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Methocarbamol

Class: Centrally Acting Skeletal Muscle Relaxants
VA Class: MS200
Chemical Name: 1-Carbamate-3-(2-methoxyphenoxy)-1,2-propanediol
Molecular Formula: C11H15NO5
CAS Number: 532-03-6
Brands: Robaxin

Medically reviewed by Drugs.com on Feb 14, 2022. Written by ASHP.

Introduction

Centrally acting skeletal muscle relaxant.

Uses for Methocarbamol

Muscular Conditions

Adjunct to rest, physical therapy, analgesics, and other measures for the relief of discomfort associated with acute, painful musculoskeletal conditions.

If pharmacologic therapy is required for acute low back pain (usually a benign and self-limiting condition), experts state that an NSAIA or skeletal muscle relaxant may be considered. Skeletal muscle relaxants may provide small improvements in pain relief, but are associated with a high incidence of adverse effects (e.g., CNS effects). Use with caution after weighing risks against benefits.

Various skeletal muscle relaxants appear to have comparable efficacy for low back pain relief.

Methocarbamol is ineffective in the treatment of skeletal muscle hyperactivity secondary to chronic neurologic disorders (e.g., cerebral palsy) and other dyskinesias.

Tetanus

Has been used as an adjunct to debridement, tetanus antitoxin, penicillin, tracheotomy, fluid and electrolyte replacement, and supportive therapy in the management of tetanus. However, most authorities prefer other sedatives or muscle relaxants (e.g., diazepam) and, in severe cases, neuromuscular blocking agents.

Methocarbamol Dosage and Administration

Administration

Administer orally; may administer IV or IM when oral administration is not feasible or for severe musculoskeletal pain. Do not administer sub-Q.

Oral Administration

NG Tube

For administration via NG tube, crush tablets and suspend in water or saline solution.

IV Administration

For solution compatibility information, see Compatibility under Stability.

Administer undiluted by direct IV injection (at a rate not exceeding 3 mL/minute), or dilute and administer by IV infusion.

Patient should be recumbent during and for 10–15 minutes following IV administration.

Avoid extravasation; solution is hypertonic.

Blood aspirated into syringe does not mix with methocarbamol injection; either inject any blood in the syringe or stop the injection when the plunger reaches the blood.

Dilution

For IV infusion, dilute 1 g with up to 250 mL of 5% dextrose or 0.9% sodium chloride injection.

Rate of Administration

For direct IV injection, maximum rate of 3 mL/minute.

IM Administration

Administer no more than 500 mg (5 mL of the 100-mg/mL injection) into each gluteal region.

Dosage

Pediatric Patients

Tetanus
IV

Recommended minimum initial dose is 15 mg/kg or 500 mg/m2; may give additional doses of 15 mg/kg or 500 mg/m2 by direct IV injection or IV infusion every 6 hours, if necessary (maximum 1.8 g/m2 daily for 3 consecutive days).

Adults

Muscular Conditions
Oral

Usual initial dosage is 1.5 g 4 times daily for 2–3 days. For maintenance, decrease dosage to 4–4.5 g daily in 3–6 divided doses.

Patients with severe symptoms may require initial dosage of 8 g daily in divided doses.

IV or IM

Usually, 1 g as a single dose, followed by oral methocarbamol to maintain relief.

For more severe conditions or when oral administration is not feasible, additional doses of 1 g may be administered every 8 hours (maximum 3 g daily for 3 consecutive days). If necessary, may readminister IM or IV after a 48-hour drug-free interval.

Tetanus
IV, then Oral

Recommended initial dose is 1–2 g by direct IV injection; may administer additional 1–2 g by IV infusion (for total initial dose of up to 3 g).

Repeat dosage regimen every 6 hours until NG tube can be inserted. May then administer crushed tablets suspended in water or saline through NG tube. Up to 24 g daily (via NG tube) may be required.

Prescribing Limits

Pediatric Patients

Tetanus
IV

Maximum 1.8 g/m2 daily for 3 consecutive days.

Adults

Muscular Conditions
IV or IM

Maximum 3 g daily for 3 consecutive days.

Cautions for Methocarbamol

Contraindications

  • Injection contraindicated in patients with impaired renal function. (See Renal Impairment under Cautions.)

  • Known hypersensitivity to methocarbamol or any ingredient in the formulation.

Warnings/Precautions

Warnings

CNS Depression

Performance of activities requiring mental alertness or physical coordination may be impaired.

Possible additive effect with other CNS depressants and/or alcohol. (See Specific Drugs and Laboratory Tests under Interactions.)

Sensitivity Reactions

Anaphylactic reactions, angioedema, urticaria, pruritus, rash, skin eruptions, and conjunctivitis with nasal congestion have occurred.

General Precautions

Seizure Disorders

Although causal relationship not established, seizures reported during IV administration. Use injection with caution in patients with known or suspected seizure disorders.

