Is Bentyl (Dicyclomine) Effective in IBS-C?
Bentyl (dicyclomine) is NOT recommended for IBS-C because it can worsen constipation through its anticholinergic effects, despite being FDA-approved for IBS and showing efficacy for abdominal pain. 1, 2
Why Dicyclomine is Problematic in IBS-C
Dicyclomine is an anticholinergic antispasmodic that reduces intestinal motility, which directly opposes the therapeutic goal in constipation-predominant IBS. 1 While the FDA label indicates dicyclomine is approved for "functional bowel/irritable bowel syndrome" generally, with 82% of patients showing favorable response versus 55% on placebo at 160 mg daily (40 mg four times daily), these trials did not specifically evaluate IBS-C patients. 2
The British Society of Gastroenterology guidelines explicitly note that anticholinergic antispasmodics like dicyclomine reduce intestinal motility and enhance water reabsorption—mechanisms that would exacerbate constipation. 1 The 2000 BSG guidelines even recommend avoiding tricyclic antidepressants "if constipation is a major feature" due to similar anticholinergic effects, and dicyclomine shares this property. 1
What Should Be Used Instead for IBS-C
First-line treatment for IBS-C should begin with soluble fiber (ispaghula/psyllium) at 3-4 g/day, gradually increased to avoid bloating. 1, 3
- Regular physical exercise should be recommended to all IBS-C patients as foundational therapy. 1, 3
- Insoluble fiber (wheat bran) must be avoided as it consistently worsens symptoms. 1, 3
- If fiber fails after 4-6 weeks, consider a low FODMAP diet supervised by a trained dietitian. 3
- Probiotics can be tried for 12 weeks and discontinued if ineffective. 1, 3
For persistent abdominal pain in IBS-C, tricyclic antidepressants (TCAs) are the most effective option, but use cautiously with adequate laxative therapy in place. 1, 3
- Start amitriptyline at 10 mg nightly and titrate slowly (by 10 mg/week) to 30-50 mg daily. 1, 3
- TCAs may worsen constipation, so ensure concurrent laxative use. 3
- Continue for at least 6 months if symptomatic response occurs. 1
For refractory constipation, secretagogues are second-line agents: 3
- Linaclotide is the preferred second-line agent with strong evidence for IBS-C. 3
- Lubiprostone is an alternative if linaclotide is not tolerated. 3
- Polyethylene glycol (osmotic laxative) can be titrated according to symptoms. 3
Common Pitfall to Avoid
The critical error is prescribing anticholinergic antispasmodics like dicyclomine for IBS-C based solely on the "IBS" diagnosis without considering the constipation subtype. 1 While dicyclomine may help abdominal pain through smooth muscle relaxation, the anticholinergic slowing of gut transit will worsen the constipation, potentially making the patient's overall symptom burden worse. 1
Review treatment efficacy after 3 months and discontinue if no response. 1, 3