RELA
Clinical safety rating: caution
Comprehensive clinical and safety monograph for RELA (RELA).
RELA (Carisoprodol) is a centrally acting muscle relaxant that modulates GABA-A receptor activity and blocks interneuronal activity in the descending reticular formation and spinal cord, resulting in muscle relaxation without directly affecting the neuromuscular junction. Its metabolite, meprobamate, contributes to anxiolytic and sedative effects.
| Metabolism | Hepatic metabolism via CYP2C19 to meprobamate (active); also minor pathways via CYP3A4 and CYP1A2. Meprobamate is further hydroxylated and glucuronidated. |
| Excretion | Primarily renal excretion of unchanged drug and metabolites; 70% to 80% eliminated via urine, remainder biliary/fecal |
| Half-life | Terminal elimination half-life approximately 20–30 hours; prolonged in elderly and renal impairment |
| Protein binding | 99% bound to albumin and alpha-1-acid glycoprotein |
| Volume of Distribution | 0.1–0.2 L/kg; indicates limited distribution in total body water |
| Bioavailability | Oral: 80–90%; Intramuscular: 100% |
| Onset of Action | Oral: 30–60 minutes; Intramuscular: 10–30 minutes |
| Duration of Action | 4–6 hours after single oral dose; may be extended with repeated dosing due to accumulation |
Adults: 250-350 mg orally 3-4 times daily.
| Dosage form | TABLET |
| Renal impairment | GFR 10-50 mL/min: administer 250 mg every 6-8 hours; GFR <10 mL/min: administer 250 mg every 12-24 hours. |
| Liver impairment | Child-Pugh Class A: no adjustment; Class B: reduce dose by 50% and monitor; Class C: avoid use. |
| Pediatric use | Children 2-12 years: 10-20 mg/kg/day divided every 6-8 hours; maximum 1.5 g/day. |
| Geriatric use | Start at lower end of adult dosing (250 mg 3 times daily); monitor for CNS effects and adjust based on renal function. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for RELA (RELA).
| Breastfeeding | Carisoprodol and its active metabolite meprobamate are excreted in human breast milk. The milk-to-plasma ratio for meprobamate is approximately 2-4. Based on limited data, a nursing infant would receive about 1-3% of the maternal weight-adjusted dose. Consider the potential for infant sedation or feeding difficulties. Use caution in breastfeeding women, especially with high doses or prolonged use. |
| Teratogenic Risk | Carisoprodol, the active ingredient in RELA, is classified as FDA Pregnancy Category C. Animal studies have shown an increased incidence of fetal resorptions and skeletal variants at doses 4-5 times the human dose. There are no adequate and well-controlled studies in pregnant women. First trimester: Risk cannot be ruled out; use only if clearly needed. Second and third trimesters: May cause neonatal withdrawal syndrome or respiratory depression if used near term. Discontinue use if pregnancy occurs. |
■ FDA Black Box Warning
None explicitly required by FDA; however, dependence, withdrawal, and abuse potential are significant due to meprobamate metabolite, leading to controlled substance scheduling (Schedule IV).
| Serious Effects |
Hypersensitivity to carisoprodol or meprobamate; acute intermittent porphyria; concurrent use with MAO inhibitors (theoretical interaction). Relative: history of substance abuse, severe hepatic or renal disease, poor CYP2C19 metabolizers.
| Precautions | Risk of dependence, withdrawal (including seizures, anxiety, insomnia) after prolonged use; CNS depression (additive with alcohol and other depressants); impaired motor skills; caution in hepatic or renal impairment; elderly patients more sensitive; avoid abrupt discontinuation. |
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| Fetal Monitoring | Monitor for signs of maternal CNS depression (e.g., sedation, dizziness, ataxia) and respiratory depression, especially near term. Monitor neonatal for symptoms of withdrawal (e.g., irritability, tremors, poor feeding) or respiratory depression if used in late pregnancy. |
| Fertility Effects | No specific human data on fertility effects. Animal studies have not reported adverse effects on fertility at doses up to 10 times the human dose. However, meprobamate (active metabolite) has been associated with anovulation in some animal models. Potential for reversible effects on ovulation with chronic use. |