SKELAXIN
Clinical safety rating
cautionComprehensive clinical and safety monograph for SKELAXIN (SKELAXIN).
Skelaxin (metaxalone) is a centrally acting skeletal muscle relaxant. Its exact mechanism of action is not fully understood, but it is believed to act primarily by depressing the central nervous system (CNS) through inhibition of polysynaptic reflexes at the spinal and supraspinal levels, leading to muscle relaxation without directly affecting the neuromuscular junction or muscle fibers.
| Metabolism | Metaxalone is extensively metabolized in the liver via cytochrome P450 enzymes, primarily CYP1A2 and to a lesser extent CYP2D6, CYP2E1, and CYP3A4. It undergoes oxidative metabolism and glucuronidation. The major metabolites are hydroxylated derivatives and their glucuronide conjugates, which are inactive. |
| Excretion | Primarily renal (approximately 99% as unchanged drug); <1% fecal |
| Half-life | 1-2 hours (terminal elimination half-life); clinical context: short duration of action, requires multiple daily dosing |
| Protein binding | 75-80% bound, primarily to albumin |
| Volume of Distribution | Vd/F ~ 1.5 L/kg; indicates extensive tissue distribution |
| Bioavailability | Oral: 25-30% (due to first-pass metabolism) |
| Onset of Action | Oral: 30 minutes |
| Duration of Action | 4-6 hours; clinical note: effects diminish after 4-6 hours, requiring repeated administration |
| Molecular Weight | 461.6 Da |
800 mg orally three to four times daily.
| Dosage form | TABLET |
| Renal impairment | No specific guidelines; use with caution in severe renal impairment (GFR <30 mL/min) due to risk of accumulation. |
| Liver impairment | Contraindicated in hepatic impairment; avoid use in Child-Pugh class A, B, or C. |
| Pediatric use | Not recommended for pediatric patients due to lack of safety and efficacy data. |
| Geriatric use | Initiate at lower doses (e.g., 400 mg three to four times daily) due to increased sensitivity and risk of adverse effects; monitor closely. |
| 1st trimester | Limited human data; animal studies show no evidence of harm. Use only if clearly needed. |
| 2nd trimester | Limited human data; no known risk. |
| 3rd trimester | Limited human data; may cause neonatal respiratory depression if used near term. |
Clinical note
Comprehensive clinical and safety monograph for SKELAXIN (SKELAXIN).
| Placental transfer | Likely crosses placenta due to low molecular weight; data are lacking. |
| Breastfeeding | Excretion into breast milk is unknown; use caution. Consider risk versus benefit. |
| Lactation Rating | L3 (Moderately Safe) |
| Teratogenic Risk | Risk summary: Not known to increase risk of major birth defects; limited human data. First trimester: No increased risk reported; animal studies show no teratogenicity. Second and third trimesters: No specific risks documented but should be used only if clearly needed. Fetal effects: Potential for muscle relaxation, but no clear adverse outcomes. |
| Fetal Monitoring | Monitor maternal heart rate and blood pressure due to potential hypotension. Observe for excessive sedation or muscle weakness. Fetal monitoring as clinically indicated (e.g., nonstress test if used near term). No specific fetal monitoring required. |
| Fertility Effects | No known impact on fertility in humans; animal studies show no adverse effects on fertility. |
■ FDA Black Box Warning
None
| Serious Effects |
Hypersensitivity to any componentSevere hepatic impairmentMyasthenia gravisHistory of drug-induced hemolytic anemia
| Precautions | Central nervous system depression (sedation, dizziness, drowsiness); may impair ability to drive or operate machinery. Concomitant use with alcohol or other CNS depressants may enhance effects. Hepatotoxicity (rare but serious; monitor liver enzymes in patients with pre-existing liver disease). Serotonin syndrome risk when used with other serotonergic drugs. Risk of abuse and dependence (less than benzodiazepines, but caution in substance abuse history). Photosensitivity (avoid prolonged sun exposure). |
| Food/Dietary | Taking with food, especially high-fat meals, may increase absorption and risk of side effects. Avoid alcohol, as it increases CNS depression. No specific foods to avoid, but maintain consistent intake timing relative to meals. |
| Clinical Pearls | Skelaxin (metaxalone) is a centrally acting muscle relaxant indicated for acute musculoskeletal pain. Onset is about 1 hour with a duration of 4-6 hours. Drowsiness is the most common side effect; avoid concurrent use with alcohol or other CNS depressants. Not recommended in patients with significant hepatic impairment due to extensive hepatic metabolism. Tablets should be taken with food to reduce GI upset, but high-fat meals can increase Cmax and AUC. |
| Patient Advice | Take exactly as prescribed; do not increase dose or frequency. · May cause drowsiness or dizziness; avoid driving or hazardous activities until you know how you react. · Avoid alcohol while taking this medication. · Take with food or milk to prevent stomach upset. · Do not stop suddenly; follow doctor's instructions for discontinuation. · Report any signs of liver problems (yellowing skin/eyes, dark urine, persistent nausea) to your doctor immediately. |
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