Why should muscle relaxants not be used long term, and what are their mechanisms of action?

Why Muscle Relaxants Should Not Be Used Long-Term

Muscle relaxants should be avoided for long-term use because they are only effective for short-term relief (2-4 weeks maximum), carry significant risks of central nervous system adverse effects (primarily sedation), have potential for abuse and dependence with certain agents, and lack evidence supporting efficacy beyond acute treatment periods. 1

Mechanism of Action

The term "skeletal muscle relaxants" is somewhat misleading—these medications do not actually relax skeletal muscle directly 2. Instead, they represent a diverse group of medications with unclear or varied mechanisms of action, grouped together primarily because they carry FDA-approved indications for musculoskeletal conditions 1.

Specific Mechanisms by Drug Class:

• Cyclobenzaprine: Acts as a centrally acting 5-HT2 receptor antagonist, structurally related to amitriptyline with similar anticholinergic properties 3
• Tizanidine: Functions as an α2-adrenergic receptor agonist (clonidine derivative) 3
• Baclofen: GABAB agonist 4, 2
• Carisoprodol: Central nervous system depressant with barbiturate-like action, metabolized to meprobamate 1, 3
• Others (methocarbamol, metaxalone, chlorzoxazone): Centrally acting with unclear precise mechanisms 3

The effects are nonspecific and not related to actual muscle relaxation—they likely work through central nervous system depression rather than peripheral muscle effects 2.

Why Long-Term Use Is Problematic

1. Limited Evidence of Efficacy Beyond Short-Term Use

• All clinical trials evaluated treatment durations of 2 weeks or less (with rare 3-week exceptions) 5
• Muscle relaxants are recommended as "an option for short-term relief of acute low back pain" only 1
• A 2024 systematic review found that long-term use for chronic pain showed minimal benefit for conditions like low back pain and fibromyalgia 6
• Despite recommendations for short-term use, real-world data shows 44.5% of users take them longer than a year, with mean duration of 2.1 years 7

2. Central Nervous System Adverse Effects

All muscle relaxants are associated with CNS adverse effects, primarily sedation 1:

• Sedation and drowsiness (most common)
• Dizziness
• Confusion and hallucinations (especially cyclobenzaprine) 3
• Increased fall risk in older adults 2
• Risk ratio of 2.04 for CNS adverse events compared to placebo 5, 8

3. Specific Drug-Related Risks

Carisoprodol carries particularly concerning risks:

• Metabolized to meprobamate (associated with abuse and overdose risk) 1
• Classified as a controlled substance due to abuse potential 3
• Removed from European market due to abuse concerns 2
• Requires slow tapering over 4-9 days to avoid severe withdrawal symptoms 3

Dantrolene: Black box warning for potentially fatal hepatotoxicity 1

Tizanidine and chlorzoxazone: Associated with hepatotoxicity (generally reversible) 1

Cyclobenzaprine:

• Anticholinergic effects including urinary retention, constipation, dry mouth 3
• Withdrawal symptoms (malaise, nausea, headache) after long-term use 3

4. Vulnerable Populations at Higher Risk

Muscle relaxants are particularly problematic in:

• Older adults: Higher risk of sedation, falls, and cognitive impairment 4, 2
• Patients with renal/hepatic dysfunction: Impaired drug clearance 3
• Real-world data shows undiminished use in elderly and those with ambulatory impairments, despite increased risk 7
• A 2023 study found cumulative muscle relaxant use associated with increased risk of Alzheimer's disease, particularly with orphenadrine 9

5. Lack of Superiority Over Alternatives

• No compelling evidence that skeletal muscle relaxants differ in efficacy or safety from each other 1
• For chronic pain, benzodiazepines appear similarly effective for short-term relief but also carry abuse risks 1
• First-line treatments (acetaminophen, NSAIDs) have better-established safety profiles for longer use 1

Clinical Pitfalls to Avoid

1. Don't prescribe believing they actually relax muscle spasm—this is a misconception; their effects are nonspecific CNS depression 2

2. Don't continue beyond 2-4 weeks without reassessment—if pain persists, consider alternative diagnoses or treatments rather than prolonging muscle relaxant use 1

3. Avoid in older adults when possible—particularly those at fall risk or with cognitive impairment 4, 2

4. Never abruptly discontinue carisoprodol or cyclobenzaprine after chronic use—taper slowly to prevent withdrawal 3

5. Don't use in patients with hepatic dysfunction (especially dantrolene, tizanidine) or renal impairment 3

6. Recognize that chronic use (>1 year) occurs in nearly half of users despite lack of evidence—actively work to prevent this pattern 7

Appropriate Short-Term Use Algorithm

For acute low back pain (<6 weeks):

• Consider muscle relaxants only after acetaminophen/NSAIDs trial
• Prescribe for maximum 2-4 weeks
• Warn patients about sedation and fall risk
• Avoid carisoprodol due to abuse potential
• Schedule reassessment before refills

For chronic pain (>12 weeks):

• Do not use muscle relaxants—evidence does not support efficacy 6
• Consider alternative adjuvant analgesics (antidepressants, gabapentinoids) for neuropathic components 1, 4