Skeletal Muscle Relaxants for Acute Upper Back Strain
For an adult with acute upper back strain and no contraindications, skeletal muscle relaxants are an appropriate short-term option (≤2 weeks) and provide moderate pain relief, though all agents in this class are associated with central nervous system adverse effects, primarily sedation. 1
Evidence-Based Recommendation
The American College of Physicians/American Pain Society guidelines establish that skeletal muscle relaxants are effective for short-term relief of acute musculoskeletal pain, including back strain 1. While these guidelines specifically address low back pain, the mechanism and pathophysiology of acute upper back strain are analogous, making the evidence applicable to thoracic spine muscle strain.
Efficacy Profile
Strong evidence demonstrates that skeletal muscle relaxants are superior to placebo for acute pain:
- Pain relief improvement of 2-4 points on a 0-10 scale within 2-4 days 2, 3, 4
- Relative risk of 0.80 (95% CI: 0.71-0.89) for achieving pain relief at 2-4 days 5, 2
- 72% increased likelihood of global improvement at 5-7 days (RR 1.72) 2
Choosing Among Muscle Relaxants
No compelling evidence demonstrates that any specific skeletal muscle relaxant is superior to others in efficacy or safety 1, 5. However, risk-benefit profiles vary substantially:
Avoid these agents due to specific safety concerns:
- Carisoprodol: Metabolized to meprobamate (abuse/overdose risk) 1
- Dantrolene: Black box warning for potentially fatal hepatotoxicity 1
- Tizanidine and chlorzoxazone: Associated with hepatotoxicity (generally reversible) 1
Reasonable first-line options:
- Cyclobenzaprine 5 mg three times daily is well-studied and demonstrated equivalent efficacy whether used alone or combined with NSAIDs 6
- Methocarbamol has a favorable safety profile in comparative trials 5
Safety Considerations
All skeletal muscle relaxants carry a 50% increased risk of adverse events compared to placebo (RR 1.50,95% CI: 1.14-1.98) 2, 3, 4. Central nervous system effects are doubled (RR 2.04,95% CI: 1.23-3.37) 5, 2:
- Sedation/somnolence (most common)
- Dizziness
- Fatigue
- Lightheadedness
Critical prescribing parameters:
- Duration: Limit to ≤2 weeks maximum 1, 5, 2
- Timing: Most trials evaluated 2-week courses; evidence beyond this timeframe is insufficient 5
- Monitoring: Warn patients about sedation and avoid concurrent CNS depressants
Alternative and Adjunctive Therapy
First-line analgesics should be considered before or alongside muscle relaxants:
- NSAIDs: More effective than acetaminophen for pain relief (moderate-certainty evidence), with small-to-moderate benefit for acute back pain 1, 7
- Acetaminophen: Weaker analgesic but favorable safety profile; however, high-certainty evidence shows no benefit over placebo for acute back pain 1
Combination therapy (muscle relaxant + NSAID) shows inconsistent evidence compared to NSAID alone, with some trials favoring combination but others showing no additional benefit 2, 6.
Clinical Algorithm
- Assess contraindications: Hepatic dysfunction, concurrent CNS depressants, substance abuse history
- Select agent: Cyclobenzaprine 5 mg TID or methocarbamol (avoid carisoprodol, dantrolene)
- Duration: Prescribe ≤7-10 days initially; maximum 14 days total
- Counsel patient: Expect sedation; avoid driving/operating machinery initially
- Consider NSAID: Add if no GI/cardiovascular contraindications
- Reassess at 3-7 days: If no improvement, discontinue and consider alternative diagnosis
Important Caveats
- Benzodiazepines are NOT recommended despite similar efficacy to muscle relaxants, due to higher abuse/addiction/tolerance risks 1
- Evidence is limited to acute pain (<4 weeks duration); chronic use is not supported 5, 2
- Older adults require extra caution due to increased fall risk and CNS sensitivity 8
- Most evidence derives from low back pain studies; direct trials for upper back strain are lacking, but pathophysiology supports extrapolation