What is the best medication for back pain, cyclobenzaprine (Flexeril) or methocarbamol (Robaxin)?

Cyclobenzaprine vs Methocarbamol for Back Pain

For acute back pain, cyclobenzaprine is the superior choice over methocarbamol based on substantially more robust clinical evidence demonstrating moderate efficacy for short-term pain relief, while methocarbamol lacks direct evidence of skeletal muscle relaxation and has minimal supporting data. 1, 2, 3

Evidence Quality Comparison

Cyclobenzaprine has the most extensive evidence base:

• Multiple systematic reviews including 36 trials demonstrate moderate superiority to placebo for short-term (2-4 days) pain relief in acute low back pain, with relative risk of 0.80 (95% CI 0.71-0.89) for not achieving pain relief 4
• Two large randomized controlled trials (N=1405) specifically evaluated cyclobenzaprine 5mg and 10mg three times daily, showing statistically significant superiority over placebo for all primary endpoints including global impression of change, medication helpfulness, and relief from starting backache 2, 5
• A meta-analysis found patients treated with cyclobenzaprine were nearly 5 times more likely to report symptom improvement by day 14 compared to placebo (odds ratio 4.7; 95% CI 2.7-8.1), with a number needed to treat of 2.7 6

Methocarbamol has minimal supporting evidence:

• The FDA label explicitly states that methocarbamol "does not directly relax tense skeletal muscles in man" and its mechanism "may be related to its sedative properties" 7
• The American Geriatrics Society guidelines explicitly state that methocarbamol has "no evidence of efficacy in chronic pain" 4
• A 2018 randomized trial (N=240) found naproxen+methocarbamol provided no additional functional improvement over naproxen+placebo for acute low back pain, with mean RMDQ improvement of 8.1 points versus 10.9 points for placebo 8

Direct Head-to-Head Evidence

No direct randomized trials comparing cyclobenzaprine versus methocarbamol exist in the evidence base. 1 However, the American College of Physicians found "insufficient evidence to conclude that any specific muscle relaxant is superior to others" when examining the entire class, though this statement is undermined by the vastly different quality and quantity of evidence for individual agents 4, 1

Dosing and Efficacy Details

Cyclobenzaprine 5mg three times daily is the optimal regimen:

• Equally effective as 10mg three times daily but with lower incidence of sedation 2, 5
• Onset of relief apparent within 3-4 doses (approximately 1 day) 5
• Effect size of 0.38-0.58 across all five outcome domains (local pain, muscle spasm, range of motion, tenderness, activities of daily living) 6
• Efficacy is independent of sedative effects, as demonstrated by subanalysis showing meaningful treatment effect in patients who did not report somnolence 5

Methocarbamol dosing per FDA label:

• 750mg taken as 1-2 tablets three times daily as needed 7
• No dose-response data or optimal dosing studies identified in the evidence

Critical Limitations for Both Agents

Duration of use must be strictly limited:

• All skeletal muscle relaxant trials were ≤2 weeks duration, with only one 3-week trial identified 4
• Treatment efficacy is greatest in the first 4 days, declining after the first week 6
• For chronic low back pain (>12 weeks), avoid both agents entirely and consider tricyclic antidepressants or duloxetine instead 1

Adverse Effect Profile

Both agents cause central nervous system effects, but cyclobenzaprine has more predictable dose-related sedation:

• Cyclobenzaprine: drowsiness/somnolence (most common), dry mouth, dizziness, fatigue—all dose-related 2, 5
• In clinical trials, ≥1 adverse event reported in 54-62% of cyclobenzaprine patients versus 35% with placebo 2
• Methocarbamol: 19% adverse event rate versus 17% with placebo in the 2018 trial, primarily sedation-related 8
• Combination therapy with NSAIDs increases central nervous system adverse events for both agents 4

Clinical Algorithm for Selection

For acute low back pain (<4 weeks):

1. Start with NSAIDs (ibuprofen 600-800mg three times daily or naproxen 500mg twice daily) as first-line therapy 4, 1
2. If severe pain persists after 2-4 days despite NSAIDs, add cyclobenzaprine 5mg three times daily for short-term use (≤1-2 weeks maximum) 1, 5
3. Avoid methocarbamol given lack of evidence demonstrating benefit over placebo when added to NSAIDs 8

For chronic low back pain (>12 weeks):

• Do not use either cyclobenzaprine or methocarbamol 4, 1
• Consider tricyclic antidepressants (amitriptyline) or duloxetine, which have moderate evidence for chronic pain 4, 1

Key Pitfalls to Avoid

Do not prescribe either agent for chronic low back pain, as no evidence supports efficacy beyond 2 weeks and the American Geriatrics Society explicitly warns against use in this setting 4, 1

Do not use in elderly patients without careful consideration of fall risk and cognitive impairment from sedation, as both agents are listed as potentially inappropriate 4

Do not expect muscle relaxants to work as monotherapy—combine with NSAIDs or acetaminophen and patient education about remaining active 1, 9

Do not prescribe extended courses without clear evidence of continued benefits, as efficacy declines after the first week 6

Avoid combining with benzodiazepines, which carry abuse and addiction risks and provide no additional benefit for back pain 4, 9