CONDITION
Small Intestinal Bacterial Overgrowth
Small intestinal bacterial overgrowth describes a situation in which bacteria that normally live in small numbers in the small intestine begin to multiply beyond their usual population. This can disrupt the digestive process, particularly the way nutrients are absorbed from food as it passes through. Owners often notice persistent or recurring diarrhoea, sometimes with a lot of gas or a particularly unpleasant smell. Weight loss may develop over time despite a normal or increased appetite, and some dogs or cats appear generally less comfortable after eating. The pattern can be subtle at first, and it may take a while before the picture becomes clear. This page explores the signs that may suggest bacterial overgrowth, what may be happening in the gut to allow it, the ways it can be investigated, and the approaches that exist to address it. The aim is to help you understand the condition and the shape of the conversations that may follow.
Why this matters now
Small intestinal bacterial overgrowth can emerge at any age, though it tends to appear more often in middle-aged and older dogs. Certain breeds, particularly German Shepherds and other large-breed dogs, may be more prone to the condition, possibly reflecting differences in gut structure or immune function. In cats the pattern is less clear, though underlying conditions that slow gut movement or alter the digestive environment may create the circumstances in which bacterial populations shift.
The pattern often develops gradually, with intermittent loose stools or mild digestive upset that may come and go over weeks or months. In some animals the signs remain relatively stable, whilst in others there may be a slow drift towards more persistent diarrhoea and weight loss as nutrient absorption becomes increasingly disrupted. The course can vary considerably between individuals, and in some cases the picture only becomes clear after other explanations have been explored.
Signals & patterns
Early signals
Intermittent soft or loose stools
The stool may appear soft, loose, or watery on some days but not others, without an obvious pattern linked to diet or routine. This inconsistency can make it difficult to identify a clear trigger.
Increased flatulence
Many owners notice more frequent or particularly strong-smelling gas, which may reflect bacterial fermentation of nutrients that would normally be absorbed higher in the gut. The smell can be unusually unpleasant.
Mild abdominal discomfort after eating
Some animals appear restless, may shift position frequently, or seem less settled after a meal. The signs are often subtle and may not suggest clear pain, but the animal's behaviour changes in a way that draws attention.
Gradual loss of body condition
Weight may decline slowly over weeks or months despite a normal or even increased appetite, as nutrients pass through without being fully absorbed. The change can be easy to miss in animals with thick coats or if seen daily.
Later signals
Persistent or chronic diarrhoea
The loose stools become more frequent and predictable, with little variation from day to day. The consistency may range from soft and unformed to watery, and the volume can be larger than expected.
Coat quality deterioration
The coat may become dull, dry, or coarse, reflecting ongoing nutritional deficiencies as vitamins and fats are poorly absorbed. In some cases the skin may also appear flaky or less healthy.
Increased frequency of defaecation
The animal may need to pass stools more often than usual, sometimes with urgency. The increased frequency can disrupt routines and may be accompanied by accidents indoors in animals that are normally reliable.
Click to read about the biological mechanisms
How this is usually investigated
The investigation typically begins with a detailed history of the digestive pattern, including the character and frequency of the diarrhoea, any relationship to feeding, and whether weight has changed despite appetite. Physical examination may reveal signs such as a thin body condition, gas-filled loops of intestine, or discomfort on palpation of the abdomen. Because the signs can overlap with many other digestive conditions, the investigation often involves ruling out more readily identifiable causes before focusing on bacterial overgrowth itself.
Faecal examination
Blood biochemistry and haematology
Serum cobalamin and folate measurement
Diagnostic imaging
Response to treatment
Options & trade-offs
Management is usually tailored to the individual animal and often involves a combination of dietary adjustment, antimicrobial treatment, and support for any underlying conditions that may be contributing. The approach that suits one owner and animal may differ from what works for another, depending on the severity of signs, the presence of other health issues, and practical constraints. Some animals respond well to a single course of treatment, whilst others require ongoing or intermittent management to keep signs controlled.
Antimicrobial therapy
Antibiotics such as metronidazole, tylosin, or oxytetracycline are often used to reduce bacterial numbers in the small intestine. The choice and duration depend on the individual animal's response and tolerance. Some animals show improvement within days, whilst others may need longer courses or repeated treatment if signs return.
Trade-offs: Prolonged or repeated antibiotic use carries the risk of altering the gut microbiome further, potentially contributing to resistance or secondary imbalances. Not all animals respond, and in some cases signs recur soon after stopping treatment, suggesting an underlying issue that has not been fully addressed.
Dietary modification
Feeding a highly digestible diet with moderate fat and easily absorbed carbohydrates can reduce the substrate available for bacterial fermentation. Some diets are formulated specifically to support animals with digestive disorders, whilst others may be home-prepared under guidance. The aim is to allow the gut to absorb nutrients more efficiently and create a less favourable environment for bacterial overgrowth.
