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methocarbamol (Robaxin)

 

Classes: Skeletal Muscle Relaxants

Dosing and uses of Robaxin (methocarbamol)

 

Adult dosage forms and strengths

tablet

  • 500mg
  • 750mg

injectable solution

  • 100mg/mL

 

Muscle Spasm

1 g IV/IM; additional doses at q8hr until PO; not to exceed 3 g/day

Total parenteral dose SHOULD NOT EXCEED 3 g/day for >3 days, unless treating tetanus; if condition persists, may repeat therapy after drug-free interval of 48 hr

1500 mg PO q6hr for 48-72 hours; not to exceed 8 g/day THEN decrease to 4-4.5 g/day divided q4-8hr

 

Tetanus

Adjunct therapy: Initial 1-2 g IV injection (at 300 mg/min), THEn

Additional 1-2 g IV infusion for total dose of 3 g initially

May repeat 1-2 g IV q6hr until can give NG or PO

Total of 24 g PO may be needed

 

Dosing Modifications

Renal failure: Not studied; parenteral dosage form contraindicated due to presence of polyethylene glycoL

Hepatic failure: Not studied

 

Pediatric dosage forms and strengths

tablet

  • 500mg
  • 750mg

injectable solution

  • 100mg/mL

 

Tetanus

15 mg/kg/dose IV q6hr PRN or 500 mg/m²/dose; not to exceed 1.8 g/m²/day for 3 days only

 

Muscle Spasm

<16 years: Safety and efficacy not established

>16 years: 1500 mg PO q6hr for 48-72 hours; not to exceed 8 g/day THEN decrease to 4-4.5 g/day divided q4-8hr

 

Geriatric dosage forms and strengths

Considered safe in elderly because of short half-life

 

Muscle Spasm

500 mg PO q6hr; may gradually titrate dose to response

 

Robaxin (methocarbamol) adverse (side) effects

Frequency not defined

Drowsiness

Dizziness

Leukopenia (IV)

Lightheadedness

Blurred vision

Headache

Fever

Nausea

Anorexia

Adynamic ileus

Dysgeusia

GI upset

Nystagmus

Diplopia

Flushing

Vertigo

Mild muscular incoordination

Syncope

Hypotension

Bradycardia

Urticaria

Pruritus

Rash

Skin eruptions

Conjunctivitis with nasal congestion

Anaphylactic reactions

Thrombophlebitis (IV)

Sloughing (IV)

Injection site pain (IV)

Hemolysis (IV)

Increased Hgb and RBCs in urine (IV)

Seizures (IV)

 

Warnings

Contraindications

Hypersensitivity

Renal impairment (parenteral)

 

Cautions

May cause drowsiness/dizziness; patients should not ingest alcohol or other CNS depressants

May take with food to avoid stomach upset

Half-life increases with hepatic impairment

IV formulation not for use in renal impairment (contains polyethylene glycol; injection rate should not exceed 3 mL/min

May interfere with screening tests for 5-HIAA and vanillylmandelic acid (VMA)

Use injection with caution in patients with a history of seizures

Sedative effects potentiated when used with other sedatives

Medication is poorly tolerated in the elderly

Pediatric IV dosing approved only for tetanus

 

Pregnancy and lactation

Pregnancy category: C

Lactation: Not known if excreted in breast milk; use caution

 

Pregnancy categories

A: Generally acceptable. Controlled studies in pregnant women show no evidence of fetal risk.

B: May be acceptable. Either animal studies show no risk but human studies not available or animal studies showed minor risks and human studies done and showed no risk.

C: Use with caution if benefits outweigh risks. Animal studies show risk and human studies not available or neither animal nor human studies done.

D: Use in LIFE-THREATENING emergencies when no safer drug available. Positive evidence of human fetal risk.

X: Do not use in pregnancy. Risks involved outweigh potential benefits. Safer alternatives exist.

NA: Information not available.

 

Pharmacology of Robaxin (methocarbamol)

Mechanism of action

Unknown; possibly general CNS depression induces skeletal muscle relaxation

 

Absorption

Onset: 30 min

Peak serum time: 1-2 hr

Peak plasma concentration: 16.5-29.8 mcg/mL

Protein binding: 46-50%

 

Metabolism

Liver (dealkylation and hydroxylation)

Metabolites: Glucuronide and sulfate conjugates of 3-(2-hydroxyphenoxy)-1,2-propanediol-1-carbamate and 3-(4-hydroxy-2-methoxyphenoxy)-1,2-propanediol-1-carbamate; unidentified metabolites

 

Elimination

Half-life: 0.9-1.8 hr

Excretion: Urine

 

Administration

IV Administration

Direct IV: Administer undiluted at not to exceed 3 mL (=300 mg undiluted)/min

Infusion: Dilute 1 g with up to 250 mL D5W or NS; avoid extravasation, since injection is hypertonic

Monitor closely for extravasation

Administer IV while in recumbent position

Maintain position 15-30 min following infusion

 

IM Administration

Not to exceed 500 mg (5 mL undiluted) should be given into each gluteal region

 

Storage

Injection when diluted to 4 mg/mL in SWI, D5W, or NS is stable for 6 days at room temperature

Do not refrigerate after dilution