METHOCARBAMOL

METHOCARBAMOL (meth-oh-kar'ba-mole) Robaxin Classifications: skeletal muscle relaxant; central-acting; Therapeutic: skeletal muscle relaxant, central acting Prototype: Cyclobenzaprine Pregnancy Category: C

Availability

500 mg, 750 mg tablet; 100 mg/mL injection

Action

Exerts skeletal muscle relaxant action by depressing multisynaptic pathways in the spinal cord and possibly by sedative effect.

Therapeutic Effect

No direct action on skeletal muscles; just on multisynaptic pathways in spinal cord that control muscular spasm.

Uses

Adjunct to physical therapy and other measures in management of discomfort associated with acute musculoskeletal disorders. Also used intravenously as adjunct in management of neuromuscular manifestations of tetanus.

Contraindications

Comatose states; CNS depression; acidosis, older adults; kidney dysfunction (injectable methocarbamol contains polyethylene glycol 300 in vehicle, which may cause urea retention and acidotic problems); pregnancy (category C).

Cautious Use

Epilepsy; females of childbearing age; renal disease, renal failure, renal impairment, seizure disorder; lactation, children less than 16 y.

Route & Dosage

Acute Musculoskeletal Disorders Adult: PO 1.5 g q.i.d. for 2–3 d, then 4–4.5 g/d in 3–6 divided doses IV/IM 1 g q8h Tetanus Adult: IV 1–3 g may be repeated q6h Child: PO 15 mg/kg repeated q6h as needed up to 1.8 g/m2/d for 3 consecutive d if necessary

Administration

Intramuscular

• Do not exceed IM dose of 5 mL (0.5 g) into each gluteal region. Insert needle deep and carefully aspirate. Inject drug slowly. Rotate injection sites and observe daily for evidence of irritation.

Intravenous PREPARE: Direct: May be given undiluted or diluted in up to 250 mL of NS or D5W.   ADMINISTER: Direct: Give at a rate of 300 mg or fraction thereof over 1 min or longer.   Keep patient recumbent during and for at least 15 min after IV injection in order to reduce possibility of orthostatic hypotension and other adverse reactions. Monitor vital signs and IV flow rate. Take care to avoid extravasation of IV solution, which may result in thrombophlebitis and sloughing.

INCOMPATIBILITIES Y-site: Furosemide.

• Store at 15°–30° C (59°–86° F).

Adverse Effects (≥1%)

Body as a Whole: Fever, anaphylactic reaction, flushing, syncope, convulsions. Skin: Urticaria, pruritus, rash, thrombophlebitis, pain, sloughing (with extravasation). Special Senses: Conjunctivitis, blurred vision, nasal congestion. CNS: Drowsiness, dizziness, light-headedness, headache. CV: Hypotension, bradycardia. GI: Nausea, metallic taste. Hematologic: Slight reduction of white cell count with prolonged therapy. Renal: Polyethylene glycol in the injection may increase preexisting acidosis and urea retention in patients with renal impairment.

Diagnostic Test Interference

Methocarbamol may cause false increases in urinary 5-HIAA (with nitrosonaphthol reagent) and VMA (Gitlow method).

Interactions

Drug: Alcohol and other cns depressants enhance CNS depression.

Pharmacokinetics

Absorption: Readily absorbed from GI tract. Onset: 30 min. Peak: 1–2 h. Metabolism: In liver. Elimination: In urine. Half-Life: 1–2 h.

Nursing Implications

Assessment & Drug Effects

• Lab tests: Obtain periodic WBC counts during prolonged therapy.
• Monitor vital signs closely during IV infusion.
• Supervise ambulation following parenteral administration.

Patient & Family Education

• Make position changes slowly, particularly from lying down to upright position; dangle legs before standing.
• Be aware that adverse reactions after oral administration are usually mild and transient and subside with dosage reduction. Use caution regarding drowsiness and dizziness. Avoid activities requiring mental alertness and physical coordination until response to drug is known.
• Urine may darken to brown, black, or green on standing.

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Common adverse effects in italic, life-threatening effects underlined; generic names in bold; classifications in SMALL CAPS; Canadian drug name; Prototype drug