Can Flexeril (cyclobenzaprine) cause constipation?

Flexeril (Cyclobenzaprine) and Constipation

Yes, Flexeril (cyclobenzaprine) can cause constipation as a documented adverse effect, occurring in 1-3% of patients taking the medication according to FDA labeling. 1

Mechanism and Incidence

Cyclobenzaprine is a centrally acting muscle relaxant with a tricyclic structure that can cause several gastrointestinal side effects, including:

• Constipation (1-3% of patients)
• Acid regurgitation
• Abdominal pain
• Dyspepsia
• Nausea

The constipation effect is likely related to cyclobenzaprine's anticholinergic properties, similar to other medications with tricyclic structures.

Risk Factors and Management

Risk Factors for Cyclobenzaprine-Induced Constipation:

• Older age
• Concurrent use of other constipating medications
• Pre-existing gastrointestinal motility disorders
• Dehydration
• Immobility (which may already be present in patients requiring muscle relaxants)

Management Strategies:

If constipation develops while taking cyclobenzaprine, consider these evidence-based approaches:

1. Prophylactic measures:

   • Maintain adequate fluid intake
   • Ensure sufficient dietary fiber
   • Encourage physical activity as tolerated

2. First-line treatment:

   • Stimulant laxatives with or without stool softeners 2
   • A capful of polyethylene glycol (PEG) with 8 oz of water twice daily 2

3. For persistent constipation:

   • Add osmotic laxatives (sorbitol, lactulose, PEG) 2
   • Consider magnesium-based products or bisacodyl 2
   • Evaluate for medication dose reduction if possible 2

4. For severe constipation:

   • Consider opioid antagonists like methylnaltrexone if constipation is related to concurrent opioid therapy 2
   • Evaluate for possible drug discontinuation or substitution

Clinical Considerations

• Constipation can be a significant adverse effect that impacts quality of life and medication adherence
• The incidence of constipation with cyclobenzaprine (1-3%) is lower than with some other medications like opioids (which cause constipation in approximately 50% of patients) 2
• Consider using the lowest effective dose of cyclobenzaprine, as adverse effects appear to be dose-dependent 3
• The 5 mg TID regimen of cyclobenzaprine has been shown to be as effective as 10 mg TID with fewer side effects 3

Alternative Approaches

If constipation becomes problematic:

1. Consider alternative muscle relaxants with potentially lower constipation risk
2. Evaluate if the patient can be managed with non-pharmacological approaches (physical therapy, heat/cold therapy)
3. Consider adjunctive analgesics to allow for lower doses of cyclobenzaprine

Monitoring

Monitor patients on cyclobenzaprine for:

• Changes in bowel habits
• Abdominal discomfort
• Signs of severe constipation or impaction

Early recognition and management of constipation can prevent more serious complications and improve medication adherence.