The decay periods of the EPCs and EPPs generated by phrenic nerve stimulation are markedly increased by methocarbamol (2 mM; for 20 min)[3]. Nav1.7 currents are unaffected by methocarbamol[3].

The muscle relaxant activity of methocarbamol (200 mg/kg; ip) is 88.96%[3].

Animal/Disease Models: Mice with weight 20-30 g[3]
Doses: 200 mg/kg
Route of Administration: IP; single dose
Experimental Results: Had Muscle relaxant activity of 88.96%.

Absorption, Distribution and Excretion
The time to maximum concentration is 1.1 hours for both healthy patients and those on hemodialysis. The maximum plasma concentration is 21.3mg/L for healthy patients and 28.7mg/L in hemodialysis patients. The area under the curve for healthy patients is 52.5mg/L\*hr and 87.1mg/L*hr in hemodialysis patients. AUC% based on terminal elimination half life is 2% for healthy patients and 4% for hemodialysis patients. Older studies report maximum plasma concentrations in 0.5 hours.
In humans the majority of the dose is eliminated in the urine. In dogs, 88.85% of the dose is eliminated in urine and 2.14% in the feces. In rats, 84.5-92.5% of the dose is eliminated in the urine and 0-13.3% is eliminated in the feces.
Volume of distribution data in humans is scarce. In horses, the volume of distribution is 515-942mL/kg at steady state or 724-1130mL/kg.
0.2-0.8L/h/kg.
Methocarbamol is rapidly and almost completely absorbed from the GI tract. Blood or serum concentrations of methocarbamol required for sedative, skeletal muscle relaxant, or toxic effects are not known.
Following oral administration of a single dose of methocarbamol, peak blood or serum concentrations of the drug appear to be attained in approximately 1-2 hours; the onset of action is usually within 30 minutes. Data from an unpublished study indicate that peak blood concentrations (measured as total carbamates and expressed in terms of methocarbamol) average 16.5 mcg/mL following a single 2-g oral dose, while data from a published study (using an assay relatively specific for methocarbamol) indicate that peak serum concentrations average 29.8 mcg/mL following the same dose. Data from the unpublished study also indicate that after IV administration of 1 g of methocarbamol at a rate of 300 mg/minute, blood concentrations of 19 mcg/mL are attained immediately and that the onset of action is almost immediate.
In dogs, methocarbamol is widely distributed, with highest concentrations attained in the kidney and liver; lower concentrations are attained in the lungs, brain, and spleen, and low concentrations are attained in heart and skeletal muscle.
The drug and/or its metabolites cross the placenta in dogs. It is not known if methocarbamol is distributed into milk in humans.
For more Absorption, Distribution and Excretion (Complete) data for METHOCARBAMOL (8 total), please visit the HSDB record page.

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Metabolism / Metabolites
Methocarbamol is metabolized in the liver by demethylation to 3-(2-hydroxyphenoxy)-1,2-propanediol-1-carbamate or hydroxylation to 3-(4-hydroxy-2-methoxyphenoxy)-1,2-propanediol-1-carbamate. Methocarbamol and its metabolites are conjugated through glucuronidation or sulfation.
Methocarbamol is extensively metabolized, presumably in the liver, by dealkylation and hydroxylation.
Based on limited data, about 10-15% of a single oral dose is excreted in urine as unchanged drug, about 40-50% as the glucuronide and sulfate conjugates of 3-(2-hydroxyphenoxy)-1,2-propanediol-1-carbamate and 3-(4-hydroxy-2-methoxyphenoxy)-1,2-propanediol-1-carbamate, and the remainder as unidentified metabolites.
In dogs, rats and in man, methocarbamol gave p-hydroxymethocarbamol and o-demethylation product. All three substances were excreted in urine as glucuronic acid and ester sulfate conjugates.
Permethylation and g.l.c.-mass spectrometric analysis of bile from an isolated rat liver perfusion to which methocarmol was added showed seven components not present in control bile: methocarbamol, glucuronides of methocarbamol and desmethyl-methocarbamol, and four glucuronides of hydroxylated methocarbamol metabolites. 2. An interesting rearrangement of a methyl group has been found in the mass spectrum of 3-(2-methoxyphenyloxy)-1,2-dimethoxypropane, the permethylation product from methocarbamol.