Specific Populations

Pregnancy

Not known whether the drug can cause fetal harm or affect reproductive capacity. Fetal and congenital abnormalities reported following in utero exposure; animal reproductive studies not conducted. Do not use during pregnancy, particularly during early pregnancy, unless potential benefits outweigh possible risks.

Lactation

Distributed into milk in dogs; not known whether distributed into human milk. Use caution.

Pediatric Use

Safety and efficacy (other than IV use in the management of tetanus) not established in pediatric patients <16 years of age.

Geriatric Use

Because of risk of injury, skeletal muscle relaxants should generally be avoided in geriatric patients.

Renal Impairment

Polyethylene glycol vehicle of methocarbamol injection may worsen preexisting acidosis and urea retention; although amount present in preparation is well within limits of safety, caution is advised. Do not administer to patients with impaired renal function. (See Contraindications under Cautions.)

Common Adverse Effects

Drowsiness, dizziness, lightheadedness.

Interactions for Methocarbamol

Specific Drugs and Laboratory Tests

Drug or Test

Interaction

Comments

Anticholinesterase agents (e.g., pyridostigmine)

Potential for severe weakness

Use with caution in patients with myasthenia gravis

CNS depressants (e.g., alcohol)

Potential for additive CNS depression

Use caution to avoid overdosage

Tests for 5-hydroxyindolacetic acid (5-HIAA) in urine (nitrosonaphthol reagent in quantitative method of Udenfriend)

False-positive results (color interference)

Tests for vanillylmandelic acid (VMA) in urine by the screening method of Gitlow

False-positive results (color interference)

Methocarbamol Pharmacokinetics

Absorption

Bioavailability

Rapidly and almost completely absorbed following oral administration.

Onset

Usually within 30 minutes following oral administration.

Almost immediate after IV administration.

Distribution

Extent

Widely distributed in dogs, with highest concentrations in the kidney and liver.

Methocarbamol and/or its metabolites cross the placenta in dogs.

Distributed into milk in dogs; not known whether distributed into human milk.

Plasma Protein Binding

46–50%.

Elimination

Metabolism

Extensively metabolized, presumably in the liver, by dealkylation and hydroxylation.

Elimination Route

Eliminated principally in urine as metabolites (40–50% as glucuronide and sulfate conjugates, remainder as unidentified metabolites); small amount (10–15%) eliminated unchanged in urine. Very small amounts excreted in feces.

Half-life

0.9–1.8 hours.

Special Populations

In geriatric patients, half-life slightly prolonged.

In patients with renal impairment on maintenance dialysis, clearance decreased by 40% but no apparent increase in half-life.

In patients with cirrhosis secondary to alcohol abuse, clearance decreased by 70% and half-life increased to about 3.4 hours.

Stability

Storage

Oral

Tablets

Tight containers at 20–25°C. Protect from light and moisture.

Parenteral

Injection

20–25°C (may be exposed to 15–30°C).

Do not refrigerate after dilution. Precipitation and haze formation may occur if diluted solution is refrigerated. Haze formation in diluted solutions may be unpredictable; visually inspect all diluted solutions prior to administration regardless of storage conditions.

Compatibility

For information on systemic interactions resulting from concomitant use, see Interactions.

Parenteral

Solution Compatibility

Compatible

Dextrose 5% in water

Sodium chloride 0.9%

Actions

  • CNS depressant with sedative and skeletal muscle relaxant effects.

  • Precise mechanism of action is not known; does not directly relax skeletal muscle and has minimal skeletal muscle relaxant effects. Beneficial effect probably is related to the drug’s sedative effect.

  • Unlike neuromuscular blocking agents, does not depress neuronal conduction, neuromuscular transmission, or muscle excitability.

Advice to Patients

  • Potential to impair mental alertness or physical coordination, especially with concomitant use of alcohol or other CNS depressants; use caution when driving or operating machinery.

  • Importance of informing clinicians of existing or contemplated concomitant therapy, including prescription and OTC drugs and dietary or herbal supplements, as well as concomitant illnesses.

  • Importance of women informing clinicians if they are or plan to become pregnant or plan to breast-feed.

  • Importance of informing patients of other important precautionary information. (See Cautions.)

Preparations

Excipients in commercially available drug preparations may have clinically important effects in some individuals; consult specific product labeling for details.

Please refer to the ASHP Drug Shortages Resource Center for information on shortages of one or more of these preparations.

* available from one or more manufacturer, distributor, and/or repackager by generic (nonproprietary) name

Methocarbamol

Routes

Dosage Forms

Strengths

Brand Names

Manufacturer

Oral

Tablets

500 mg*

Methocarbamol Tablets

750 mg*

Methocarbamol Tablets

Tablets, film-coated

500 mg

Robaxin

Endo

750 mg

Robaxin-750

Endo

Parenteral

Injection

100 mg/mL

Robaxin

West-ward

AHFS DI Essentials™. © Copyright 2022, Selected Revisions February 24, 2020. American Society of Health-System Pharmacists, Inc., 4500 East-West Highway, Suite 900, Bethesda, Maryland 20814.

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