Methocarbamol

Usage

Methocarbamol is primarily prescribed for the short-term relief of acute, painful musculoskeletal conditions such as muscle spasms, strains, and sprains. It’s classified as a central muscle relaxant and is often used adjunctively with rest, physical therapy, and other pain management strategies. Its mechanism of action involves depressing the central nervous system, which may interrupt reflexes that cause muscle spasms. The exact mechanism remains incompletely understood, but it is thought to act at the level of the spinal cord and possibly at subcortical areas of the brain.

Alternate Names

While “methocarbamol” is the generic name, it’s marketed under several brand names, most notably Robaxin and Robaxin-750. Other brand names include Carbacot, Skelex and Tanlor. There is a combination brand name available as well, which combines aspirin with methocarbomal called Robaxisal.

How It Works

Pharmacodynamics: Methocarbamol acts centrally to reduce muscle spasms, likely through CNS depression involving the spinal cord and potentially subcortical brain regions. Although not fully understood, it is thought that the primary mode of action is by blocking nerve impulses (or pain sensations) that are sent to the brain.

Pharmacokinetics:

• Absorption: Readily absorbed after oral administration.
• Metabolism: Extensively metabolized in the liver.
• Elimination: Primarily excreted by the kidneys as metabolites. About 85%-95% of methocarbamol is cleared renally in rats. Some excretion may occur through feces or bile, as well as through breastmilk, as per animal studies. The reported elimination half-life is about 1 to 2 hours.

Dosage

Standard Dosage

Adults:

• Oral: Initial dose: 1500 mg four times a day for 2-3 days. Maintenance dose: 1000 mg four times a day or 4000 mg to 4500mg per day in divided doses.
• IM/IV: 1 gram every 8 hours, not to exceed 3 grams daily for more than 3 consecutive days, except for tetanus.

Children:

• Oral: Safety and efficacy not established in children under 16 for muscle spasm.
• IV: For tetanus: 15 mg/kg every 6 hours or 500 mg/m² every 6 hours; not to exceed 1.8 g/m²/day for 3 days.

Special Cases:

• Elderly Patients: Start with a lower dose (e.g., 500 mg four times daily) and titrate cautiously due to increased sensitivity to side effects.
• Patients with Renal Impairment: Use with caution; IV formulation is contraindicated due to polyethylene glycol content. Oral use warrants close monitoring and possible dosage adjustment.
• Patients with Hepatic Dysfunction: Use with caution; dosage adjustments may be necessary.
• Patients with Comorbid Conditions: Use with caution in patients with seizure disorders or myasthenia gravis receiving cholinesterase inhibitors.

Clinical Use Cases

• Intubation/Surgical Procedures/Mechanical Ventilation/ICU Use: Information is limited regarding these clinical use cases. Dosage should be adjusted based on patient needs and response. Consult with a specialist or pharmacist if you are considering methocarbamol.
• Tetanus: Initially, 1-2 g IV, followed by 1-2 g IV infusion (total 3 g). Repeat every 6 hours until oral administration is possible. Up to 24 g orally may be needed.
• Emergency Situations: Data is not sufficient. However, dosage and administration route should be under the direction of an experienced physician or pharmacist.

Dosage Adjustments

Dosage adjustments should be made based on patient-specific factors such as age, renal or hepatic function, and concomitant medications. It’s important to avoid exceeding the maximum daily dose, especially in elderly patients and those with renal or hepatic impairment. There is no specific information available on adjustments based on genetic polymorphisms.

Side Effects

Common Side Effects

Drowsiness, dizziness, lightheadedness, headache, blurred vision, metallic taste, nausea, and upset stomach.

Rare but Serious Side Effects

Allergic reactions (rash, itching, hives, swelling), seizures, CNS depression (slowed breathing, confusion, fainting), bradycardia, hypotension, jaundice. Angioedema, amnesia, and adynamic ileus have also been reported but are very rare.

Long-Term Effects

Limited information is available on the long-term effects of methocarbamol.

Adverse Drug Reactions (ADR)

Anaphylaxis, angioedema, severe CNS depression requiring respiratory support.

Contraindications

Hypersensitivity to methocarbamol, renal impairment (for IV formulation), comatose or pre-comatose states. Use with extreme caution in patients with epilepsy, myasthenia gravis taking cholinesterase inhibitors and those with brain damage.

