Carisoprodol is an FDA-approved drug indicated for the relief of discomfort associated with acute, painful musculoskeletal conditions.  It only has approval for up to three weeks. It is not recommended in pediatric patients less than 16 years of age since it has not had evaluations in this population. In the United States, it is a schedule IV controlled substance. Carisoprodol was approved for medical use in the United States in 1959 and is available as a generic medication. It comes in tablet form and can be taken by mouth up to three times a day and before bed.  According to the package insert, carisoprodol is intended to be used together with rest, physical therapy, and other measures to relax muscles after strains, sprains, and muscle injuries.

As per the package insert, the exact mechanism of action of carisoprodol in relieving discomfort associated with acute painful musculoskeletal conditions has not been identified. The believed to be a centrally acting skeletal muscle relaxant that does not directly relax skeletal muscles. The muscle relaxation effects induced by carisoprodol in animal studies is associated with altered interneuronal activity in the spinal cord and the descending reticular formation of the brain. Its primary metabolite, meprobamate, is believed to work at the GABA receptors and is believed to be responsible for carisoprodol’s therapeutic effects as well as its abuse potential.[1] Meprobamate is a benzodiazepine-type anxiolytic and has sedative properties.    

Carisoprodol is metabolized in the liver primarily by the cytochrome P450 oxidase isozyme CYP2C19, and it gets excreted by the kidneys. The absolute bioavailability of carisoprodol remains undetermined. Carisoprodol’s primary metabolite is meprobamate, a known drug of abuse and dependence.[2][3][4][5][6][7] Meprobamate was classified as a schedule IV controlled substance in 1970 and is in the tranquilizer medication class, and its marketing was under several names.  As per the package insert, carisoprodol after oral ingestion has a quick onset of action with the time to maximum plasma concentration being approximately 1.5 to 1.7 hours for the 250-milligram strength and the 350-milligram strength respectively. The elimination half-life for carisoprodol is 1.7 to 2 hours, whereas the half-life for the meprobamate metabolite is approximately 10 hours.  The time to maximum plasma concentration for the meprobamate metabolite is approximately 3.6 to 4.5 hours.    

Given the significantly prolonged half-life of meprobamate compared to the carisoprodol, there is a risk of meprobamate bio-accumulation following extended periods of carisoprodol administration.  Also, patients with reduced CYP2C19 activity are poor metabolizers of the carisoprodol resulting in up to a 4-fold increase in exposure to carisoprodol and a 50% reduced exposure to meprobamate.  

Carisoprodol is a racemic mixture, only slightly soluble in water, but freely soluble in alcohol. The solubility of carisoprodol is practically independent of pH.  Furthermore, as per the package insert, taking this medication with fatty food did not appear to have any effect on its pharmacokinetics.

Carisoprodol is administered as an oral tablet 250 mg or 350 mg taken by the mouth up to three times a day and before bed. It is also available in various co-formulated forms: carisoprodol combined with acetaminophen and caffeine, carisoprodol combined with gamma-aminobutyric acid, and carisoprodol with acetylsalicylic acid and codeine.

Although the safety and pharmacokinetics of carisoprodol in patients with renal impairment have not undergone evaluation, caution is a recommendation since the kidney excretes this medication.  Of note, carisoprodol is dialyzable by hemodialysis and peritoneal dialysis.[8] Carisoprodol gets metabolized by CYP2C19 in the liver. As per the package insert, caution is necessary if administered to patients with impaired hepatic function or reduced CYP2C19 activity since this could result in higher exposure to carisoprodol. Co-administration of CYP2C19 inhibitors could result in increased levels of carisoprodol as well as decreased levels of the meprobamate metabolite. Common CYP2C19-inhibitors include omeprazole, ticlopidine, fluoxetine, fluvoxamine, topiramate, sertraline, and tricyclic antidepressants. Conversely, co-administration of CYP2C19-inducers could result in decreased levels of carisoprodol as well as increased levels of meprobamate. Common CYP2C19-inducers include rifampin, carbamazepine, phenobarbital, aspirin, and St. John’s Wort.

