METHOCARBAMOL — methocarbamol injection
Somerset Therapeutics, LLC
Methocarbamol injection, USP a carbamate derivative of guaifenesin, is a central nervous system (CNS) depressant with sedative and musculoskeletal relaxant properties. It is a sterile, pyrogen-free solution intended for intramuscular or intravenous administration.
Each mL contains: methocarbamol, USP 100 mg, polyethylene glycol 300, NF 0.5 mL, Water for Injection, USP q. s. The pH is adjusted, when necessary, with hydrochloric acid and/or sodium hydroxide. The chemical name of methocarbamol is 3-(2-methoxyphenoxy)-1,2-propanediol 1-carbamate and has the empirical formula of C11 H15 NO5. Its molecular weight is 241.24. The structural formula is shown below.
Methocarbamol is a white powder, sparingly soluble in water and chloroform, soluble in alcohol (only with heating) and propylene glycol, and insoluble in benzene and n -hexane.
Methocarbamol injection has a pH between 3.5 and 6.0.
AFTER MIXING WITH I.V. INFUSION FLUIDS, DO NOT REFRIGERATE.
The mechanism of action of methocarbamol in humans has not been established, but may be due to general CNS depression. It has no direct action on the contractile mechanism of striated muscle, the motor end plate or the nerve fiber.
In healthy volunteers, the plasma clearance of methocarbamol ranges between 0.20 and 0.80 L/h/kg, the mean plasma elimination half-life ranges between 1 and 2 hours, and the plasma protein binding ranges between 46% and 50%.
Methocarbamol is metabolized via dealkylation and hydroxylation. Conjugation of methocarbamol also is likely. Essentially all methocarbamol metabolites are eliminated in the urine. Small amounts of unchanged methocarbamol also are excreted in the urine.
The mean (±SD) elimination half-life of methocarbamol in elderly healthy volunteers (mean (±SD) age, 69 (±4) years) was slightly prolonged compared to a younger (mean (±SD) age, 53.3 (±8.8) years), healthy population (1.5 (±0.4) hours versus 1.1 (±0.27) hour, respectively). The fraction of bound methocarbamol was slightly decreased in the elderly versus younger volunteers (41 to 43% versus 46 to 50%, respectively).
The clearance of methocarbamol in 8 renally-impaired patients on maintenance hemodialysis was reduced about 40% compared to 17 normal subjects, although the mean (±SD) elimination half-life in these two groups was similar (1.2 (±0.6) versus 1.1 (± 0.3) hours, respectively).
In 8 patients with cirrhosis secondary to alcohol abuse, the mean total clearance of methocarbamol was reduced approximately 70% compared to that obtained in 8 age- and weight-matched normal subjects. The mean (±SD) elimination half-life in the cirrhotic patients and the normal subjects was 3.38 (±1.62) hours and 1.11 (±0.27) hours respectively. The percent of methocarbamol bound to plasma proteins was decreased to approximately 40 to 45% compared to 46 to 50% in the normal subjects.
INDICATIONS AND USAGE
The injectable form of methocarbamol is indicated as 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 this drug has not been clearly identified, but may be related to its sedative properties. Methocarbamol does not directly relax tense skeletal muscles in man.
Methocarbamol injection should not be administered to patients with known or suspected renal pathology. This caution is necessary because of the presence of polyethylene glycol 300 in the vehicle.
A much larger amount of polyethylene glycol 300 than is present in recommended doses of Methocarbamol injection is known to have increased pre-existing acidosis and urea retention in patients with renal impairment. Although the amount present in this preparation is well within the limits of safety, caution dictates this contraindication.
Methocarbamol injection is contraindicated in patients hypersensitive to methocarbamol or to any of the injection components.
Since methocarbamol may posses a general CNS depressant effect, patients receiving Methocarbamol injection should be cautioned about combined effects with alcohol and other CNS depressants.
Safe use of Methocarbamol injection has not been established with regard to possible adverse effects upon fetal development. There have been very rare reports of fetal and congenital abnormalities following in utero exposure to methocarbamol. Therefore, Methocarbamol injection 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 (see PRECAUTIONS, Pregnancy).
Use in Activities Requiring Mental Alertness
Methocarbamol may impair mental and/or physical abilities required for performance of hazardous tasks, such as operating machinery or driving a motor vehicle. Patients should be cautioned about operating machinery, including automobiles, until they are reasonably certain that methocarbamol therapy does not adversely affect their ability to engage in such activities.
As with other agents administered either intravenously or intramuscularly, careful supervision of dose and rate of injection should be observed. Rate of injection should not exceed 3 mL per minute-i.e., one 10 mL vial in approximately three minutes. Since Methocarbamol injection is hypertonic, vascular extravasation must be avoided. A recumbent position will reduce the likelihood of side reactions.
Blood aspirated into the syringe does not mix with the hypertonic solution. This phenomenon occurs with many other intravenous preparations. The blood may be injected with the methocarbamol, or the injection may be stopped when the plunger reaches the blood, whichever the physician prefers.
The total dosage should not exceed 30 mL (three vials) a day for more than three consecutive days except in the treatment of tetanus.
Caution should be observed in using the injection form in patients with suspected or known seizure disorders.
Information for Patients
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.
See WARNINGS and PRECAUTIONS for interaction with CNS drugs and alcohol.
Methocarbamol may inhibit the effect of pyridostigmine bromide. Therefore, methocarbamol should be used with caution in patients with myasthenia gravis receiving anticholinesterase agents.
Drug/Laboratory Test Interactions
Methocarbamol may cause a color interference in certain screening tests for 5-hydroxyindoleacetic acid (5-HIAA) using nitrosonaphthol reagent and in screening tests for urinary vanillylmandelic acid (VMA) using the Gitlow method.
Carcinogenesis, Mutagenesis Impairment of Fertility
Long-term studies to evaluate the carcinogenic potential of methocarbamol have not been performed. No studies have been conducted to assess the effect of methocarbamol on mutagenesis or its potential to impair fertility.
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 injection should be given to a pregnant woman only if clearly needed.
Safe use of Methocarbamol injection 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 injection 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 (see WARNINGS).
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