What is the recommended muscle relaxer, such as Flexeril (cyclobenzaprine) or Skelaxin (metaxalone), for a patient with hamstring pain, considering their medical history, including any history of liver or kidney disease?

Muscle Relaxers Are Not Recommended for Hamstring Pain

Muscle relaxers such as cyclobenzaprine (Flexeril) or metaxalone (Skelaxin) should not be used for hamstring pain, as they lack evidence of efficacy for chronic musculoskeletal pain and carry significant risks of sedation and falls, particularly in older adults. 1, 2

Why Muscle Relaxers Are Inappropriate for Hamstring Pain

Lack of Evidence for Efficacy

• So-called "muscle relaxants" including metaxalone, cyclobenzaprine, methocarbamol, carisoprodol, and chlorzoxazone are FDA-approved only for acute musculoskeletal pain, not chronic conditions, and have no evidence of efficacy in chronic pain. 1
• These medications do not directly relax skeletal muscle in humans despite their name—their mechanism is related to sedative properties rather than actual muscle relaxation. 3, 4
• The American College of Physicians and American Pain Society guidelines state there is no compelling evidence that skeletal muscle relaxants differ in efficacy or safety from one another, and their use should be limited to short-term relief of acute low back pain only. 1

Significant Safety Concerns

• All muscle relaxants are associated with central nervous system adverse effects, primarily sedation, which can impair function and increase fall risk. 1
• The American Geriatrics Society specifically warns that muscle relaxants should not be prescribed in the mistaken belief that they relieve muscle spasm, and they carry greater risk for falls in older persons. 2
• Specific safety issues include: carisoprodol is metabolized to meprobamate (associated with abuse and overdose risk), dantrolene carries a black box warning for potentially fatal hepatotoxicity, and both tizanidine and chlorzoxazone are associated with hepatotoxicity. 1

FDA Labeling Restrictions

• Cyclobenzaprine is indicated only for short periods (up to 2-3 weeks) because adequate evidence of effectiveness for more prolonged use is not available. 4
• Metaxalone is indicated only as an adjunct to rest and physical therapy for acute, painful musculoskeletal conditions, and does not directly relax tense skeletal muscles. 3

Recommended Treatment Approach for Hamstring Pain

First-Line Management

• NSAIDs (oral or topical) are the appropriate first-line pharmacologic option for acute musculoskeletal pain including hamstring injuries, with moderate to high-certainty evidence showing pain reduction at less than 2 hours. 1
• Acetaminophen alone or combined with NSAIDs can be used for pain relief, though evidence is moderate-certainty. 1

Non-Pharmacologic Interventions (Primary Treatment)

• Rest, ice, compression, and elevation (RICE) immediately following hamstring injury. 5, 6
• Progressive stretching exercises are essential—one trial showed stretching 4 times daily versus once daily resulted in significantly faster return to function (mean difference -1.8 days). 7
• Progressive agility and trunk stabilization exercises should be incorporated after initial healing, as lumbar stability and pelvic motor control may reduce recurrence rates. 7
• Physical therapy focusing on strength, flexibility, endurance, coordination, and agility is the cornerstone of hamstring rehabilitation. 5, 8

Special Considerations Based on Medical History

For patients with liver disease:

• Avoid metaxalone entirely—it is metabolized by the liver via multiple CYP450 enzymes, and its impact on hepatic insufficiency has not been determined. 3
• Avoid chlorzoxazone and tizanidine due to hepatotoxicity risk. 1

For patients with kidney disease:

• Metaxalone should be used with caution as the impact of renal disease on its pharmacokinetics has not been determined. 3
• NSAIDs should be used cautiously given renovascular risks. 1

For older adults:

• Muscle relaxants pose particularly high risk due to increased sedation and fall risk. 1, 2
• If NSAIDs are necessary, assess cardiovascular and gastrointestinal risk factors first, and use the lowest effective dose for the shortest period. 1

When Pharmacologic Options Are Exhausted

• Opioids or tramadol may be considered only when pain is severe and disabling, not controlled by acetaminophen and NSAIDs, and used judiciously for time-limited courses. 1
• Tricyclic antidepressants may be considered for chronic pain if no contraindications exist, though evidence is primarily for low back pain rather than hamstring-specific pain. 1

Common Pitfalls to Avoid

• Do not prescribe muscle relaxants believing they will relieve muscle spasm—this is a common misconception as they do not directly affect skeletal muscle. 2, 3, 4
• Do not use muscle relaxants for chronic or recurrent hamstring pain—they are only indicated for acute conditions and lack evidence beyond 2-3 weeks. 1, 4
• Do not allow athletes to return to activity before complete rehabilitation—inadequate rehabilitation is thought to cause many recurrent hamstring injuries. 5, 6
• Do not overlook lumbar spine, sacroiliac, and pelvic alignment issues—these may be contributing factors requiring specific intervention. 7