Treatment of Irritable Bowel Syndrome with Constipation (IBS-C)
Begin with soluble fiber (ispaghula/psyllium) at 3-4 g/day, titrated upward gradually, combined with regular aerobic exercise; if symptoms persist after 4-6 weeks, add polyethylene glycol (PEG) as an osmotic laxative; for refractory cases, escalate to linaclotide 290 mcg once daily on an empty stomach as the preferred prescription agent. 1, 2, 3
First-Line Treatment: Lifestyle and Soluble Fiber
Start all IBS-C patients on regular aerobic exercise, as this independently improves global symptom scores and should form the foundation of treatment. 1, 2
Initiate soluble fiber (ispaghula/psyllium) at 3-4 g/day and build up gradually to avoid bloating and gas. This is effective for both global symptoms and abdominal pain with moderate-quality evidence. 1, 2
Critically, avoid insoluble fiber such as wheat bran, as it consistently aggravates bloating, pain, and overall symptom burden in IBS-C patients. 1, 2
Provide basic dietary counseling to identify and reduce excessive intake of caffeine, lactose, fructose, or alcohol, and ensure patients allow adequate time for regular defecation. 1
Consider a 12-week trial of probiotics for global symptoms and abdominal pain, though no specific strain can be recommended; discontinue if no improvement occurs. 1, 2
Second-Line Treatment: Osmotic Laxatives
If symptoms persist after 4-6 weeks of fiber therapy, add polyethylene glycol (PEG) and titrate the dose to symptom response. Abdominal discomfort is the most common adverse effect. 2, 4
Re-evaluate efficacy after 3 months of PEG therapy; discontinue if no meaningful improvement is observed. 2
As a second-line dietary intervention, consider a low FODMAP diet supervised by a trained dietitian with planned reintroduction of foods according to tolerance, though this has very low-quality evidence. 1, 2
Third-Line Treatment: Prescription Secretagogues
Linaclotide 290 mcg once daily on an empty stomach (at least 30 minutes before the first meal) is the preferred prescription agent after failure of first-line therapies, with high-quality evidence showing significant benefit for both constipation and abdominal pain. 2, 3, 5
Linaclotide must be taken on an empty stomach to maximize efficacy, and diarrhea is the most common adverse event, occurring as the mechanism of action. 2, 3
Plecanatide 3 mg daily is an alternative secretagogue with efficacy comparable to linaclotide for patients who cannot tolerate or afford linaclotide. 2
Lubiprostone 8 mcg twice daily with food is a conditional third-line option for women with IBS-C, with moderate-certainty evidence showing modest benefit but a higher rate of nausea (approximately 19% versus 14% with placebo). 2, 6, 5 Lubiprostone should be taken with food and water, and capsules must be swallowed whole. 6
Managing Abdominal Pain in IBS-C
For meal-related abdominal pain, use peppermint oil as an antispasmodic before escalating to other agents, as it has a favorable side-effect profile. 1, 2
Critically, avoid anticholinergic antispasmodics (dicyclomine, hyoscyamine) in IBS-C patients, as they reduce intestinal motility and enhance water reabsorption, which will worsen the constipation. 2, 4
For persistent abdominal pain after adequate constipation treatment, tricyclic antidepressants (amitriptyline) are the most effective option. Start at 10 mg nightly and titrate slowly (by 10 mg/week) to 30-50 mg daily. 1, 2 Continue for at least 6 months if symptomatic response occurs, but use cautiously in IBS-C and ensure adequate laxative therapy is in place, as TCAs may worsen constipation through anticholinergic effects. 2
When tricyclics are not tolerated or worsen constipation, selective serotonin reuptake inhibitors (SSRIs) may be considered, although supporting evidence is weaker. 1, 2
Explain to patients that amitriptyline is being used as a gut-brain neuromodulator, not as an antidepressant, to improve adherence. 7 Common adverse effects include dry mouth, visual disturbances, and dizziness. 1
Fourth-Line: Psychological Therapies for Refractory Symptoms
IBS-specific cognitive-behavioral therapy (CBT) and gut-directed hypnotherapy should be offered when symptoms remain refractory after at least 12 months of optimal pharmacologic management, as both modalities reduce overall symptom burden with moderate-quality evidence. 1, 2
Critical Pitfalls to Avoid
Do not prescribe anticholinergic antispasmodics (dicyclomine, hyoscyamine) in IBS-C, as the critical error is prescribing these based solely on the "IBS" diagnosis without considering the constipation subtype, which will worsen the constipation. 2, 4
Do not continue docusate (Colace), as it lacks efficacy for constipation and adds no benefit to other laxative therapy. 2
Do not recommend IgG-based food elimination diets or gluten-free diets unless celiac disease is confirmed, as current evidence does not support their use in IBS-C and may lead to unnecessary dietary restrictions. 1, 2
Do not use opioids for chronic abdominal pain management in IBS-C due to risks of dependence, complications, and worsening constipation. 2, 8
Review treatment efficacy after 3 months and discontinue therapies that lack efficacy. 1, 2 Avoid extensive investigations once the diagnosis of IBS-C is established, as this can reinforce illness behavior. 1