Carisoprodol is pregnancy category C. However; animal studies indicate that carisoprodol adversely affected fetal growth and postnatal survival.  Based on the limited respective post-marketing data, the primary metabolite meprobamate did not show any increased risk for particular congenital malformations.[1]  

Although Carisoprodol prescribing appears to be on the decline dropping from number 214 the previous year down to number 181 of the top 300 commonly prescribed medications, it is still a commonly prescribed medication in the U.S., with over 3.4 million prescriptions in 2017. 

The efficacy, safety, and pharmacokinetics of carisoprodol have not been established in patients under the age of 16 nor patients over the age of 65.

The most common side effects of carisoprodol include drowsiness, dizziness, and headache.  According to the package insert, up to 17% of patients experienced sedation after taking carisoprodol compared to 6% of patients who received a placebo. This side effect can potentially impair the mental and physical abilities necessary for the performance of potentially hazardous work such as driving a vehicle or operating heavy machinery. There are post-marketing reports of motor vehicle accidents correlated with the use of carisoprodol. Since the sedative effects CNS depressants may be additive, patients should be cautioned to avoid or minimize taking other CNS depressants such as alcohol, opioids, or benzodiazepines simultaneously, and to take necessary precautions not to drive or engage in other potentially dangerous activities if experiencing sedation. 

Significant hypotension can also occur with carisoprodol overdose and is usually treated with supportive care and possibly dialysis.[8][9][10]

Dependence, withdrawal, and abuse of carisoprodol has been reported with prolonged use, especially in patients with a history of addiction or when used in combination with other drugs with abuse potential.[1][11] Furthermore, withdrawal symptoms have been reported after abrupt discontinuation after prolonged use.  Therefore it is recommended that carisoprodol not be used for more than two to three weeks for the relief of acute musculoskeletal discomfort.  The belief is that carisoprodol elicits barbiturate-like effects where animal studies show that this medication can produce rewarding effects, like other drugs of abuse.[1] In addition to the potential for somnolence, a normal prescribed dosage of carisoprodol may result in short-lived mild to moderate euphoria or dysphoria due to carisoprodol's potent anxiolytic effects. Carisoprodol, more so than the meprobamate, may be responsible for the euphoria.  Tolerance to the side effects of carisoprodol can develop and lessens over time.

Meprobamate was a frequently misused drug in the 1950s and 1960s, and there have been reported overdoses.[3][6] With prolonged usage, carisoprodol and meprobamate can produce physical dependence of the barbiturate type as well as withdrawal symptoms similar to those of alcohol withdrawal. Similar to benzodiazepines, potential psychological dependence can result in withdrawal symptoms switch persist for significantly longer periods lasting months or even years. These symptoms may include anxiety and depression, long term memory loss, chronic insomnia, social withdrawal, agitation and aggression, and other potential symptoms.[1][6][7] The severity of the symptoms appears to be magnified in patients with a history of substance abuse as well as patients concomitantly on other drugs with sedatives or benzodiazepine or opioid-like effects.  The combination of carisoprodol with opioids and benzodiazepines has been referred to as "the Holy Trinity," reportedly to increase the power of the "high.”[11] Because of its significant abuse potential due to its sedating, relaxant, and anxiolytic effects, in December of 2011, the drug enforcement agency classified carisoprodol as a schedule IV medication according to controlled substance act.

In March of 2007 Norwegian medical regulatory authorities conducted a review of Carisoprodol and the European Medicines Agency (EMEA) Committee for Medicinal Products for Human Use (CHMP) concluded that the benefits of medicines containing carisoprodol no longer outweigh their risks and that all marketing authorizations for carisoprodol be suspended throughout Europe.

Another potential significant reported adverse effect is seizures. In Indonesia, in September 2017, close to 100 people suffered seizures with at least one fatality when PCC, standing for "paracetamol-caffeine-carisoprodol," was mixed into student's drinks. 

Important contraindications listed in the package insert are acute intermittent porphyria as well as hypersensitivity reactions to a carbamate such as meprobamate. Patients with a sulfa allergy can develop a reaction after the carisoprodol is converted into meprobamate.