What is the difference between Robaxin (Methocarbamol) and Flexeril (Cyclobenzaprine)?

From the Guidelines

Robaxin and Flexeril are both effective for short-term pain relief, but Flexeril is moderately superior to placebo for short-term pain relief, with a relative risk of 0.80 for not achieving pain relief at 2 to 4 days, as found in a higher-quality Cochrane review 1. When considering the treatment of acute low back pain, it's essential to weigh the benefits and harms of each medication.

• Robaxin (methocarbamol) acts by depressing the central nervous system, while Flexeril (cyclobenzaprine) blocks pain sensations sent to the brain.
• Flexeril is generally considered more effective for acute muscle spasms, with typical dosing of 5-10mg taken up to three times daily for 2-3 weeks.
• Robaxin is often prescribed at 1500mg four times daily for 2-3 days, then reduced to 750mg every 4 hours.
• Side effects overlap between the medications, with both causing drowsiness, dizziness, and headache, though Flexeril tends to cause more dry mouth and sedation.
• Neither medication should be taken with alcohol or other CNS depressants.
• The choice between them depends on individual factors like the patient's response to previous treatments, other medications being taken, and specific health conditions.
• Both medications are intended for short-term use (2-3 weeks maximum) alongside other treatments like physical therapy and rest, as there is insufficient evidence to conclude that any specific muscle relaxant is superior to others for benefits or harms in the long term 1.

From the FDA Drug Label

Methocarbamol Tablets, USP, 500 mg and 750 mg, a carbamate derivative of guaifenesin, is a central nervous system (CNS) depressant with sedative and musculoskeletal relaxant properties. Cyclobenzaprine hydrochloride is a white, crystalline tricyclic amine salt with the empirical formula C20H21N • HCl and a molecular weight of 311.9. It has a melting point of 217°C, and a pKa of 8. 47 at 25°C.

Comparison of Robaxin (Methocarbamol) and Flexeril (Cyclobenzaprine)

• Mechanism of Action: Both are central nervous system (CNS) depressants with musculoskeletal relaxant properties, but they have different chemical structures and mechanisms.
• Key Differences:
  • Methocarbamol is a carbamate derivative of guaifenesin.
  • Cyclobenzaprine is a tricyclic amine salt. The FDA drug label does not provide a direct comparison of the two drugs, therefore no conclusion can be drawn about which one is more effective or has fewer side effects 2 3.

From the Research

Comparison of Robaxin and Flexeril

• Robaxin (methocarbamol) and Flexeril (cyclobenzaprine) are both skeletal muscle relaxants used to treat musculoskeletal conditions and spasticity.
• According to a study published in 2004 4, cyclobenzaprine has been evaluated in the most clinical trials and has consistently been found to be effective in treating musculoskeletal conditions.
• The same study found that methocarbamol has very limited or inconsistent data regarding its effectiveness in treating musculoskeletal conditions.
• A review of commonly used muscle relaxant therapies for acute low back pain published in 2004 5 found that cyclobenzaprine hydrochloride has the most recent and largest clinical trials demonstrating its benefit.
• The review also found that carisoprodol and metaxalone appear to be effective, but carisoprodol's usefulness is mitigated by its potential for abuse.
• A study published in 2021 6 found that cyclobenzaprine was the dominant agent, accounting for more than 50% of prescriptions, and that the use of methocarbamol has declined in recent years.

Efficacy and Safety

• The 2004 study 4 found that there is fair evidence that cyclobenzaprine, carisoprodol, and tizanidine are effective compared to placebo in patients with musculoskeletal conditions.
• The study also found that there is insufficient evidence to determine the relative efficacy or safety of cyclobenzaprine, carisoprodol, and methocarbamol.
• The 2021 study 6 found that the median treatment duration for skeletal muscle relaxants was 14 days, and that 4.0% of individuals used them for more than 90 days.

Usage Patterns

• The 2021 study 6 found that the prevalence of skeletal muscle relaxant treatment ranged from 61.5 to 68.3 per 1,000 individuals, and that about one-third of users did not have a preceding musculoskeletal disorder diagnosis.
• The study also found that the use of skeletal muscle relaxants varied by age group, sex, geographic region, and individual agent.