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Biological Half-Life
The elimination half life is 1.14 hours in healthy subjects and 1.24 hours in subjects with renal insufficiency. Older studies report half lives of 1.6-2.15 hours.
Methocarbamol has a serum half-life of 0.9-1.8 hours.
... Pharmacokinetics of methocarbamol were studied in eight healthy, adult horses after intravenous (iv) and oral administration of large dosages. ... Plasma methocarbamol concentration declined very rapidly during the initial or rapid disposition phase after iv administration; the terminal elimination half-life ranged from 59 to 90 mins. ...
/Investigators/ determined plasma methocarbamol concentrations over 24 hr following a 1.5 g methocarbamol dose (off-dialysis day) to 8 chronic hemodialysis patients and compared these results to those from 17 healthy male volunteers. The harmonic mean elimination half-life was similar between the two groups, 1.24 and 1.14 hr, respectively. ...
... Pharmacokinetics of methocarbamol were studied in eight healthy, adult horses after intravenous (iv) and oral administration of large dosages. ... Plasma methocarbamol concentration declined very rapidly during the initial or rapid disposition phase after iv administration; the terminal elimination half-life ranged from 59 to 90 mins. ...

Hepatotoxicity
While the product label for methocarbamol states that it can cause jaundice (including cholestatic jaundice), there is little published evidence to suggest that methocarbamol is a cause of hepatic injury or clinically apparent drug induced liver disease. During clinical trials of methocarbamol, some patients had to stop treatment because of nausea, dizziness, or other nonspecific complaints, but no serum aminotransferase levels or other laboratory results were reported. Methocarbamol appears to be well tolerated, but the lack of monitoring of serum aminotransferase levels during clinical trials with methocarbamol makes it impossible to rule out the possibility of mild liver injury occurring with treatment.
Likelihood score: E (Unlikely cause of clinically apparent liver injury).
Drug Class: Muscle Relaxants

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Protein Binding
Methocarbamol is 46-50% protein bound in healthy patients and 47.3-48.9% protein bound in hemodialysis patients.

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Interactions
Additive CNS depression may occur when methocarbamol is administered concomitantly with other CNS depressants, including alcohol. If methocarbamol is used concomitantly with other depressant drugs, caution should be used to avoid overdosage.
A case is presented of a fatal drug interaction caused by ingestion of methocarbamol (Robaxin) and ethanol. ... Therapeutic concentrations of methocarbamol are reported to be 24 to 41 ug/mL. Biological fluids were screened for ethanol ... and quantitated by gas-liquid chromatography (GLC). Determination of methocarbamol concentrations in biological tissue homogenates and fluids were obtained by colorimetric analysis of diazotized methocarbamol. Blood ethanol concentration was 135 mg/dL (0.135% w/v) and urine ethanol was 249 mg/dL (0.249% w/v). Methocarbamol concentrations were: blood, 257 ug/mL; bile, 927 ug/L; urine, 255 ug/L; gastric, 3.7 g; liver, 459 ug/g; and kidney, 83 ug/g. The combination of ethanol and carbamates is contraindicated since acute alcohol intoxication combined with carbamate usage can lead to combined central nervous system depression as a result of the interactive sedative-hypnotic properties of the compound
.../Methocarbamol/ is capable of inducing hepatic microsomal enzymes that metabolize warfarin in animals.
Imipramine enhances CNS effect of.../methocarbamol/ in animals...
Methocarbamol may inhibit the effects of pyridostigmine bromide. Use with caution in patients with myasthenia gravis receiving anticholinesterase agents.

[1]. Bruce, R.B., L.B. Turnbull, and J.H. Newman, Metabolism of methocarbamol in the rat, dog, and human. J Pharm Sci, 1971. 60(1): p. 104-6.

[2]. Pharmacokinetics and protein binding of methocarbamol in renal insufficiency and normals. Eur J Clin Pharmacol, 1990. 39(2): p. 193-4.