Trade-offs: Dietary change alone may not resolve overgrowth in all cases, particularly if an underlying structural or motility problem persists. Some animals are reluctant to accept unfamiliar foods, and trial and error may be needed to find a diet that is both effective and palatable.
Probiotic supplementation
Probiotics are live bacteria or yeasts intended to help restore a more balanced gut microbiome. They are often used alongside other treatments in the hope of supporting the re-establishment of a healthier bacterial population after antimicrobial therapy. The evidence for their benefit varies, and responses can be inconsistent.
Trade-offs: Not all probiotic products are equivalent, and the specific strains and doses that may be helpful are not fully established for this condition. Some animals show no clear change, and probiotics are rarely sufficient as a sole treatment.
Vitamin and nutrient supplementation
Animals with low cobalamin levels, or those showing signs of fat-soluble vitamin deficiency, may be given supplements by injection or orally to restore normal levels. Cobalamin is often given as a course of injections over several weeks, with response monitored through repeat blood tests and observation of clinical signs.
Trade-offs: Supplementation addresses the consequence of malabsorption rather than the overgrowth itself, so it is usually combined with other measures. Some animals require long-term supplementation if the underlying problem cannot be fully resolved.
Management of underlying conditions
When bacterial overgrowth is secondary to another issue such as exocrine pancreatic insufficiency, chronic pancreatitis, or a motility disorder, addressing that condition becomes central to management. This may involve enzyme supplementation, medications to improve gut movement, or treatment of inflammatory disease, depending on what has been identified.
Trade-offs: Identifying and managing an underlying cause can be more complex and time-consuming than treating the overgrowth alone, and not all underlying conditions can be fully corrected. In some animals, the precipitating factor remains unclear even after thorough investigation.
Common misconceptions
"Small intestinal bacterial overgrowth means the gut is infected with harmful bacteria that must be eliminated completely."
The bacteria involved are often normal inhabitants of the digestive system, present in all healthy animals but usually in much smaller numbers. The problem is not that the bacteria are inherently harmful, but that their population has grown beyond the level the small intestine can accommodate without disrupting digestion. Management aims to restore balance rather than sterilise the gut.
"Once bacterial overgrowth is treated, it will not return."
In some animals the condition can recur, particularly if an underlying cause such as reduced gut motility, immune dysfunction, or structural abnormality has not been identified or cannot be fully corrected. Some individuals require intermittent or ongoing management to keep signs controlled, whilst others remain stable after a single course of treatment. The course varies considerably.
"Bacterial overgrowth can be diagnosed with a single definitive test."
There is no single gold-standard test that confirms bacterial overgrowth with certainty in dogs and cats. Diagnosis is often based on a combination of clinical signs, exclusion of other conditions, supporting laboratory findings such as low cobalamin, and sometimes the response to treatment. The picture is built up over time rather than established in a single moment.
Related conditions
Exocrine Pancreatic Insufficiency (EPI)
Exocrine pancreatic insufficiency can present with similar signs—persistent diarrhoea, weight loss despite good appetite, and poor nutrient absorption—because both conditions disrupt the normal digestive process, though the underlying mechanisms differ. Distinguishing between the two often requires specific testing of pancreatic enzyme levels.
Inflammatory Bowel Disease in Cats
Inflammatory bowel disease in cats can alter the normal intestinal environment in ways that may predispose to bacterial overgrowth, and both conditions can produce overlapping signs of chronic diarrhoea, weight loss, and digestive upset. In some cases, the two may occur together, complicating the clinical picture.
Inflammatory Bowel Disease in Dogs
Inflammatory bowel disease in dogs can disrupt the normal structure and function of the small intestine, creating conditions that may allow bacterial populations to increase beyond their usual numbers. The two can share similar signs, and chronic intestinal inflammation may predispose to bacterial overgrowth in some individuals.
Megaoesophagus
Megaoesophagus can lead to small intestinal bacterial overgrowth when delayed gastric emptying or changes in gut motility allow bacteria to multiply in the small intestine. Both conditions can contribute to malabsorption and weight loss, though they affect different parts of the digestive tract.
Gastroenteritis in Cats
Gastroenteritis in cats may present with acute signs that can resemble a flare of bacterial overgrowth, and recurrent or persistent gastroenteritis may sometimes create intestinal changes that predispose to bacterial population shifts. Distinguishing between transient inflammation and an underlying overgrowth pattern often depends on the duration and response to initial measures.
Small intestinal bacterial overgrowth often sits within a broader context of metabolic and digestive health, and understanding how nutrient absorption, gut motility, and the intestinal environment interact can be useful background for conversations over time. If weight loss, appetite changes, or persistent digestive signs are part of the picture, exploring related conditions within the metabolic health pillar may offer additional perspective. The shape of long-term management, and the balance between control and relapse, can become clearer as the individual animal's pattern emerges.
Last reviewed: 1 July 2026 · Dr Alastair Greenway MRCVS