Why Muscle Relaxers Are Prescribed for Pain
Muscle relaxers are prescribed for pain relief in acute musculoskeletal conditions, but their mechanism is not actually through muscle relaxation—their effects are nonspecific and likely related to central nervous system sedation rather than direct action on muscle spasm. 1
The Mechanism Is Misunderstood
Muscle relaxants do not directly relax tense skeletal muscles in humans. 2 This is a critical misconception that leads to inappropriate prescribing.
The pain relief mechanism remains unclear, but may be related to sedative properties and inhibition of polysynaptic reflexes in animal models—whether this translates to human pain relief is unknown. 1
These drugs should not be prescribed under the mistaken belief that they relieve muscle spasm. 1 If true muscle spasm is suspected, consider benzodiazepines or baclofen instead, which have known effects on spasticity. 1
Evidence for Efficacy in Acute Pain
For acute low back pain, skeletal muscle relaxants are moderately superior to placebo for short-term pain relief (2-4 days), with relative risk for not achieving pain relief of 0.80 at 2-4 days. 1
Cyclobenzaprine 5 mg three times daily is as effective as 10 mg three times daily but causes less sedation, making it the preferred dosing regimen. 3
Adding a muscle relaxant to NSAIDs or acetaminophen provides greater short-term pain relief than analgesic monotherapy, though this increases CNS adverse events. 4
FDA approval is only for short-term use (2-3 weeks) as an adjunct to rest and physical therapy for acute, painful musculoskeletal conditions. 5 Evidence for prolonged use does not exist.
Evidence for Chronic Pain Is Lacking
Only one lower-quality trial of cyclobenzaprine evaluated chronic low back pain, and it did not report pain intensity or global efficacy. 1
For chronic musculoskeletal pain, muscle relaxants are not favored given potential adverse effects and lack of evidence. 1
These medications have not been found effective for spasticity associated with cerebral or spinal cord disease. 5
Adverse Effects Are Significant
Muscle relaxants are associated with 50% higher total adverse events (RR 1.50) and double the CNS adverse events (RR 2.04) compared to placebo. 1
Common adverse effects include somnolence, dizziness, dry mouth, and increased fall risk—particularly concerning in older adults. 1
Cyclobenzaprine is essentially identical to amitriptyline with similar anticholinergic side effects and potential toxicity in overdose. 1
Carisoprodol has been removed from European markets due to abuse concerns and has significant potential for physical and psychological dependence. 1, 6
Clinical Decision Algorithm
For acute musculoskeletal pain (< 2 weeks duration):
- Start cyclobenzaprine 5 mg three times daily (not 10 mg) for 7-14 days maximum. 3
- Combine with NSAIDs or acetaminophen for enhanced pain relief. 4
- Avoid in older adults due to fall risk and anticholinergic effects. 1
For suspected true muscle spasm:
- Consider baclofen or benzodiazepines instead, which have actual effects on spasticity. 1
- Tizanidine is an alternative to cyclobenzaprine for acute conditions, starting at 2-4 mg with upward titration. 4
Avoid entirely:
- Chronic pain conditions (no evidence of benefit). 1
- Benzodiazepines as first-line (abuse potential despite similar efficacy). 4
- Carisoprodol (removed from European markets, dependence risk). 1, 6
- Patients at high fall risk or with cognitive impairment. 1
Common Prescribing Pitfalls
Prescribing beyond 2-3 weeks: No evidence supports longer duration, and muscle spasm from acute conditions is typically short-lived. 5
Using 10 mg dosing when 5 mg is equally effective: The higher dose only increases sedation without additional benefit. 3
Believing these drugs actually relax muscles: They do not directly affect skeletal muscle tension in humans. 2
Prescribing for chronic pain: Evidence is insufficient, and risks outweigh potential benefits. 1
Ignoring fall risk in older adults: Many muscle relaxants significantly increase fall risk in this population. 1