[3]. Methocarbamol blocks muscular Na v 1.4 channels and decreases isometric force of mouse muscles. Muscle Nerve. 2020 Oct 11.

2-hydroxy-3-(2-methoxyphenoxy)propyl carbamate is a carbamate ester that is glycerol in which one of the primary alcohol groups has been converted to its 2-methoxyphenyl ether while the other has been converted to the corresponding carbamate ester. It is a carbamate ester, a secondary alcohol and an aromatic ether.
Methocarbamol was developed in the early 1950s as a treatment for muscle spasticity and the associated pain. It is a guaiacol glyceryl ether. Methocarbamol tablets and intramuscular injections are prescription medicines indicated in the United States as an adjunct to rest, physical therapy, and other measures for the relief of discomforts associated with acute, painful musculoskeletal conditions. In Canada, methocarbamol can be sold as an over the counter oral medicine at a lower dose that may be combined with [acetaminophen] or [ibuprofen]. A combination product with [acetylsalicylic acid] and [codeine] is available in Canada by prescription. Methocarbamol was FDA approved on 16 July 1957.
Methocarbamol is a Muscle Relaxant. The physiologic effect of methocarbamol is by means of Centrally-mediated Muscle Relaxation.
Methocarbamol is a commonly used, centrally acting muscle relaxant and has not been linked to instances of liver injury.
Methocarbamol is a carbamate with centrally acting muscle relaxant properties. Though the exact mechanism of action of methocarbamol was not established, it's postulated to be via a mechanism similar of carbamate, inhibition of acetylcholinesterase at synapses in the autonomic nervous system, neuromuscular junction, and central nervous system. Methocarbamol has no direct effect on the contractile mechanism of striated muscle, the motor end plate or the nerve fiber.
A centrally acting muscle relaxant whose mode of action has not been established. It is used as an adjunct in the symptomatic treatment of musculoskeletal conditions associated with painful muscle spasm. (From Martindale, The Extra Pharmacopoeia, 30th ed, p1206)
See also: Aspirin; methocarbamol (component of).

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Drug Indication
Methocarbamol tablets and intramuscular injections are indicated in the United States as an adjunct to rest, physical therapy, and other measures for the relief of discomforts associated with acute, painful musculoskeletal conditions. Oral methocarbamol in America may be given up to 1500mg 4 times daily for 2-3 days. In Canada, methocarbamol containing oral formulations are sold over the counter for pain associated with muscle spasm. However, if these combination formulations include codeine, they are prescription only.
FDA Label

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Mechanism of Action
The mechanism of action of methocarbamol is thought to be dependant on its central nervous system depressant activity. This action may be mediated through blocking spinal polysynaptic reflexes, decreasing nerve transmission in spinal and supraspinal polysynaptic pathways, and prolonging the refractory period of muscle cells. Methocarbamol has been found to have no effect on contraction of muscle fibres, motor end plates, or nerve fibres.
Precise mechanism of action has not been determined. These agents act in the central nervous system (CNS) rather than directly on skeletal muscle. Several of these medications have been shown to depress polysynaptic reflexes preferentially. The muscle relaxant effects of most of these agents may be related to their CNS depressant (sedative) effects. /Skeletal Muscle Relaxants/

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Therapeutic Uses
Muscle Relaxants, Central
Skeletal muscle relaxants are indicated as adjunts to other measures, such as rest and physical therapy, for the relief of muscle spasm associated with acute, painful musculoskeletal conditions. /Included in US product label/
Methcarbamol is also FDA-approved for control of the neuromuscular manifestations of tetanus. However it has largely been replaced in the treatment of tetanus by diazepam, or, in severe cases a neuromuscular blocking agent such as pancuronium. Such therapy is used as an adjunct to other measures, such as debridement, tetanus antitoxin, penicillin, tracheotomy, fluid and electrolyte replacement, and supportive treatment.
VET: In dogs, cats, and horses, methocarbamol is indicated as adjunct therapy of acute inflammatory and traumatic conditions of skeletal muscle and to reduce muscle spasms.
For more Therapeutic Uses (Complete) data for METHOCARBAMOL (6 total), please visit the HSDB record page.

