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Pimentel M, Lembo A, Chey WD, et al. Rifaximin therapy for patients with irritable bowel syndrome without constipation. N Engl J Med. 2011;364(1):22-32. PMID 21208106

One-paragraph summary

Two identically designed phase 3 double-blind placebo-controlled trials (TARGET 1, NCT00731679; TARGET 2, NCT00724126) randomised patients with IBS without constipation to rifaximin 550 mg three times daily or placebo for two weeks, then followed them for a further ten weeks. The primary endpoint was the proportion achieving adequate relief of global IBS symptoms, defined as self-reported relief for at least two of the first four weeks after treatment; the key secondary endpoint was adequate relief of IBS-related bloating. Rifaximin beat placebo for global symptom relief in both trials — 40.8% vs 31.2% (p=0.01) in TARGET 1, 40.6% vs 32.2% (p=0.03) in TARGET 2, and 40.7% vs 31.7% (p<0.001) combined — and for bloating (39.5% vs 28.7%, p=0.005; 41.0% vs 31.9%, p=0.02; combined 40.2% vs 30.3%, p<0.001). Daily self-ratings of global symptoms, bloating, abdominal pain and stool consistency all favoured rifaximin. Adverse-event incidence was similar between groups. The studies were funded by Salix Pharmaceuticals.

Key findings

  • Absolute treatment–placebo difference of 9 percentage points on global symptom relief, sustained through the ten-week post-treatment follow-up.
  • A two-week course produced measurable benefit for weeks afterwards — a durability pattern unlike any other IBS drug.
  • Bloating, the symptom most often attributed to fermentation, improved by a comparable margin.
  • Safety was indistinguishable from placebo, a finding that has held: rifaximin is the safest IBS drug in the 54-trial safety meta-analysis, with a negative and non-significant number needed to harm (Busam 2026, PMID 40471839).
  • The follow-on trial, TARGET 3 (NCT01543178), showed retreatment of relapsers works but less well: 38.1% vs 31.5% (p=0.03), with abdominal pain improving (50.6% vs 42.2%, p=0.018) but stool consistency no better than placebo (51.8% vs 50.0%, p=0.42) (Lembo 2016, PMID 27528177).

Limitations

  • Modest absolute effect: nine percentage points, i.e. roughly one additional responder for every eleven treated.
  • The endpoint was "adequate relief" for ≥2 of 4 weeks, a less stringent definition than the FDA composite later adopted; placebo response was 31.7%.
  • Industry-sponsored, with several authors holding relevant relationships.
  • The trials did not test for, or stratify by, small intestinal bacterial overgrowth — so they cannot support the mechanistic story used to justify them.
  • The 16S substudy of TARGET 3 found only "modest, largely transient" effects on stool microbiota, with little difference from baseline by 46 weeks (Fodor 2019, PMID 29708822).

Why it matters

TARGET established that a non-absorbed antibiotic relieves symptoms in non-constipated IBS, and in doing so gave the strongest available empirical argument for a microbial contribution to the condition. It also created the field's sharpest epistemic problem: the drug works, its microbial effect is transient, retreatment preserves the pain benefit while losing the stool benefit, and the diagnostic construct invoked to explain it — small intestinal bacterial overgrowth — is present in 35.5% of IBS patients and 29.7% of controls by breath testing (Shah 2020, PMID 31913194) and in 4% by jejunal culture (Ford 2009, PMID 19602448). Every subsequent guideline recommendation for rifaximin rests on TARGET; no guideline recommends testing to select who receives it.

Cited by wiki pages

  • rifaximin-and-the-sibo-question
  • microbiome
  • diarrhoea-predominant-pharmacotherapy
  • diagnosis-and-rome-criteria
  • guidelines
  • overview