Chapter 204. Methocarbamol

Pharmacology. Methocarbamol is a centrally acting muscle relaxant. It does not directly relax skeletal muscle and does not depress neuromuscular transmission or muscle excitability; muscle relaxation is probably related to its sedative effects. After intravenous administration, the onset of action is nearly immediate. Elimination occurs by hepatic metabolism, with a serum half-life of 0.9–2.2 hours.

Indications

1. Control of painful muscle spasm caused by black widow spider envenomation (See Spiders). Methocarbamol should be used as an adjunct to other medications (eg, morphine, diazepam) that are considered more effective.

2. Management of muscle spasm caused by mild tetanus (See Tetanus) or strychnine (See Strychnine) poisoning.

Contraindications

1. Known hypersensitivity to the drug.

2. History of epilepsy (intravenous methocarbamol may precipitate seizures).

Adverse effects

1. Dizziness, drowsiness, nausea, flushing, and metallic taste may occur.

2. Extravasation from an intravenous site may cause phlebitis and sloughing. Do not administer subcutaneously.

3. Hypotension, bradycardia, and syncope have occurred after intramuscular or intravenous administration. Keep the patient in a recumbent position for 10–15 minutes after injection.

4. Urticaria and anaphylactic reactions have been reported.

5. Use in pregnancy. There is no reported experience, and no FDA category has been assigned.

Drug or laboratory interactions

1. Methocarbamol produces additive sedation with alcohol and other CNS depressants.

2. Methocarbamol may cause false-positive urine test results for 5-hydroxyindole acetic acid (5-HIAA) and vanillylmandelic acid (VMA).

3. The urine may turn brown, black, or blue after it stands.

Dosage and method of administration

1. Parenteral

   1. Administer 1 g (children: 15 mg/kg) IV over 5 minutes, followed by 0.5 g in 250 mL of 5% dextrose (children: 10 mg/kg in 5 mL of 5% dextrose) over 4 hours. Repeat every 6 hours to a maximum of 3 g daily.

   2. For tetanus, higher doses are usually recommended. Give 1–2 g IV initially, no faster than 300 mg/min; if needed, use a continuous infusion up to a total of 3 g.

   3. The usual intramuscular dose is 500 mg every 8 hours for adults and 10 mg/kg every 8 hours for children. Do not inject more than 5 mL into the gluteal region. Do not give subcutaneously.

2. Oral. Switch to oral administration as soon as tolerated. Give 0.5–1 g (children: 10–15 mg/kg) PO or by gastric tube every 6 hours. The maximum dose is 1.5 g every 6 hours; for tetanus, the maximum adult dose is 24 g/d.

Formulations

1. Parenteral. Methocarbamol (Robaxin, others), 100 mg/mL in 10-mL vials (in solution with polyethylene glycol).

2. Oral. Methocarbamol (Robaxin, others), 500- and 750-mg tablets.

3. Suggested minimum stocking levels to treat a 100-kg adult for the first 8 hours and 24 hours: methocarbamol, first 8 hours: 8 g or eight vials (100 mg/mL, 10 mL each); first 24 hours: 24 g or 24 vials (100 mg/mL, 10 mL each).