What is the comparison between Robaxin (methocarbamol) and Flexeril (cyclobenzaprine) for treating back pain?

Robaxin vs Flexeril for Back Pain

For acute low back pain, choose Flexeril (cyclobenzaprine) over Robaxin (methocarbamol) based on substantially stronger evidence—cyclobenzaprine has been studied in 20 trials with 1,553 patients demonstrating moderate short-term efficacy, while methocarbamol showed no benefit over placebo when added to naproxen in the only high-quality head-to-head trial. 1, 2

Evidence Quality Comparison

Cyclobenzaprine has far superior evidence:

• Pooled data from 20 trials (n=1,553) demonstrates cyclobenzaprine is superior to placebo for short-term global improvement (2-4 days), with patients nearly 5 times more likely to report symptom improvement by day 14 (odds ratio 4.7; 95% CI 2.7-8.1) 1, 3
• The number needed to treat is 2.7, meaning fewer than 3 patients need treatment for 1 to improve 3
• Effect sizes range from 0.38 to 0.58 across all outcome domains (local pain, muscle spasm, range of motion, tenderness, activities of daily living) 3

Methocarbamol has minimal supporting evidence:

• A 2018 randomized controlled trial (n=240) found that adding methocarbamol to naproxen provided no functional improvement over naproxen plus placebo for acute nonradicular low back pain 2
• The American College of Physicians found insufficient evidence to conclude methocarbamol is effective, with no direct trials demonstrating superiority 1

Dosing Algorithm

For cyclobenzaprine:

• Start with 5 mg three times daily (TID) for 7-14 days maximum 4, 5
• The 5 mg TID dose is as effective as 10 mg TID but causes less sedation (54% vs 62% reporting adverse events) 4
• Onset of relief occurs within 3-4 doses 4
• Do not use the 2.5 mg TID dose—it was not significantly more effective than placebo 4

For methocarbamol (if cyclobenzaprine is contraindicated):

• Standard dosing is 750 mg, 1-2 tablets three times daily as needed 2
• However, recognize this choice lacks evidence of efficacy 2

Treatment Efficacy Timeline

Peak effectiveness occurs early:

• Greatest benefit is seen in the first 4 days of treatment with cyclobenzaprine 3
• Efficacy declines after the first week, supporting shorter treatment courses 3
• All muscle relaxant trials were ≤2 weeks duration, with no evidence supporting longer use 1

Safety Profile Comparison

Both agents cause CNS adverse effects:

• Cyclobenzaprine: drowsiness/sedation is most common (dose-related), but efficacy is independent of sedative effects 4, 3
• Methocarbamol: 19% reported adverse events (vs 17% with placebo), primarily drowsiness and dizziness 2
• All skeletal muscle relaxants increase total adverse events (RR 1.50; 95% CI 1.14-1.98) and CNS adverse events (RR 2.04; 95% CI 1.23-3.37) compared to placebo 1

Critical safety consideration:

• Methocarbamol has potential for hepatotoxicity (usually reversible), requiring monitoring 6
• Both agents increase fall risk in elderly patients and should be avoided per the American Geriatrics Society 1

Combination Therapy Approach

Always combine with NSAIDs or acetaminophen:

• Muscle relaxants should not be used as monotherapy 1
• NSAIDs are slightly more effective than acetaminophen for pain relief and should be first-line 1
• Adding a muscle relaxant to NSAIDs provides greater short-term pain relief than monotherapy, but increases CNS adverse events (RR 2.44; 95% CI 1.05-5.63) 1

When NOT to Use Either Agent

Avoid both methocarbamol and cyclobenzaprine in:

• Chronic low back pain (>12 weeks)—no evidence supports efficacy beyond 2 weeks; use tricyclic antidepressants or duloxetine instead 1, 6
• Radicular pain/sciatica—use gabapentin as first-line for the neuropathic component 1, 6
• Elderly patients—high risk of falls, cognitive impairment, and sedation 1

Common Pitfalls to Avoid

• Do not prescribe for longer than 1-2 weeks—all trial evidence is limited to ≤2 weeks duration 1
• Do not expect muscle relaxants to work alone—they must be combined with NSAIDs/acetaminophen and patient education about remaining active 1
• Do not use in chronic pain—the American Geriatrics Society explicitly states muscle relaxants have no evidence of efficacy in chronic pain 1
• Warn patients about sedation—this is the most common adverse effect and affects ability to drive or operate machinery 4, 3, 5