SIBO, or small intestinal bacterial overgrowth, happens when bacteria that normally live in the large intestine migrate into or accumulate in the small intestine, where they ferment food and produce excess gas. The underlying causes fall into a few major categories: impaired gut motility, structural abnormalities, reduced digestive secretions, and medications that change the intestinal environment. Most cases involve more than one of these factors working together.
Impaired Gut Motility
Your small intestine has a built-in cleaning mechanism called the migrating motor complex. Between meals, this wave of muscular contractions sweeps undigested food particles and bacteria downward toward the colon, preventing buildup. It’s essentially a housekeeper that works during fasting periods, particularly during sleep and between meals.
When the migrating motor complex malfunctions, food residues sit in the small intestine longer than they should. Bacteria feed on those residues and multiply. This is considered the primary driver of excessive fermentation in the small intestine, and disrupted motility is also a leading reason SIBO tends to relapse after treatment. Conditions that impair this cleaning wave include diabetes (which can damage the nerves controlling gut movement), scleroderma, hypothyroidism, and prior food poisoning. Post-infectious SIBO is particularly common: certain foodborne pathogens trigger an autoimmune response that damages the nerves responsible for these contractions, leaving a lasting motility deficit even after the original infection resolves.
Structural Problems in the Small Intestine
Anything that creates a physical obstruction or a pocket where bacteria can hide increases the risk of overgrowth. Scar tissue from previous abdominal surgeries (adhesions) can wrap around the small bowel and slow transit. Small pouches that bulge through the intestinal wall, called diverticula, create dead-end spaces where food accumulates and ferments. Abnormal connections between two segments of bowel (fistulas) can also redirect bacteria into areas they don’t belong.
The ileocecal valve, a one-way gate between the small and large intestine, plays a critical role in keeping colonic bacteria out of the small bowel. When this valve is removed during surgery or stops functioning properly, bacteria from the colon can flow backward into the small intestine. People who have had a right hemicolectomy (removal of part of the colon along with this valve) often develop chronic, recurring SIBO. Even when the valve is intact, if the surgical reconnection leaves a pocket or “blind loop,” food can collect in that dead end and become a breeding ground for bacteria. One patient’s case illustrates this clearly: a 10-centimeter pocket left after a bowel resection harbored bacteria and pushed them back into the intestine for years before the problem was identified.
Low Stomach Acid
Stomach acid is your first line of defense against bacteria you swallow. A healthy stomach, with a pH low enough to kill most microorganisms, prevents oral bacteria from reaching the small intestine alive. When acid production drops, a condition called hypochlorhydria, that barrier weakens significantly.
Research has shown that people with low stomach acid develop noticeably different bacterial populations in their upper small intestine. Specifically, oral bacteria like certain strains of Streptococcus and Rothia, which would normally be destroyed in the stomach, survive the journey and take up permanent residence in the duodenum (the first section of the small intestine). The duodenal microbiota in these patients shows increased biodiversity, which in this context is not a good thing. It reflects colonization by organisms that don’t belong there. Chronic atrophic gastritis, a condition where the stomach lining thins and produces less acid over time, is a well-documented pathway to this kind of disruption.
Medications That Raise Risk
Acid-Suppressing Drugs
Proton pump inhibitors (PPIs) are among the most widely used medications in the world, and they substantially reduce stomach acid production. A large meta-analysis found that SIBO prevalence among PPI users was about 37%, compared to roughly 20% in people not taking them. That translates to approximately double the odds of developing SIBO.
The risk increases with duration. Taking PPIs for less than a month showed no statistically significant increase, but one to six months of use raised the odds roughly fourfold. Beyond six months, the risk was even higher. The analysis estimated that each additional month of PPI therapy increased SIBO prevalence by about 4 percentage points. This doesn’t mean everyone on a PPI will develop SIBO, but long-term use is a meaningful contributor, especially when combined with other risk factors.
Opioids
Opioid medications slow the entire digestive tract. They reduce peristalsis (the rhythmic contractions that push food forward), delay gastric emptying, and increase resting muscle tone in the intestinal wall. The result is significantly longer transit times through the small intestine, giving bacteria more opportunity to multiply. This effect is similar to what’s seen in constipation-predominant irritable bowel syndrome, where prolonged transit alone causes measurable shifts in the gut microbiome. Long-term opioid use compounds this by chronically impairing the coordinated muscle activity needed to keep the small intestine clear.
Reduced Pancreatic Function
The pancreas contributes to small intestinal defense in ways most people don’t realize. Beyond producing digestive enzymes, it secretes antimicrobial peptides that help keep bacterial populations in check, and it releases bicarbonate that helps maintain the right chemical environment in the small intestine.
When the pancreas underperforms, a condition called pancreatic exocrine insufficiency, several things go wrong at once. Undigested carbohydrates, proteins, and fats accumulate in the small intestine, providing an abundant food source for bacteria. Antimicrobial peptide production drops. The chemical balance of the intestinal contents shifts. And motility patterns change due to abnormal formation of the semi-liquid mixture (chyme) moving through the gut. Chronic pancreatitis is the most common cause of this kind of pancreatic insufficiency, and SIBO is a frequent complication.
How Multiple Causes Overlap
In practice, SIBO rarely comes from a single isolated cause. Someone with diabetes may have impaired motility and be taking a PPI for acid reflux. A person who had abdominal surgery may have both adhesions and a compromised ileocecal valve. An older adult may have naturally declining stomach acid, slower motility, and be on multiple medications. This layering of risk factors explains why SIBO is so common in certain populations and why it frequently recurs after antibiotic treatment: if the underlying causes aren’t addressed, the bacteria simply grow back.
How SIBO Is Identified
The standard diagnostic tool is a breath test. You drink a sugar solution (either glucose or lactulose), then breathe into collection tubes at regular intervals. Bacteria in the small intestine ferment the sugar and produce gases that enter your bloodstream and get exhaled through your lungs. A rise in exhaled hydrogen of at least 20 parts per million above your baseline within 90 minutes is considered diagnostic. Methane levels at or above 10 parts per million indicate a specific type of overgrowth involving methane-producing organisms, now sometimes called intestinal methanogen overgrowth. The type of gas produced matters because hydrogen-dominant and methane-dominant overgrowth tend to cause different symptom patterns, with methane more closely linked to constipation and hydrogen to diarrhea.