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Drug Warnings
The most frequent adverse effects of methocarbamol are drowsiness, dizziness, and lightheadedness. Blurred vision, headache, fever, and nausea may occur after oral, IM, or IV administration of the drug. Anorexia has been reported after oral administration. Adynamic ileus occurred in one patient who received a total of 10 g of methocarbamol orally. Metallic taste, GI upset, nystagmus, diplopia, flushing, vertigo, mild muscular incoordination, syncope, hypotension, and bradycardia have occurred in patients receiving the drug IM or IV.
Allergic reactions such as urticaria, pruritus, rash, skin eruptions, and conjunctivitis with nasal congestion may occur in patients receiving methocarbamol. Anaphylactic reactions have occurred following IM or IV administration of the drug. Although most patients with methocarbamol-induced syncope recover with supportive treatment, epinephrine, corticosteroids, and/or antihistamines have been used to increase the rate of recovery in some of these patients.
When methocarbamol is administered IV, thrombophlebitis, sloughing, and pain at the injection site may result from extravasation. IM injection of the drug may also cause local irritation. IV injection of methocarbamol may cause a small amount of hemolysis and increased hemoglobin and red blood cells in the urine. Leukopenia may occur rarely.
Parenteral dosage forms should be used with caution in patients with epilepsy.
For more Drug Warnings (Complete) data for METHOCARBAMOL (14 total), please visit the HSDB record page.

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Pharmacodynamics
Methacarbamol is a skeletal muscle relaxant with an unknown mechanism of action. Methacarbamol has been shown to block spinal polysynaptic reflexes, decrease nerve transmission in spinal and supraspinal polysynaptic pathways, and prolong the refractory period of muscle cells. Methocarbamol does not act as a local anesthetic upon injection. In animal studies, methocarbamol also prevents convulsions after electric shock.

C11H15NO5

241.24

241.095

532-03-6

Methocarbamol-d5;1189699-70-4;Methocarbamol-d3;1346600-86-9;Methocarbamol-13C,d3;2747917-88-8

4107

White to off-white solid powder

1.3±0.1 g/cm3

472.5±40.0 °C at 760 mmHg

95-97ºC

239.6±27.3 °C

0.0±1.2 mmHg at 25°C

1.541

0.55

2

5

7

17

236

0

GNXFOGHNGIVQEH-UHFFFAOYSA-N

InChI=1S/C11H15NO5/c1-15-9-4-2-3-5-10(9)16-6-8(13)7-17-11(12)14/h2-5,8,13H,6-7H2,1H3,(H2,12,14)

[2-hydroxy-3-(2-methoxyphenoxy)propyl] carbamate

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

Room temperature (This product is stable at ambient temperature for a few days during ordinary shipping and time spent in Customs)

Solubility in Formulation 1: ≥ 3.5 mg/mL (14.51 mM) (saturation unknown) in 10% DMSO + 40% PEG300 + 5% Tween80 + 45% Saline (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 35.0 mg/mL clear DMSO stock solution to 400 μL PEG300 and mix evenly; then add 50 μL Tween-80 to the above solution and mix evenly; then add 450 μL normal saline to adjust the volume to 1 mL.
Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH₂ O to obtain a clear solution.

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Solubility in Formulation 2: ≥ 3.5 mg/mL (14.51 mM) (saturation unknown) in 10% DMSO + 90% (20% SBE-β-CD in Saline) (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 35.0 mg/mL clear DMSO stock solution to 900 μL of 20% SBE-β-CD physiological saline solution and mix evenly.
Preparation of 20% SBE-β-CD in Saline (4°C，1 week): Dissolve 2 g SBE-β-CD in 10 mL saline to obtain a clear solution.

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Solubility in Formulation 3: ≥ 3.5 mg/mL (14.51 mM) (saturation unknown) in 10% DMSO + 90% Corn Oil (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 35.0 mg/mL clear DMSO stock solution to 900 μL of corn oil and mix evenly.

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Solubility in Formulation 4: 25 mg/mL (103.63 mM) in PBS (add these co-solvents sequentially from left to right, and one by one), clear solution; with ultrasonication (<60°C).

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 (Please use freshly prepared in vivo formulations for optimal results.)