Metaxalone 400
Reading File
Finding Sources
Searching the Web
Metaxalone 400mg skeletal muscle relaxant pharmacology dosage mechanism
Reading File
Reading File
Reading File
I now have comprehensive information from multiple authoritative sources. Here is a complete overview of Metaxalone 400 mg:
Metaxalone 400 mg (Skelaxin)
Drug Classification
Metaxalone is a centrally acting skeletal muscle relaxant (antispasmodic). It belongs to the oxazolidinone chemical class: 5-[(3,5-dimethylphenoxy)methyl]-2-oxazolidinone. Molecular formula: C₁₂H₁₅NO₃; MW: 221.25.
Indication
Adjunct to rest, physical therapy, and other measures for relief of discomfort associated with acute, painful musculoskeletal conditions (strains, sprains, muscle spasm). It does not replace physical therapy or rehabilitation.
It is used in conditions such as:
- Nonspecific back pain
- Myofascial pain
- Fibromyalgia (adjunct)
- Tension headaches (muscle component)
Mechanism of Action
The exact mechanism has not been fully established. Key points:
-
No direct action on striated muscle, the motor end plate, or the nerve fiber
-
Acts via generalized CNS depression, likely at the brainstem and higher CNS centers
-
May disrupt the spasm-pain-spasm cycle
-
Sedative properties are thought to be the primary contributor to muscle relaxation
-
Firestein & Kelley's Textbook of Rheumatology, p. 1406
-
Katzung's Basic and Clinical Pharmacology, 16th Ed., p. 764
Dosage & Administration
| Parameter | Details |
|---|---|
| Standard adult dose | 800 mg (two 400 mg tablets) 3-4 times daily |
| 400 mg tablet use | Historically available; the 400 mg strength was not marketed by King Pharmaceuticals but is used in some formulations |
| Age restriction | Adults and children > 12 years old |
| Route | Oral |
| Food effect | Food increases bioavailability - take consistently with or without food |
The recommended clinical dose is 800 mg TID-QID (not 400 mg alone, though 400 mg tablets are used to compose the 800 mg dose).
Pharmacokinetics
| Parameter | Value |
|---|---|
| Onset | Within 1 hour |
| Tmax | ~3 hours (fasted) |
| Duration | 4-6 hours |
| Half-life | 9.0 ± 4.8 hours (range 4-14 hr) |
| Volume of distribution | ~800 L |
| Metabolism | Hepatic - CYP1A2, CYP2D6, CYP2E1, CYP3A4 (major); CYP2C8, CYP2C9, CYP2C19 (minor) |
| Excretion | Urine |
| Bioavailability | Not precisely established; food significantly increases absorption |
| Dose proportionality | Doubling 400 mg to 800 mg gives roughly proportional increase in Cmax and AUC |
Metaxalone does not significantly inhibit or induce major CYP enzymes, giving it a limited drug interaction profile compared to peers like cyclobenzaprine.
Adverse Effects
Common (CNS-related):
- Drowsiness / sedation (notably less than other muscle relaxants in this class)
- Dizziness
- Headache
- Nausea, vomiting
- Irritability, nervousness
Serious (less common):
- Hemolytic anemia (rare)
- Leukopenia
- Hepatotoxicity / elevated liver enzymes
- Hypersensitivity reactions (rash, urticaria, anaphylaxis)
Contraindications
- Known drug-induced, hemolytic, or other anemias
- Significant renal impairment (dose adjustment not established; avoid)
- Significant hepatic impairment (drug is hepatically metabolized; avoid)
- Hypersensitivity to metaxalone
Drug Interactions
- Metaxalone has limited drug-drug interactions compared to other muscle relaxants (e.g., no significant CYP inhibition)
- Additive CNS depression with: alcohol, benzodiazepines, opioids, other CNS depressants, antihistamines
- Serotonin syndrome risk when combined with serotonergic agents (MAOIs, SSRIs, SNRIs, triptans)
Special Populations
| Population | Consideration |
|---|---|
| Elderly | Increased sedation, fall risk, fracture risk - use with caution |
| Pregnancy | Category not clearly established; limited data |
| Lactation | No data available; weigh benefits vs. risks |
| Renal impairment | Contraindicated in significant impairment |
| Hepatic impairment | Contraindicated in significant impairment |
| Children < 12 years | Safety and efficacy not established |
Comparative Advantages
Relative to other muscle relaxants (cyclobenzaprine, carisoprodol, methocarbamol):
- Less sedating than most alternatives in its class
- Fewer drug interactions (minimal CYP inhibition/induction)
- No anticholinergic effects (unlike cyclobenzaprine, orphenadrine)
- No abuse potential like carisoprodol (which is metabolized to meprobamate)
Efficacy Note
Clinical trial data for metaxalone are described as mixed in controlled studies. Evidence for cyclobenzaprine, tizanidine, carisoprodol, and orphenadrine is somewhat more consistent than for metaxalone in musculoskeletal pain. - Firestein & Kelley's Textbook of Rheumatology, p. 1406
Monitoring
- Liver function tests (with prolonged use)
- CBC (watch for hemolytic anemia)
- Monitor CNS depression, especially if combined with other sedatives
This is a shared conversation. Sign in to Orris to start your own chat.