Robaxin

Robaxin,

trademark for a skeletal muscle relaxant (methocarbamol).

---

methocarbamol

Methocarbamol (CA), Robaxin

Pharmacologic class: Autonomic nervous system agent

Therapeutic class: Skeletal muscle relaxant (centrally acting)

Pregnancy risk category C

Action

Unknown. Thought to depress central perception of pain without directly relaxing skeletal muscles or directly affecting motor endplate or motor nerves.

Availability

Injection: 100 mg/ml in 10-ml ampules, 100 mg/ml in 10-ml vials

Tablets: 500 mg, 750 mg

⊘Indications and dosages

➣ Adjunct in muscle spasms caused by acute, painful musculoskeletal conditions

Adults: Initially, 1.5 g P.O. q.i.d. (up to 8 g/day) for 2 to 3 days, then 4 to 4.5 g/day P.O. in three to six divided doses; or 750 mg P.O. q 4 hours or 1 g P.O. q.i.d. or 1.5 g P.O. t.i.d. If oral dosing isn't feasible or if condition is severe, give 1 to 3 g/day I.M. or I.V. for maximum of 3 days.

Off-label uses

• Tetanus

Contraindications

• Hypersensitivity to drug, its components, or polyethylene glycol (with parenteral form)
• Renal impairment (with parenteral form)

Precautions

Use cautiously in:
• seizure disorders (with parenteral use)
• pregnant or breastfeeding patients
• children (safety not established).

Administration

• For direct I.V. injection, administer slowly. Keep patient supine for 10 to 15 minutes afterward.
• For I.V. infusion, dilute 1 g with up to 250 ml 5% dextrose or 0.9% sodium chloride injection.
• Avoid extravasation; drug is hypertonic.
• Don't give subcutaneously.
• For I.M. use, inject no more than 500 mg (5 ml of 10% injection) into each gluteal area.
• Don't use parenteral form in patients with renal impairment. Polyethylene glycol vehicle may irritate kidneys.
• When giving for tetanus, crush and suspend tablets in water or saline solution, and give via nasogastric tube, if necessary.
• Be aware that drug is usually given as part of regimen that includes rest and physical therapy.

Route	Onset	Peak	Duration
P.O.	30 min	2 hr	Unknown
I.V.	Immediate	End of infusion	Unknown
I.M.	Unknown	Unknown	Unknown

Adverse reactions

CNS: dizziness, light-headedness, drowsiness, syncope, seizures (with I.V. use)

CV: bradycardia or hypotension (with I.V. use)

EENT: blurred vision, conjunctivitis, nasal congestion

GI: nausea, GI upset, anorexia

GU: brown, black, or green urine

Musculoskeletal: mild muscle incoordination (with I.V. or I.M. use)

Skin: flushing (with I.V. use), pruritus, rash, urticaria

Other: fever, pain at I.M. injection site, phlebitis at I.V. site, allergic reactions including anaphylaxis (with I.M. or I.V. use)

Interactions

Drug-drug. Antihistamines, CNS depressants (such as opioids, sedative-hypnotics): additive CNS depression

Drug-diagnostic tests. Urinary 5-hydroxyindoleacetic acid, urine vanillylmandelic acid: false elevations

Drug-herbs. Chamomile, hops, kava, skullcap, valerian: increased CNS depression

Drug-behaviors. Alcohol use: increased CNS depression

Patient monitoring

• Assess for orthostatic hypotension, especially with parenteral use. Keep patient supine for 10 to 15 minutes after I.V. administration.
☞ Watch for anaphylaxis after I.M. or I.V. administration.
☞ Stay alert for bradycardia and syncope after I.V. or I.M. dose. As needed and prescribed, give epinephrine, corticosteroids, or antihistamines.
• Monitor I.V. site frequently to prevent sloughing and thrombophlebitis.

Patient teaching

• Tell patient that drug may turn urine brown, black, or green.
• Caution patient to avoid driving and other hazardous activities, because drug may cause drowsiness or dizziness.
• Instruct patient to move slowly when changing position, to avoid dizziness from sudden blood pressure decrease.
• As appropriate, review all other significant and life-threatening adverse reactions and interactions, especially those related to the drugs, tests, herbs, and behaviors mentioned above.