SIBO stands for small intestinal bacterial overgrowth, a condition where abnormally large numbers of bacteria colonize the small intestine, causing digestive symptoms like bloating, gas, diarrhea, and malabsorption. Your small intestine normally hosts relatively few bacteria compared to the large intestine, so when bacterial populations grow out of control in that space, they interfere with normal digestion and nutrient absorption.
The Medical Definition
In clinical terms, SIBO is defined as a syndrome of maldigestion caused by excessive or regionally inappropriate bacteria in the small intestine. “Regionally inappropriate” is a key phrase here: the bacteria involved aren’t necessarily harmful on their own. Many are normal residents of the large intestine that have simply ended up in the wrong place or multiplied beyond what the small intestine can handle.
Diagnosis typically relies on a breath test. After drinking a sugar solution (glucose or lactulose), you breathe into a collection device at regular intervals. The bacteria in your small intestine ferment the sugar and produce gases that enter your bloodstream and travel to your lungs. A rise in exhaled hydrogen of at least 20 parts per million above your baseline within 90 minutes is considered diagnostic. For methane, any reading of 10 ppm or above at any point during the test indicates a related but distinct condition now called intestinal methanogen overgrowth (IMO).
How the Small Intestine Normally Keeps Bacteria in Check
Your small intestine has a built-in cleaning mechanism called the migrating motor complex, or MMC. Between meals, this wave-like muscular contraction sweeps residual food particles and bacteria downward toward the large intestine. Think of it as a housekeeping cycle that runs every 90 to 120 minutes when you’re not eating. Stomach acid, bile, and the immune lining of the intestine also help keep bacterial counts low.
When any of these defenses falter, bacteria have the opportunity to settle and multiply. Dysmotility, where the muscular contractions of the small intestine slow down or become disorganized, is one of the most common underlying mechanisms. Without those regular sweeping waves, bacteria accumulate instead of being pushed along.
What Causes It
SIBO rarely appears out of nowhere. It almost always develops on top of an underlying condition or structural change that disrupts the small intestine’s normal defenses. The causes generally fall into two categories: structural problems and motility disorders.
Structural causes include intestinal adhesions (scar tissue from surgery or inflammation), small bowel diverticula (small pouches that bulge outward from the intestinal wall and trap bacteria), and surgical changes like gastric bypass that alter the normal anatomy. Blind loops, where a section of intestine is bypassed and becomes stagnant, are a classic example.
Motility-related causes include diabetes (which can damage the nerves controlling gut movement), scleroderma (an autoimmune condition that stiffens intestinal tissue), Crohn’s disease, celiac disease, and radiation enteritis. Opioid medications, which slow gut motility, can also contribute. Any condition that reduces the frequency or strength of those migrating motor complex waves raises the risk.
Symptoms and How They Develop
The hallmark symptoms are bloating, abdominal distension, gas, diarrhea, and abdominal pain. These happen because the overgrown bacteria ferment carbohydrates that would normally be absorbed further down the digestive tract, producing hydrogen, methane, or both. That gas production is what makes bloating so prominent.
The type of gas matters. Hydrogen-dominant SIBO tends to be associated with diarrhea, while methane-dominant overgrowth (IMO) is more strongly linked to constipation. Methane itself slows intestinal transit, which is why practitioners increasingly treat IMO as a clinically distinct condition rather than just a subtype of SIBO.
Beyond gas and bloating, the excess bacteria can interfere with nutrient absorption. Bacteria in the small intestine compete with your body for nutrients and can break down bile acids that you need to absorb dietary fat. When bile acids are degraded prematurely, fat passes through undigested, sometimes causing greasy, foul-smelling stools and deficiencies in fat-soluble vitamins like A, D, E, and K. Bacteria can also compete with the body for vitamin B12, though the relationship between SIBO and B12 deficiency is more complex than once thought. Recent research suggests B12 levels don’t always drop predictably across all SIBO subtypes.
How SIBO Is Diagnosed
The breath test is the most widely used diagnostic tool because it’s noninvasive and accessible. You fast overnight, drink a sugar substrate, and then provide breath samples over two to three hours. The test measures hydrogen and methane in your exhaled breath. A hydrogen rise of 20 ppm or more within the first 90 minutes, or a methane level of 10 ppm or more at any point, is considered positive based on the North American Consensus guidelines.
The breath test does have limitations. It can produce false positives (especially with lactulose, which can reach the large intestine quickly in people with fast transit) and false negatives. The older gold standard, small bowel aspirate culture, involves threading a tube into the small intestine and directly sampling its contents. This is more invasive and less commonly performed, but it remains valuable in cases where breath testing is inconclusive.
Treatment and Recurrence
The primary treatment is a course of antibiotics designed to reduce the bacterial load in the small intestine. The most commonly prescribed option is a gut-specific antibiotic that stays largely within the intestinal tract rather than being absorbed into the bloodstream. Treatment courses typically last 7 to 14 days, though the optimal dose and duration haven’t been firmly established. Some patients need repeat courses.
This is where SIBO gets frustrating for many people: recurrence is common. Up to two-thirds of patients experience a return of symptoms within several months of successful treatment. That high recurrence rate exists because antibiotics address the overgrowth itself but not the underlying reason it developed. If slow motility, structural abnormalities, or another predisposing factor remains, the bacteria tend to creep back.
Managing the root cause is just as important as clearing the bacteria. For motility issues, medications that stimulate the migrating motor complex (called prokinetics) are sometimes used after antibiotics to help prevent relapse. Dietary modifications, particularly reducing fermentable carbohydrates that feed the bacteria, can help manage symptoms between episodes. Spacing meals to allow the migrating motor complex to complete its cleaning cycles, rather than snacking continuously, is another practical strategy some gastroenterologists recommend.
SIBO vs. IMO: An Important Distinction
The terminology around SIBO has shifted in recent years. Traditional SIBO refers to bacterial overgrowth producing excess hydrogen. But the organisms responsible for elevated methane aren’t actually bacteria. They’re archaea, a separate domain of life, with the primary culprit being a species that thrives in the gut by consuming the hydrogen other microbes produce. Because archaea aren’t bacteria, and because methane-producing organisms can overgrow in the large intestine as well as the small intestine, the term “intestinal methanogen overgrowth” was introduced to describe this condition more accurately.
The distinction matters clinically because IMO responds differently to treatment and presents with different symptoms, predominantly constipation rather than diarrhea. If your breath test shows elevated methane, your treatment approach will likely differ from someone with hydrogen-dominant SIBO.