Drug Interactions

Methocarbamol may interact with other CNS depressants (e.g., alcohol, opioids, benzodiazepines, sedative-hypnotics, antihistamines, tricyclic antidepressants, some antipsychotics) leading to additive CNS depression, and potentially dangerous consequences such as respiratory depression. Caution is advised when using with cholinesterase inhibitors, general and local anesthetics, and muscle relaxants used during surgery. Methocarbamol can also interfere with laboratory tests for vanillylmandelic acid (VMA) and 5-hydroxyindoleacetic acid (5-HIAA), causing false-positive test results.

Pregnancy and Breastfeeding

Pregnancy Safety Category: C. Methocarbamol has been associated with fetal and congenital abnormalities in animal and some human case studies. It’s generally not recommended during pregnancy, especially during the first trimester. It is not known if methocarbamol passes into human breast milk. Caution is advised when using it while breastfeeding.

Drug Profile Summary

• Mechanism of Action: Central muscle relaxant, possibly acting at the spinal cord and subcortical levels.
• Side Effects: Drowsiness, dizziness, headache, blurred vision, metallic taste, nausea; rarely, allergic reactions, seizures, CNS depression.
• Contraindications: Hypersensitivity, renal impairment (IV), coma.
• Drug Interactions: Other CNS depressants, cholinesterase inhibitors.
• Pregnancy & Breastfeeding: Category C; use with caution.
• Dosage: See detailed dosage section above.
• Monitoring Parameters: Respiratory rate, heart rate, blood pressure, liver function tests, signs of allergic reactions.

Popular Combinations

Methocarbamol is often used in combination with NSAIDs like ibuprofen or naproxen for pain relief. It may also be prescribed with other muscle relaxants or physical therapy modalities for managing musculoskeletal conditions. Occasionally, methocarbamol may be given with aspirin for pain and inflammation management.

Precautions

• General Precautions: Screen for allergies, renal/hepatic impairment, and seizure disorders. Monitor for CNS depression, especially with concomitant CNS depressant use.
• Specific Populations: Pregnant women, breastfeeding mothers, children and elderly, consult a physician.
• Lifestyle Considerations: Avoid alcohol and operating machinery until effects are known.

FAQs (Frequently Asked Questions)

Q1: What is the recommended dosage for Methocarbamol?

A: Please refer to the detailed dosage guidelines provided above, which cover adult, pediatric, and special population dosing.

Q2: Is Methocarbamol a narcotic?

A: No, methocarbamol is not a narcotic. It’s a central muscle relaxant.

Q3: Can I drink alcohol while taking Methocarbamol?

A: No, it’s advised to avoid alcohol while taking methocarbamol, as it can potentiate the CNS depressant effects, leading to excessive drowsiness, dizziness, or impaired coordination.

Q4: How long can I take Methocarbamol?

A: Methocarbamol is generally prescribed for short-term use (2-3 days initially), with the possibility of continued treatment at lower doses. It is not recommended for chronic conditions without careful physician supervision.

Q5: What should I do if I miss a dose?

A: If you miss a dose, take it as soon as you remember. However, if it’s close to your next scheduled dose, skip the missed dose and resume your regular schedule. Do not double the dose.

Q6: Is Methocarbamol safe during pregnancy?

A: Methocarbamol is a Pregnancy Category C drug. Consult your physician, as the potential risks and benefits during pregnancy must be carefully assessed.

Q7: Can Methocarbamol be used for nerve pain?

A: Methocarbamol primarily targets muscle pain and spasms. While it might provide some relief for pain related to musculoskeletal conditions that also affect the nerves, it’s essential to consult with your physician or pharmacist to find the appropriate drug for nerve pain if necessary.

Q8: Can methocarbamol be used for long-term treatment of muscle spasticity in conditions like cerebral palsy?

A: Methocarbamol is not typically used for long-term treatment of muscle spasticity in conditions like cerebral palsy. Its efficacy in these conditions hasn’t been established, and other medications are usually preferred for managing chronic spasticity.

Q9: What are the key drug interactions to be aware of with Methocarbamol?

A: The most significant interactions are with other CNS depressants, such as alcohol, opioids, benzodiazepines, and certain antidepressants or antipsychotics. These combinations can lead to increased sedation and potentially dangerous respiratory depression. Always inform your physician about all other medications you are taking.

Q10: Are there any specific monitoring requirements while a patient is on Methocarbamol?

A: Monitoring should focus on vital signs, especially respiratory rate, heart rate, and blood pressure, as well as observing for signs of CNS depression or allergic reactions. Liver function tests may be necessary in some cases. Close monitoring is needed for patients with renal impairments, liver diseases, and comobid conditions, along with elderly patients.