In an adult with acute back muscle spasms and no contraindications, is baclofen or cyclobenzaprine (Flexeril) the preferred oral muscle relaxant?

Cyclobenzaprine (Flexeril) is Preferred Over Baclofen for Acute Back Muscle Spasms

For an adult with acute back muscle spasms, cyclobenzaprine (Flexeril) is the better choice because baclofen has minimal evidence for musculoskeletal pain and is primarily indicated for spasticity from upper motor neuron disorders, whereas cyclobenzaprine has consistent evidence for short-term relief of acute musculoskeletal conditions. 1, 2

Why Cyclobenzaprine is Preferred

Cyclobenzaprine has the most robust evidence base for acute musculoskeletal pain:

• Multiple randomized trials consistently demonstrate cyclobenzaprine's superiority to placebo for short-term pain relief in acute back and neck pain with muscle spasm 3, 4, 5, 6
• Cyclobenzaprine is specifically FDA-approved for acute musculoskeletal conditions, whereas baclofen is FDA-approved only for spasticity from upper motor neuron lesions 2, 7
• The American College of Physicians/American Pain Society guidelines explicitly state that skeletal muscle relaxants like cyclobenzaprine are an option for short-term relief of acute low back pain, while noting there is "little evidence for the efficacy of baclofen" in this setting 1

Why Baclofen is Not Appropriate

Baclofen should be reserved for true spasticity, not musculoskeletal pain:

• Baclofen is a GABA-B agonist designed for upper motor neuron spasticity (stroke, multiple sclerosis, spinal cord injury) with fair evidence only in those conditions 8, 2, 4
• For musculoskeletal conditions, there is "very limited or inconsistent data" regarding baclofen's effectiveness compared to placebo 4
• Recent evidence shows baclofen carries higher risks in older adults, with a 54% increased risk of injury and 233% increased risk of delirium compared to other muscle relaxants when used for musculoskeletal pain 9

Comparative Safety Profile

Both agents cause CNS adverse effects, but baclofen has a worse safety profile for musculoskeletal pain:

• Cyclobenzaprine's most common adverse effects are drowsiness, dry mouth, and dizziness—dose-related but generally well-tolerated 10, 5, 6
• Baclofen is associated with significantly higher rates of encephalopathy and composite injury outcomes, particularly in older adults 9, 11
• A 2022 emergency department study found no skeletal muscle relaxant (including baclofen, cyclobenzaprine, or others) provided benefit over placebo when combined with NSAIDs, though cyclobenzaprine had more adverse effects than placebo 12

Evidence Quality Considerations

The evidence hierarchy strongly favors cyclobenzaprine:

• The 2017 American College of Physicians guideline (the most recent high-quality guideline) recommends skeletal muscle relaxants like cyclobenzaprine for acute low back pain when NSAIDs/acetaminophen are insufficient, but does not recommend baclofen 13
• A 2020 JAGS guideline explicitly states that drugs like cyclobenzaprine "are approved for acute musculoskeletal pain in the United States" but "do not directly relax skeletal muscle and have no evidence of efficacy in chronic pain" 8
• The same guideline notes baclofen has only "anecdotal reports" and "minimal data" for pain management outside spasticity 8

Practical Prescribing Algorithm

For acute back muscle spasms without true spasticity:

1. First-line: NSAIDs or acetaminophen 13, 2

2. If inadequate relief after 48-72 hours: Add cyclobenzaprine 5 mg three times daily for ≤2 weeks 3, 5, 14

   • Start with lower doses (5 mg) rather than standard 10 mg to minimize sedation 14
   • Limit duration to 7-14 days maximum 3, 2

3. Do NOT use baclofen unless there is documented upper motor neuron spasticity (e.g., from stroke, MS, spinal cord injury) 2, 4

Critical Caveats

Important warnings for both agents:

• Neither medication should be used long-term for musculoskeletal pain—there is no evidence of sustained benefit and adverse effects outweigh any modest analgesia 2
• Avoid combining either agent with benzodiazepines or opioids due to markedly increased risk of respiratory depression and profound sedation 2
• In older adults (≥65 years), use extreme caution with either agent due to fall risk, cognitive impairment, and anticholinergic effects, though baclofen carries higher delirium risk 9, 11, 15
• Cyclobenzaprine is structurally similar to amitriptyline and carries similar anticholinergic risks (dry mouth, urinary retention, confusion) 16, 2
• If baclofen has been used chronically, it must be tapered slowly to avoid withdrawal phenomena including seizures and delirium 16, 2, 7

Bottom line: Cyclobenzaprine has consistent evidence for acute musculoskeletal pain relief, whereas baclofen lacks efficacy data for this indication and carries higher safety risks. Reserve baclofen exclusively for documented spasticity from neurological conditions.