What Causes IBS and Which Treatments Actually Help

Research-informed explainer — last updated April 12, 2026

Irritable bowel syndrome affects an estimated 10 to 15 percent of adults worldwide, making it one of the most common conditions a gastroenterologist sees — yet it remains one of the least understood. The old idea that IBS is "just stress" is outdated. Research over the past two decades points to a cluster of overlapping biological mechanisms: altered gut motility, changes in intestinal permeability, shifts in the gut microbiome, and a disrupted gut-brain communication loop. Treatments that work target one or more of these pathways, which is why no single approach helps everyone.

This explainer draws on research from four specialists in the Convene directory. Michael Camilleri at Mayo Clinic has published foundational work on intestinal barrier function and the biology of gut motility, including the AGA technical review that for years defined the scientific understanding of IBS. Anthony Lembo at Cleveland Clinic was a lead investigator on the Rome IV bowel disorders criteria — the current diagnostic standard — and the pivotal rifaximin trial in the New England Journal of Medicine. Prateek Sharma at the University of Kansas contributed to research on gastrointestinal motility overlap and endoscopic evaluation of upper GI conditions that frequently co-occur with IBS. Stuart Spechler at Baylor University Medical Center has published on the intersection of reflux disorders and functional bowel disease.

What IBS actually is — and is not

IBS is a functional bowel disorder: your gut produces symptoms without any structural abnormality that shows up on imaging or in biopsies. No ulcer, no tumor, no visible inflammation. This is not the same as saying nothing is wrong — it means the problem lies in how the gut functions rather than in its structure.

The Rome IV criteria, developed through a consensus process that included Anthony Lembo and published in Gastroenterology in 2016, define IBS as recurrent abdominal pain at least one day per week over the past three months, associated with two or more of the following: pain related to defecation, a change in stool frequency, or a change in stool form [4]. Subtypes are based on stool pattern: IBS with constipation (IBS-C), IBS with diarrhea (IBS-D), mixed IBS (IBS-M), and unclassified IBS.

What research says causes IBS

No single cause has been identified. Current evidence points to several overlapping mechanisms, most of which involve the gut-brain axis — the two-way communication network between your intestinal nervous system and your brain.

Intestinal permeability changes. The gut lining is normally a selective barrier that lets nutrients through while keeping bacteria and inflammatory compounds out. In a substantial subset of IBS patients, this barrier is leakier than normal. Michael Camilleri's review of intestinal permeability research found that increased paracellular transport (fluid leaking between cells) and altered tight junction proteins are measurable in IBS patients compared to healthy controls [2]. This allows low-grade immune activation that may drive both pain and motility changes.

Altered gut motility. The muscular contractions that move food through your intestines are regulated by a complex network of nerves and hormones. In IBS-D, transit is often faster than normal; in IBS-C, it is slower. The mechanisms are partly hormonal: serotonin, which is produced in large quantities in the gut, plays a major role in coordinating intestinal movement. Drugs that modulate serotonin — like alosetron for IBS-D — work specifically on this pathway.

Post-infectious IBS. A specific and well-documented trigger is a bacterial or viral gastroenteritis. Research consistently shows that approximately 10 percent of people who develop acute infectious gastroenteritis go on to develop IBS symptoms that persist long after the infection resolves. This post-infectious IBS is thought to involve persistent low-grade inflammation and changes to the gut microbiome that outlast the original infection.

Small intestinal bacterial overgrowth (SIBO). In some patients, bacteria that normally live in the large intestine colonize the small intestine, producing excess hydrogen or methane gas. Breath testing, which Anthony Lembo and colleagues helped standardize in the North American Consensus document, can identify these patients [8]. Methane-dominant SIBO, in particular, is associated with constipation-predominant symptoms. This overlap between SIBO and IBS is part of why antibiotics have emerged as a treatment option.

The gut-brain connection. Many IBS patients have heightened sensitivity in the gut — a phenomenon called visceral hypersensitivity — meaning normal amounts of gas or stool feel painful. Stress, anxiety, and depression do not cause IBS outright, but they modulate the gut-brain axis in ways that amplify symptoms. This is why psychological treatments have measurable benefit, not because the symptoms are imaginary, but because the nervous system is genuinely involved.

