Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
RELA vs CARISOPRODOL COMPOUND
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
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.
Carisoprodol is a centrally acting muscle relaxant that acts as a prodrug for meprobamate, a barbiturate-like compound with sedative and anxiolytic properties. Its mechanism is thought to involve GABA-A receptor modulation and depression of polysynaptic reflexes in the spinal cord and reticular formation. Aspirin provides analgesic and anti-inflammatory effects via irreversible inhibition of cyclooxygenase (COX-1 and COX-2), reducing prostaglandin synthesis. Codeine is an opioid agonist at mu-opioid receptors, producing analgesia by mimicking endogenous endorphins.
Adjunctive treatment for acute musculoskeletal pain due to muscle spasm (FDA approved),Off-label: Tension headaches, fibromyalgia, myofascial pain syndrome
Relief of discomfort associated with acute, painful musculoskeletal conditions,As an adjunct to rest, physical therapy, and other measures
Adults: 250-350 mg orally 3-4 times daily.
1-2 tablets (carisoprodol 200 mg/aspirin 325 mg) orally 4 times daily.
Terminal elimination half-life approximately 20–30 hours; prolonged in elderly and renal impairment
Carisoprodol has a terminal elimination half-life of approximately 1.5–2 hours; its active metabolite meprobamate has a half-life of 9–12 hours, which may lead to prolonged effects with chronic use.
Hepatic metabolism via CYP2C19 to meprobamate (active); also minor pathways via CYP3A4 and CYP1A2. Meprobamate is further hydroxylated and glucuronidated.
Carisoprodol is metabolized by CYP2C19 to meprobamate (active metabolite). Aspirin is hydrolyzed by esterases in the liver and plasma to salicylic acid, which is further conjugated. Codeine is metabolized by CYP2D6 to morphine (active) and by CYP3A4 to norcodeine.
Primarily renal excretion of unchanged drug and metabolites; 70% to 80% eliminated via urine, remainder biliary/fecal
Carisoprodol is primarily metabolized in the liver, with about 50% excreted renally as unchanged drug and metabolites; the major metabolite meprobamate is also renally excreted. Fecal excretion is negligible (<2%).
99% bound to albumin and alpha-1-acid glycoprotein
Carisoprodol is approximately 60% bound to plasma proteins, mainly albumin.
0.1–0.2 L/kg; indicates limited distribution in total body water
Volume of distribution is approximately 0.6–0.8 L/kg, indicating distribution into total body water.
Oral: 80–90%; Intramuscular: 100%
Oral bioavailability is nearly complete (close to 100%) due to rapid and extensive absorption.
GFR 10-50 m L/min: administer 250 mg every 6-8 hours; GFR <10 m L/min: administer 250 mg every 12-24 hours.
Contraindicated in severe renal impairment (Cr Cl <30 m L/min). No specific dose adjustment for mild-moderate impairment; use caution.
Child-Pugh Class A: no adjustment; Class B: reduce dose by 50% and monitor; Class C: avoid use.
Contraindicated in severe hepatic impairment (Child-Pugh class C). For moderate impairment, reduce dose or increase interval; specific guidelines not established.
Children 2-12 years: 10-20 mg/kg/day divided every 6-8 hours; maximum 1.5 g/day.
Not recommended for pediatric patients due to aspirin content and risk of Reye syndrome.
Start at lower end of adult dosing (250 mg 3 times daily); monitor for CNS effects and adjust based on renal function.
Initiate at lowest effective dose; monitor for CNS depression, falls, and aspirin-related bleeding. Avoid in patients ≥65 years due to risks of dizziness, sedation, and GI bleeding.
None explicitly required by FDA; however, dependence, withdrawal, and abuse potential are significant due to meprobamate metabolite, leading to controlled substance scheduling (Schedule IV).
None
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.
