under 12 years. For ages 12 and older, same as adult dosing. |
| Geriatric use | Initiate at 15 mg once daily. Due to higher incidence of anticholinergic effects and falls, monitor closely; consider lower doses in frail elderly. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for AMRIX (AMRIX).
| Breastfeeding | 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. |
| Teratogenic Risk | 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). |
■ FDA Black Box Warning
None
| Serious Effects |
["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."]
| Precautions | ["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."] |
| Food/Dietary | 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. |
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| Fetal Monitoring |
| Monitor maternal vital signs, sedation level, and signs of anticholinergic effects. Fetal monitoring is not routinely indicated unless maternal overdose or adverse effects occur. |
| Fertility Effects | No data on human fertility impairment. In animal studies, no effect on fertility was observed. Use in women of childbearing potential should weigh risks and benefits. |
| Clinical Pearls | 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. |
| Patient Advice | 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. |