How IBS is diagnosed

There is no blood test, imaging study, or biopsy that diagnoses IBS. The diagnosis is clinical, based on the Rome IV symptom criteria and the exclusion of other conditions that could explain the symptoms. Your doctor will typically rule out celiac disease (blood test), inflammatory bowel disease (colonoscopy or stool markers in appropriate cases), and thyroid dysfunction. In patients with alarm features — unexplained weight loss, blood in stool, family history of colorectal cancer, or symptoms starting after age 50 — more investigation is warranted before attributing symptoms to IBS.

Breath testing for SIBO or carbohydrate malabsorption (lactose, fructose) may be appropriate depending on your symptom pattern [8]. The North American Consensus on breath testing provides standardized protocols for how these tests should be conducted and interpreted, reducing the variability that has historically made results difficult to compare.

Treatments with real evidence

Dietary changes. The low-FODMAP diet — which restricts fermentable short-chain carbohydrates found in wheat, dairy, onions, garlic, and many fruits — has the strongest dietary evidence for IBS. In well-conducted trials, around 50 to 75 percent of patients report meaningful symptom improvement. The diet is typically used as a short-term elimination protocol followed by systematic reintroduction to identify personal triggers, not as a permanent lifestyle change.

Rifaximin. This antibiotic works differently from most: it is not well-absorbed into the bloodstream and acts locally in the gut. The pivotal trial, a pair of large randomized controlled trials including Anthony Lembo as a co-investigator, showed that a two-week course of rifaximin significantly reduced global IBS symptoms, bloating, and loose stools compared to placebo in IBS-D patients [5]. Symptom relief was sustained for up to 10 weeks after treatment ended — consistent with the idea that the antibiotic is resetting the gut microbial environment rather than just suppressing symptoms temporarily.

Antispasmodics. Medications like dicyclomine and hyoscine reduce gut muscle spasms and can help with pain and urgency, particularly in IBS-D. Effect sizes are modest but meaningful for patients with cramping as their primary symptom.

Low-dose antidepressants. Tricyclic antidepressants (like amitriptyline) and SSRIs are often used in IBS at doses below those used for depression. They work through gut-brain axis modulation, not primarily through mood effects. TCAs are generally preferred for IBS-D (they slow transit), while SSRIs may be more appropriate for IBS-C.

Psychological treatments. Gut-directed cognitive behavioral therapy (CBT) and gut-directed hypnotherapy have demonstrated benefit in multiple randomized trials and are endorsed by major GI societies. The mechanism appears to be real neurobiological modulation of the gut-brain axis, not simply placebo. Research by Anthony Lembo's group on placebo components in IBS found that even when patients knew they were receiving a placebo, symptom improvement occurred — suggesting that the therapeutic encounter and expectation of relief have genuine physiological effects [7].

What about "leaky gut"?

This term has become widespread in consumer wellness culture, often used to sell supplements with dubious claims. The underlying biology is real — changes in intestinal permeability are measurable and associated with several GI conditions including IBS. Michael Camilleri's review in Gut provides a useful framework: the barrier components include mucus, the epithelial layer, and immune defenses, and each can be altered in ways that cause symptoms [2]. But the evidence does not support the idea that a generic "gut repair" supplement will fix this. Treatment approaches that have shown effect on permeability include dietary change (particularly reducing ultra-processed foods), specific probiotic strains, and in some patients, treating underlying SIBO.

Prognosis and what to expect

IBS is chronic, but it is not progressive in the sense that IBD or cancer is. Many patients have waxing and waning symptoms that are manageable with appropriate treatment. Some patients experience significant improvement over years, particularly after identifying and avoiding dietary triggers. IBS does not increase the risk of colorectal cancer, inflammatory bowel disease, or other structural GI conditions.

Finding the right treatment often takes time and some trial. Because the condition involves multiple mechanisms, what works for one person may not work for another. A gastroenterologist with experience in motility and functional GI disorders can help sequence treatments in a logical way rather than cycling through options at random.

Questions to ask your doctor

• Which IBS subtype do I have, and does that change which treatments are most likely to help?
• Should I be tested for celiac disease, lactose intolerance, or SIBO before starting treatment?
• Is a low-FODMAP diet appropriate for me, and should I work with a dietitian to do it correctly?
• Would rifaximin or another targeted treatment be appropriate given my symptom pattern?
• Are there any symptoms in my history that warrant further testing before a diagnosis of IBS is confirmed?
• What is a realistic timeline for finding a treatment combination that reduces my symptoms significantly?