For an adult with irritable bowel syndrome and no contraindications, should I start Bentyl (dicyclomine) or Levsin (hyoscyamine) as first‑line antispasmodic?

Bentyl vs Levsin for IBS: Which Antispasmodic to Start?

Start with either Bentyl (dicyclomine) or Levsin (hyoscyamine) only for intermittent, as-needed relief of acute pain flares—but recognize that both are inferior to tricyclic antidepressants for chronic IBS management and should not be first-line therapy for daily symptoms. 1

Evidence Quality and Positioning

• The 2022 AGA guidelines give antispasmodics only a conditional recommendation with low-certainty evidence for IBS, noting that only hyoscine, dicyclomine, and peppermint oil are available in the United States (hyoscyamine falls under the hyoscine category). 1

• Tricyclic antidepressants (amitriptyline 10–50 mg at bedtime) are more effective than any antispasmodic for both global IBS symptoms and abdominal pain, with moderate-quality evidence supporting their use as true first-line therapy. 2

• A Cochrane meta-analysis of 22 RCTs (2,983 patients) showed antispasmodics as a class improved abdominal pain (RR 0.74; 95% CI 0.59–0.93) and global symptoms (RR 0.67; 95% CI 0.55–0.80) versus placebo, but individual drug effects were difficult to interpret due to small study numbers and high risk of bias. 1

Choosing Between Dicyclomine and Hyoscyamine

Dicyclomine (Bentyl)

• Dose: 10–20 mg before meals or four times daily for scheduled use; 40 mg four times daily was studied in older trials. 3, 4

• Best for: Patients who need scheduled dosing around meals for predictable postprandial cramping. 3

• Anticholinergic burden: Dicyclomine is a tertiary amine that crosses the blood-brain barrier, causing more systemic anticholinergic effects (dry mouth in 2.6–3.5%, dizziness, blurred vision, cognitive impairment in elderly). 5, 6

• Contraindication: Avoid in constipation-predominant IBS (IBS-C), as anticholinergic effects worsen constipation. 1, 6

Hyoscyamine (Levsin)

• Dose: 0.125–0.25 mg sublingual for acute, unpredictable pain episodes; reaches peak effect in 2–2.5 hours. 3, 7

• Best for: Patients with infrequent but severe, unpredictable pain flares who need rapid relief and portability (sublingual formulation). 3

• Anticholinergic burden: Hyoscyamine is a quaternary ammonium compound with fewer systemic/CNS effects than dicyclomine, though dry mouth and thirst (7–7.8%) remain common. 5

• Contraindication: Also avoid in IBS-C for the same anticholinergic constipation risk. 1

Practical Algorithm for Antispasmodic Selection

1. First, attempt a tricyclic antidepressant (amitriptyline 10 mg at bedtime, titrate to 30–50 mg) for any patient with chronic daily IBS symptoms; this is the most effective pharmacologic option. 2

2. If the patient refuses a TCA or has only intermittent symptoms:

   • Choose hyoscyamine 0.125–0.25 mg sublingual for unpredictable, severe pain episodes that require rapid relief (e.g., before social events, travel). 3
   • Choose dicyclomine 10–20 mg before meals for predictable postprandial cramping that occurs daily or several times per week. 3

3. Avoid both agents in IBS-C; their anticholinergic effects will worsen constipation. 1, 6

4. Avoid dicyclomine in elderly patients with cognitive impairment due to delirium risk from CNS penetration. 2

5. Reassess after 2–4 weeks:

   • If no benefit, discontinue the antispasmodic and escalate to a TCA. 2
   • If partial benefit, consider adding (not substituting) a TCA for superior pain control. 2

Common Pitfalls

• Do not use antispasmodics as monotherapy for severe or chronic IBS symptoms; they are less effective than TCAs and the evidence supporting them is very low quality. 2, 6

• Do not combine dicyclomine with hyoscyamine without first optimizing TCA monotherapy, as the additive anticholinergic burden increases side effects without proven incremental benefit. 2

• Do not prescribe antispasmodics indefinitely; they are intended for intermittent use during symptom flares, not chronic daily dosing. 5, 3

• Screen for glaucoma before starting either agent, as anticholinergics increase intraocular pressure. 2

Safety Profile

• Both drugs share anticholinergic adverse effects: dry mouth (most common), dizziness, blurred vision, urinary retention, and constipation. 1, 5, 4

• No serious adverse events were reported in the Cochrane meta-analysis, but tolerability limits long-term adherence. 1

• Hyoscyamine's quaternary structure results in less CNS penetration and fewer cognitive/sedative effects compared to dicyclomine. 5

When Antispasmodics Are Appropriate

• Diarrhea-predominant IBS (IBS-D) with intermittent pain flares is the best indication, as anticholinergic slowing of transit may provide dual benefit. 1

• Postprandial pain and urgency that is predictable and meal-related, though this specific indication has not been studied in RCTs. 1

• Patients who decline or cannot tolerate TCAs and need symptomatic relief while dietary modifications are optimized. 2