Risk of dependence, abuse, and withdrawal with carisoprodol and codeine,CYP2D6 ultrarapid metabolizers may have morphine toxicity from codeine,Reye's syndrome risk in children with viral illness (aspirin),GI bleeding risk with aspirin,Respiratory depression with codeine,Sedation and impaired motor function,Hepatic impairment,Renal impairment
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.
Hypersensitivity to carisoprodol, meprobamate, aspirin, codeine, or any component,Porphyria,Acute intermittent porphyria,Children with viral illness (aspirin) due to Reye's syndrome risk,Breastfeeding (codeine),Severe renal or hepatic impairment,GI bleeding or peptic ulcer disease (aspirin),Concurrent use of MAOIs or within 14 days,Respiratory depression (codeine)
No specific food interactions. However, avoid alcohol as it increases sedation and respiratory depression risk.
Avoid alcohol and grapefruit juice. Alcohol increases CNS depression and risk of hepatotoxicity. Grapefruit juice may inhibit metabolism, leading to increased levels and toxicity.
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.
Carisoprodol is a pregnancy category C drug. Data from animal studies are insufficient or show adverse effects, but no adequate human studies exist. There is a potential risk of fetal harm if used during the first trimester due to possible neural tube defects based on limited reports. In the second and third trimesters, maternal use may cause neonatal withdrawal symptoms (e.g., irritability, feeding difficulties) and respiratory depression if used near term. Carisoprodol is not recommended during pregnancy unless benefit outweighs risk.
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.
Carisoprodol is excreted into human breast milk. The milk-to-plasma (M/P) ratio is approximately 2-4 based on small studies. An infant would receive a weight-adjusted dose of about 4-8% of the maternal dose, which may cause sedation, drowsiness, or irritability in the neonate. Breastfeeding is not recommended during carisoprodol use, especially in premature infants or those with hepatic impairment. If used, monitor infant for signs of CNS depression.
Pharmacokinetic changes in pregnancy (increased volume of distribution, increased renal clearance, altered hepatic metabolism) may increase carisoprodol clearance. Reduce steady-state concentrations; however, no specific dosing adjustments are recommended due to lack of data. Use the lowest effective dose for the shortest duration. Avoid during labor and delivery due to risk of neonatal respiratory depression.
No specific dosing adjustments for carisoprodol are established in pregnancy. However, due to increased plasma volume and altered hepatic metabolism in pregnancy, the drug's half-life may be reduced. Clinical monitoring for efficacy and maternal side effects (e.g., drowsiness, dizziness) is recommended. Use the lowest effective dose for the shortest duration. Consider avoidance of the compound formulation with aspirin or other NSAIDs, which have additional risks.
RELA (carisoprodol) is a centrally acting muscle relaxant metabolized to meprobamate, a controlled substance. Monitor for sedation and abuse potential. Avoid concurrent use with CNS depressants including alcohol. Sudden discontinuation after prolonged use may precipitate withdrawal symptoms.
Carisoprodol is metabolized to meprobamate, a controlled substance with abuse potential; use cautiously in patients with history of substance abuse. Combination with other CNS depressants (e.g., alcohol, benzodiazepines) increases sedation risk. Limit use to 2-3 weeks due to lack of efficacy beyond that and risk of dependence. Avoid in patients with porphyria because carisoprodol may be porphyrinogenic.
This medication may cause drowsiness or dizziness. Do not drive or operate heavy machinery until you know how it affects you.,Avoid alcohol and other CNS depressants while taking this drug.,Take only as prescribed; do not increase dose or duration due to risk of dependence.,Do not stop abruptly after long-term use; consult your doctor for a taper schedule.
This medication may cause drowsiness, dizziness, or blurred vision; avoid driving or operating machinery until you know how it affects you.,Do not consume alcohol or other CNS depressants while taking this drug.,Take only as prescribed; do not increase dose or frequency. This drug has abuse potential.,Inform your doctor if you have a history of drug or alcohol abuse, seizures, or liver/kidney disease.,Do not use for longer than 2-3 weeks unless directed by your doctor.
