Flexeril (Cyclobenzaprine) Dosing for Back Strain
Start cyclobenzaprine 5 mg three times daily for a maximum of 1-2 weeks, as this dose provides equivalent efficacy to 10 mg with significantly less sedation, and no evidence supports use beyond 2-3 weeks. 1
Standard Dosing Protocol
Begin with 5 mg three times daily rather than the traditional 10 mg dose, as clinical trials demonstrate equivalent pain relief and functional improvement with 5 mg TID compared to 10 mg TID, but with substantially lower rates of somnolence (the most common adverse effect) 2
The FDA-approved dosing allows escalation to 10 mg three times daily based on individual response, but this higher dose should be reserved only for patients who fail to respond adequately to 5 mg and can tolerate increased sedation 1
Limit treatment duration to 1-2 weeks maximum, as the American College of Physicians guidelines explicitly state muscle relaxants should not be used beyond 2 weeks due to lack of efficacy evidence in chronic pain and increased risks of sedation, falls, and cognitive impairment 3
Clinical Efficacy and Timing
Onset of relief typically occurs within 3-4 doses of the 5 mg regimen, with peak efficacy demonstrated in the first 4-7 days of treatment 2, 4
Meta-analysis shows patients treated with cyclobenzaprine are nearly 5 times more likely to report symptom improvement by day 14 compared to placebo (odds ratio 4.7), with a number needed to treat of approximately 3 patients 4
The magnitude of benefit is modest (effect size 0.38-0.58) across all outcome domains including local pain, muscle spasm, range of motion, tenderness, and activities of daily living, with greatest efficacy in the first 4 days 4
Combination Therapy Considerations
Combine cyclobenzaprine with an NSAID (such as naproxen 500 mg twice daily) rather than prescribing it alone, as combination therapy targeting both inflammatory and muscle spasm components demonstrates superior outcomes for objective muscle spasm, tenderness, and range of motion compared to NSAID monotherapy 5
A large randomized trial (N=323) found that adding cyclobenzaprine to naproxen did not improve functional outcomes at 1 week compared to naproxen alone, but this study used a 5 mg dose "as needed" rather than scheduled dosing, which may have underdosed patients 6
Avoid combining cyclobenzaprine with opioids (such as oxycodone/acetaminophen), as this combination provides no additional benefit over NSAID therapy alone and substantially increases adverse effects 6
Special Populations and Contraindications
In elderly patients or those with hepatic impairment, use less frequent dosing (consider 5 mg once or twice daily rather than three times daily) due to prolonged drug elimination and increased sensitivity to anticholinergic and sedative effects 1
In patients with substance abuse history or concerns, cyclobenzaprine is preferable to carisoprodol, which has significant abuse potential and is a controlled substance in many states 7
Cyclobenzaprine is contraindicated in patients taking MAO inhibitors, those with recent myocardial infarction, arrhythmias, heart block, conduction disturbances, or hyperthyroidism due to its tricyclic antidepressant-like structure 1
Monitoring and Adverse Effects
Warn patients about dose-dependent sedation and drowsiness, which occurs in approximately 30-40% of patients and is the primary reason for treatment discontinuation 2, 4
Dry mouth is the second most common adverse effect, occurring in approximately 9% of patients 8
The efficacy of cyclobenzaprine is independent of its sedative effects, meaning patients who do not experience somnolence still achieve meaningful pain relief 2
What NOT to Do
Do not prescribe cyclobenzaprine beyond 2-3 weeks, as the FDA label explicitly states use for longer periods is not recommended, and guidelines emphasize no evidence supports chronic use 1, 3
Do not use benzodiazepines (such as diazepam) instead of cyclobenzaprine, as they are ineffective for radiculopathy and carry substantial risks of abuse, addiction, and tolerance 3
Do not prescribe opioids as first-line therapy for acute back strain, as guidelines from the American College of Physicians and American Pain Society recommend opioids only for severe, disabling pain not controlled with acetaminophen or NSAIDs 8
Avoid the 2.5 mg TID dose, as it was not significantly more effective than placebo in clinical trials, with ineffectiveness being the main reason for treatment discontinuation 2