Dysbiotic Gut Flora in IBS
Overgrown bacteria crowd the gut, triggering IBS symptoms.
- 10-15%: IBS prevalence (of adults worldwide)
- ~60%: SIBO overlap (of IBS-D patients)
- High: Gas production (from bacterial fermentation)
- ~1,000: Gut microbiome species (in a healthy gut)
Bacterial overgrowth basics
Excess bacteria ferment food into bloating gas.
IBS symptom link
Overgrowth correlates with pain, bloating, irregular bowel habits.
Why treat locally
Systemic antibiotics disrupt the whole-body microbiome unnecessarily.
SIBO breath tests detect excess hydrogen or methane gas.
Rifaximin Administered Orally
Patient swallows rifaximin; almost none reaches the bloodstream.
- <0.4%: Oral bioavailability (systemic absorption)
- 550 mg: Typical dose (three times daily)
- 14 days: Standard course (for IBS-D)
- 2015: FDA approval (for IBS-D indication)
Rifamycin derivative
Rifaximin is a non-absorbed rifamycin antibiotic analog.
Gut-restricted design
A pyridoimidazole ring keeps the molecule out of blood.
Less than 0.4% of an oral dose enters circulation.
Dosing regimen
Three daily doses sustain drug levels in the lumen.
Rifaximin Stays in the Gut Lumen
The drug concentrates in stool, not in the blood.
- High: Fecal concentration (vs undetectable plasma)
- ~4-9h: Plasma half-life (for absorbed trace amount)
- Lumen: Site of action (entire gut length)
- Rare: Systemic side effects (minimal drug exposure)
RNA polymerase blockade
Rifaximin blocks bacterial RNA polymerase, halting protein synthesis.
Broad-spectrum, local only
It targets gram-positive and gram-negative gut bacteria alike.
No systemic resistance driver
Minimal absorption limits pressure on distant bacterial strains.
Fecal drug levels stay high through the entire course.
Pathogenic Overgrowth Is Reduced
Overgrown bacteria shrink while beneficial flora is spared.
- Up to 90%: Pathogenic reduction (with full course)
- Minimal: Beneficial flora impact (largely spared)
- Increases: Diversity recovery (toward baseline balance)
- Falls: Fermentation gas (less hydrogen and methane)
Selective pressure
Rifaximin favors survival of normal commensal populations.
Course duration matters
Longer, consistent dosing drives deeper bacterial rebalancing.
Adherence effect
Missed doses let pathogenic bacteria rebound between doses.
Full adherence over a full course maximizes rebalancing.
IBS Symptoms Ease With Rebalanced Flora
Less gas and bacteria mean less bloating and pain.
- ~65%: Symptom responders (adequate relief in trials)
- Yes: Retreatment option (symptoms can recur later)
- ~2-4 wks: Time to relief (after course completion)
- TARGET 3: Pivotal trial (phase 3 rifaximin study)
Bloating and gas relief
Reduced fermentation lowers bloating and abdominal gas.
Durable but not permanent
Relief can last months before symptoms gradually return.
Retreatment protocol
Repeat courses reuse the same gut-restricted dosing strategy.
About two in three patients get lasting symptom relief.