Can Gabapentin (Neurontin) cause constipation?

Gabapentin and Constipation

Yes, gabapentin can cause constipation as a documented side effect, occurring in approximately 4% of patients taking the medication for postherpetic neuralgia and 2% of patients taking it for epilepsy according to FDA data. 1

Evidence for Gabapentin-Induced Constipation

The FDA drug label for gabapentin clearly lists constipation as an adverse effect:

• In postherpetic neuralgia patients: 4% of gabapentin users experienced constipation vs. 2% in placebo group 1
• In epilepsy patients (>12 years): 2% of gabapentin users experienced constipation vs. 1% in placebo group 1

This side effect is well-documented in clinical guidelines as well. The 2017 ophthalmology guidelines for neuropathic pain management specifically mention constipation as one of the common side effects of gabapentin and pregabalin, alongside dizziness, somnolence, and dry mouth. 2

Mechanism and Risk Factors

Gabapentin's constipating effect likely stems from:

1. Its action on calcium channels that may affect gut motility
2. Central nervous system effects that alter autonomic function
3. Potential interaction with GABA receptors in the enteric nervous system

Risk factors that may increase the likelihood or severity of gabapentin-induced constipation include:

• Advanced age
• Reduced kidney function (gabapentin is exclusively eliminated renally) 3
• Concurrent use of other constipating medications
• Dehydration
• Immobility
• Pre-existing gastrointestinal disorders

Clinical Significance

The severity of gabapentin-induced constipation can range from mild to severe:

• Most cases are mild to moderate and manageable with standard interventions
• However, severe cases have been documented with the related medication pregabalin, where constipation was dose-dependent and the most frequent adverse effect requiring drug discontinuation (6.3% of patients) 4
• In patients with chronic kidney disease, gabapentin accumulation can lead to increased toxicity, potentially worsening constipation 3

Management Approaches

For patients experiencing constipation while on gabapentin:

1. Preventive measures:

   • Ensure adequate hydration
   • Increase physical activity when appropriate
   • Consider prophylactic laxative therapy when starting gabapentin

2. First-line treatment:

   • Stimulant laxatives to increase bowel motility
   • Add osmotic laxatives if constipation persists

3. For refractory cases:

   • Consider dose reduction if clinically appropriate
   • Evaluate for medication alternatives
   • Assess for other causes of constipation

4. Monitoring:

   • Regular assessment of bowel movements
   • Patient education about reporting constipation promptly

Special Considerations

In patients with neuropathic pain, it's worth noting that gabapentin has been shown to reduce rectal mechanosensitivity and increase rectal compliance in some studies 5, which could potentially contribute to constipation in certain patient populations.

For patients with chronic kidney disease, gabapentin dosing should be adjusted according to renal function to prevent toxicity, which can manifest as various side effects including constipation 3.

Conclusion

When prescribing gabapentin, clinicians should:

• Be aware of constipation as a potential side effect
• Proactively discuss this possibility with patients
• Consider preventive measures in high-risk patients
• Monitor for and promptly address constipation if it develops
• Consider dose adjustments or alternative medications if constipation becomes problematic

This approach will help minimize the impact of this adverse effect on patient quality of life and medication adherence.