Methocarbamol

Uses

Methocarbamol is used for an adjunct to rest, physical therapy, and other measures for the relief of discomfort associated with acute, painful musculoskeletal conditions. The mode of action of methocarbamol has not been clearly identified, but may be related to its sedative properties. Methocarbamol does not directly relax tense skeletal muscles in man

Methocarbamol is also used to associated treatment for these conditions: Arthritis, Discomfort, Gouty Arthritis, Muscle Spasms, Pain, Rheumatism, Soreness, Muscle, Tetanus

How Methocarbamol works

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.

Dosage

Methocarbamol dosage

Methocarbamol 500 mg:

• Initial dosage: 1500 mg (3 tablets) q.i.d.
• Maintenance dosage: 2 tablets q.i.d.

Methocarbamol 750 mg:

• Initial dosage: 1500 mg (2 tablets) q.i.d.
• Maintenance dosage: 1 tablet q.4h. or 2 tablets t.i.d.

Six grams a day are recommended for the first 48 to 72 hours of treatment. (For severe conditions 8 grams a day may be administered). Thereafter, the dosage can usually be reduced to approximately 4 grams a day.

Side Effects

Body as a whole: Anaphylactic reaction, angioneurotic edema, fever, headache

Cardiovascular system: Bradycardia, flushing, hypotension, syncope, thrombophlebitis

Digestive system: Dyspepsia, jaundice (including cholestatic jaundice), nausea and vomiting

Hemic and lymphatic system: Leukopenia

Immune system: Hypersensitivity reactions

Nervous system: Amnesia, confusion, diplopia, dizziness or lightheadedness, drowsiness, insomnia, mild muscular incoordination, nystagmus, sedation, seizures (including grand mal), vertigo

Skin and special senses: Blurred vision, conjunctivitis, nasal congestion, metallic taste, pruritus, rash, urticaria

Toxicity

Overdose of methocarbamol may be associated with alcohol and other central nervous system depressants. Patients may experience nausea, drowsiness, blurred vision, hypotension, seizures, and coma. Treatment of overdose is generally through airway maintenance, monitoring urinary output, vital signs, and giving fluid intravenously if necessary.

The oral LD50 in rats is 3576.2mg/kg.

The FDA has classified methocarbamol as pregnancy category C. Animal and human studies have not been performed to determine the risks to a fetus, however fetal and congenital abnormalities have been reported. Methocarbamol is excreted in the milk of dogs, however it is unknown if this is also the case for humans. Caution should be exercised when taking methocarbamol while breastfeeding.

Studies to assess the carcinogenicity, mutagenicity, or effects on fertility of methocarbamol have not been performed.

Precaution

Patients should be cautioned that methocarbamol may cause drowsiness or dizziness, which may impair their ability to operate motor vehicles or machinery. Because methocarbamol may possess a general CNS-depressant effect, patients should be cautioned about combined effects with alcohol and other CNS depressants.

Since methocarbamol may possess a general CNS depressant effect, patients receiving methocarbamol should be cautioned about combined effects with alcohol and other CNS depressants. Safe use of methocarbamol has not been established with regard to possible adverse effects upon fetal development. There have been reports of fetal and congenital abnormalities following in utero exposure to methocarbamol. Therefore, methocarbamol should not be used in women who are or may become pregnant and particularly during early pregnancy unless in the judgment of the physician the potential benefits outweigh the possible hazards

Interaction

Methocarbamol may inhibit the effect of pyridostigmine bromide. Therefore, methocarbamol should be used with caution in patients with myasthenia gravis receiving anticholinesterase agents.

Food Interaction

• Avoid alcohol.
• Take with or without food. The absorption is unaffected by food.

[Moderate] GENERALLY AVOID: Alcohol may potentiate some of the pharmacologic effects of CNS-active agents.

Use in combination may result in additive central nervous system depression and
MANAGEMENT: Patients receiving CNS-active agents should be warned of this interaction and advised to avoid or limit consumption of alcohol.

Ambulatory patients should be counseled to avoid hazardous activities requiring complete mental alertness and motor coordination until they know how these agents affect them, and to notify their physician if they experience excessive or prolonged CNS effects that interfere with their normal activities.

Methocarbamol Drug Interaction

Major: acetaminophen / hydrocodoneModerate: zolpidem, diphenhydramine, duloxetine, escitalopram, pregabalin, metoprolol, alprazolam, cetirizineUnknown: amphetamine / dextroamphetamine, aspirin, celecoxib, omega-3 polyunsaturated fatty acids, cyclobenzaprine, albuterol, levothyroxine, acetaminophen, cyanocobalamin, ascorbic acid, cholecalciferol

Methocarbamol Disease Interaction

Major: renal dysfunctionModerate: seizures

Volume of Distribution

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.

Elimination Route

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.

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.

Clearance

0.2-0.8L/h/kg.

Elimination Route

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.

Pregnancy & Breastfeeding use

Pregnancy Category C. Animal reproduction studies have not been conducted with methocarbamol. It is also not known whether methocarbamol can cause fetal harm when administered to a pregnant woman or can affect reproduction capacity. methocarbamol should be given to a pregnant woman only if clearly needed.

Safe use of methocarbamol has not been established with regard to possible adverse effects upon fetal development. There have been reports of fetal and congenital abnormalities following in utero exposure to methocarbamol. Therefore, methocarbamol should not be used in women who are or may become pregnant and particularly during early pregnancy unless in the judgment of the physician the potential benefits outweigh the possible hazards

Nursing Mothers: Methocarbamol and/or its metabolites are excreted in the milk of dogs; however, it is not known whether methocarbamol or its metabolites are excreted in human milk. Because many drugs are excreted in human milk, caution should be exercised when Methocarbamol is administered to a nursing woman.

Contraindication

Methocarbamol is contraindicated in patients hypersensitive to methocarbamol or to any of the tablet components.

Special Warning

Pediatric Use: Safety and effectiveness of Methocarbamol in pediatric patients below the age of 16 have not been established.

Acute Overdose

Limited information is available on the acute toxicity of methocarbamol. Overdose of methocarbamol is frequently in conjunction with alcohol or other CNS depressants and includes the following symptoms: nausea, drowsiness, blurred vision, hypotension, seizures, and coma. In post-marketing experience, deaths have been reported with an overdose of methocarbamol alone or in the presence of other CNS depressants, alcohol or psychotropic drugs.

Management of overdose includes symptomatic and supportive treatment. Supportive measures include maintenance of an adequate airway, monitoring urinary output and vital signs, and administration of intravenous fluids if necessary. The usefulness of hemodialysis in managing overdose is unknown.

Storage Condition

Store at controlled room temperature, between 20°C and 25°C

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