CARISOPRODOL
Clinical safety rating: safe
Animal studies have demonstrated safety
Carisoprodol is a centrally acting skeletal muscle relaxant that exerts its effects via modulation of GABA-A receptors, possibly through its active metabolite meprobamate, which is a controlled substance with barbiturate-like activity. It also inhibits interneuronal activity in the descending reticular formation and spinal cord, leading to muscle relaxation.
| Metabolism | Primarily hepatic via CYP2C19; partially metabolized to meprobamate (a Schedule IV controlled substance) by N-dealkylation; also undergoes hydrolysis and subsequent conjugation. |
| Excretion | Renal: >99% as metabolites (hydroxycarisoprodol and meprobamate) and minor unchanged drug. Fecal: <1%. Biliary: negligible. |
| Half-life | Terminal elimination half-life is approximately 2.0 hours for carisoprodol; the active metabolite meprobamate has a half-life of 6-12 hours. Clinical context: Short half-life supports three-times-daily dosing; accumulation of meprobamate with repeated dosing or renal impairment may prolong effects. |
| Protein binding | Carisoprodol: approximately 60% bound to plasma proteins (predominantly albumin). Meprobamate: ~20% bound. |
| Volume of Distribution | Apparent Vd: approximately 0.8 L/kg for carisoprodol (total body water distribution). Clinical meaning: Extensive distribution into tissues; consistent with moderate lipophilicity. |
| Bioavailability | Oral: Approximately 95% absorbed from the GI tract; extensive first-pass metabolism converts ~50% to meprobamate; net bioavailability of parent drug is ~50-60%. |
| Onset of Action | Oral: Onset of muscle relaxation within 30 minutes; peak plasma levels at 1-2 hours. |
| Duration of Action | Oral: Duration of muscle relaxant effect is 4-6 hours, corresponding to the elimination half-life. Clinical note: Sedative effects may persist longer due to meprobamate accumulation. |
250-350 mg orally 3 times daily and at bedtime
| Dosage form | TABLET |
| Renal impairment | No specific guidelines; use with caution in severe renal impairment (CrCl <30 mL/min) due to increased risk of accumulation. |
| Liver impairment | Child-Pugh A: no dose adjustment; Child-Pugh B: reduce dose by 50%; Child-Pugh C: avoid use. |
| Pediatric use | Not recommended for use in children under 16 years due to lack of safety and efficacy data. |
| Geriatric use | Initiate at 250 mg 3-4 times daily; monitor for sedation and falls; consider reducing dose in frail elderly. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
CNS depressants including alcohol can cause additive sedation Metabolism produces meprobamate a controlled substance with abuse potential.
| Breastfeeding | Carisoprodol and its active metabolite meprobamate are excreted into human breast milk. The milk-to-plasma ratio (M/P) is not well established but considered low. However, potential adverse effects in nursing infants include sedation and withdrawal symptoms. The manufacturer recommends caution; avoid breastfeeding while using carisoprodol due to risk of neonatal sedation. |
| Teratogenic Risk | Carisoprodol is classified as FDA Pregnancy Category C. Data from animal studies have shown fetal harm, but no adequate well-controlled studies in pregnant women. First trimester: Limited data suggest a possible increased risk of congenital anomalies, particularly with first-trimester exposure. Second and third trimesters: Use may be associated with neonatal withdrawal syndrome including irritability, tremors, and poor feeding. Avoid use during pregnancy, especially during the first trimester. |
■ FDA Black Box Warning
None
| Common Effects | spasms |
| Serious Effects |
["Hypersensitivity to carisoprodol or meprobamate","Acute intermittent porphyria","Concomitant use with MAOIs (potential for hypertensive crisis)"]
| Precautions | ["Risk of sedation and dizziness, impairing ability to drive or operate machinery","Potential for abuse and dependence, especially with long-term use; meprobamate is a controlled substance","Withdrawal symptoms including anxiety, insomnia, and seizures upon abrupt discontinuation","Hepatic impairment may alter metabolism; use with caution","May cause serotonin syndrome when used with other serotonergic drugs","Respiratory depression with concurrent use of CNS depressants"] |
| Food/Dietary | Avoid alcohol. No specific food interactions known, but CNS depressant effects may be exacerbated by alcohol or other sedating substances. |
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| Fetal Monitoring | Monitor for signs of neonatal withdrawal if used near term (including irritability, tremors, poor feeding). Assess fetal growth and development with ultrasound if used during first trimester. Monitor maternal CNS depression and fall risk. |
| Fertility Effects | No specific studies on human fertility. Animal studies have not reported impairment of fertility. Theoretical effects: central nervous system depressant effects may alter hormonal regulation indirectly, but evidence is lacking. |
| Clinical Pearls | Carisoprodol is centrally acting muscle relaxant that is metabolized to meprobamate, a controlled substance with abuse potential. Avoid in patients with history of substance abuse. Use short-term (2-3 weeks) due to lack of evidence for long-term efficacy. Monitor for sedation and dizziness; avoid concomitant use with other CNS depressants. Taper to discontinue after prolonged use to prevent withdrawal symptoms. |
| Patient Advice | Take only as prescribed for short-term relief (usually 2-3 weeks). · Do not increase dose or stop abruptly without consulting doctor. · May cause drowsiness, dizziness, or blurred vision; avoid driving or operating machinery until you know how you react. · Avoid alcohol and other sedatives while taking this medication. · Report any signs of abuse or dependence (e.g., craving, needing higher doses). · Do not share this medication with others due to abuse potential. · Seek medical attention if you experience allergic reactions (rash, itching, swelling) or seizures. |