No interactions on record
"The co-administration of pentobarbital, a barbiturate and potent CYP3A4 inducer, with carisoprodol, a prodrug that is metabolized to its active form, meprobamate, via CYP2C19, may lead to reduced plasma concentrations of meprobamate due to pentobarbital-induced upregulation of CYP2C19, potentially diminishing the sedative and muscle relaxant effects of carisoprodol. However, pentobarbital also acts as a central nervous system (CNS) depressant, and additive CNS depression can occur, increasing the risk of excessive sedation, respiratory depression, and impairment of psychomotor function. Clinical outcomes may include altered therapeutic efficacy of carisoprodol and heightened risk of CNS and respiratory adverse effects."
"Carisoprodol, a centrally acting skeletal muscle relaxant, is metabolized primarily by CYP2C19 to its active metabolite meprobamate. Isoniazid, a first-line antitubercular agent, is a known inhibitor of CYP2C19. When coadministered, isoniazid can decrease the metabolism of carisoprodol, leading to increased plasma concentrations of both carisoprodol and meprobamate. This elevation raises the risk of dose-related adverse effects such as sedation, dizziness, and respiratory depression, and may prolong the duration of muscle relaxant action."
"The combination of sulpiride, an atypical antipsychotic with dopamine D2 receptor antagonism and mild serotonin 5-HT4 agonist properties, and carisoprodol, a centrally acting muscle relaxant metabolized to meprobamate (a barbiturate-like sedative-hypnotic), can result in additive central nervous system (CNS) depression, including sedation, dizziness, and psychomotor impairment. Additionally, both drugs may lower the seizure threshold, increasing the risk of seizures. Sulpiride can also prolong the QT interval, and carisoprodol's sedative effects may mask or exacerbate this cardiotoxicity, potentially leading to ventricular arrhythmias such as torsade de pointes."
Explore head-to-head clinical comparisons of other medications in the same therapeutic classes.
Common clinical questions about RELA vs CARISOPRODOL COMPOUND, answered by our medical review team.
RELA is a Skeletal Muscle Relaxant that works by 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.. CARISOPRODOL COMPOUND is a Skeletal Muscle Relaxant that works by Carisoprodol is a centrally acting muscle relaxant that acts as a prodrug for meprobamate, a barbiturate-like compound with sedative and anxiolytic properties. Its mechanism is thought to involve GABA-A receptor modulation and depression of polysynaptic reflexes in the spinal cord and reticular formation. Aspirin provides analgesic and anti-inflammatory effects via irreversible inhibition of cyclooxygenase (COX-1 and COX-2), reducing prostaglandin synthesis. Codeine is an opioid agonist at mu-opioid receptors, producing analgesia by mimicking endogenous endorphins.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between RELA and CARISOPRODOL COMPOUND depend on the specific clinical indication. These are both Skeletal Muscle Relaxant agents and are not directly interchangeable by dose. A physician or clinical pharmacist should guide any therapeutic switching decisions.
The standard adult dose of RELA is: Adults: 250-350 mg orally 3-4 times daily.. The standard adult dose of CARISOPRODOL COMPOUND is: 1-2 tablets (carisoprodol 200 mg/aspirin 325 mg) orally 4 times daily.. Dosing should always be individualized based on indication, renal and hepatic function, age, and other patient factors.
No direct drug-drug interaction has been formally documented between RELA and CARISOPRODOL COMPOUND in current clinical databases. However, individual patient risk factors including other medications, organ function, and comorbidities should always be evaluated by a qualified healthcare provider.
The maternal-fetal safety profiles differ. RELA is classified as Category C. 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 . CARISOPRODOL COMPOUND is classified as Category A/B. Carisoprodol is a pregnancy category C drug. Data from animal studies are insufficient or show adverse effects, but no adequate human studies exist. There is a potential risk of fe. Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.