What Is Functional Diarrhea?

Functional diarrhea is chronic diarrhea without any clear cause. It is a functional gastrointestinal disorder (FGD), meaning that it occurs without any other known signs of disease, injury, or structural problem.

Even if the cause of your functional diarrhea is unknown, your symptoms are real and should be treated. Over time, functional diarrhea can cause negative health effects, and you may benefit from lifestyle modifications or medical treatment.

This article explains what functional diarrhea is, how it is diagnosed, and how it is treated.

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Functional Diarrhea Criteria

The Rome IV diagnostic criteria categorize FGD, including functional diarrhea, according to specific definitions.

Functional diarrhea criteria are:

  • The experience of loose or watery stools
  • Lack of pain with the diarrhea
  • Diarrhea occurring in at least 75% of bowel movements
  • At least six months of symptoms, at least three months of which must be consecutive
  • No identifiable cause (structural or biochemical)

Functional Diarrhea and Irritable Bowel Syndrome

Irritable bowel syndrome (IBS) is a type of FGD. When chronic diarrhea is the predominant symptom of IBS, diarrhea-predominant irritable bowel syndrome (IBS-D) may be diagnosed.

Functional diarrhea is characterized by a lack of abdominal pain, while IBS-D can cause abdominal pain.

Both disorders involve frequent loose stools and may also involve such symptoms as gas, bloating, mucus in the stool, and feelings of urgency or incomplete evacuation, in which it feels like you have to use the toilet even if you just went.

Functional diarrhea is often considered a subtype of IBS-D.


Functional diarrhea is diagnosed through a process of exclusion. This means that you can be diagnosed with functional diarrhea only after other digestive disorders or health problems have been ruled out.

Typically, your healthcare provider will take your health history, do a physical examination, and may order diagnostic lab tests, such as blood work and a stool sample analysis.

Other possible tests can include imaging examinations, such as abdominal computed tomography (CT), ultrasound, or magnetic resonance imaging (MRI). Invasive tests such as a colonoscopy or an endoscopy can also help identify a cause.

Differential Diagnoses

Possible causes of diarrhea that need to be ruled out before a diagnosis of functional diarrhea is made include:

  • Gastrointestinal (GI) infection, including chronic infection. Infections notoriously cause loose stools. Most GI infections last no longer than a few weeks and typically resolve on their own. But chronic infections, which can be diagnosed with a stool sample, may cause prolonged symptoms.
  • Medication side effects. A number of medications can cause diarrhea, and you may develop this side effect even if you have taken medication for a while without diarrhea.
  • Diet. Various food allergies and sensitivities can cause loose stools without pain. You may notice a pattern of diarrhea occurring after you eat certain foods.
  • Celiac disease. This can develop with age and may be associated with gluten sensitivity, or may cause diarrhea regardless of your diet.
  • Gluten sensitivity. This is becoming a more common problem. You can try a gluten-free diet for a few weeks to see if there is a decrease in your diarrhea.
  • Lactose intolerance. A relatively common problem, lactose intolerance is characterized by diarrhea and cramping after eating or drinking dairy products.
  • Fructose malabsorption. It can be more difficult to identify than lactose intolerance, but cutting out foods that contain fructose for a few weeks can help you assess whether fructose contributes to your diarrhea.
  • Malabsorption due to bariatric surgery. Malabsorption is a common problem caused by bariatric surgery, in which parts of the GI system are removed to prevent over-eating and excessive absorption of calories.
  • Gastrointestinal cancer. Cancer may cause bleeding and weight loss and may manifest with a variety of less-common symptoms, such as diarrhea.
  • Autonomic neuropathy. Often caused by chronic alcohol use, neuropathy can impair the function of nerves throughout the body, including the nerves of the GI system, potentially causing diarrhea.

You may also be at higher risk of developing functional diarrhea after having your gallbladder removed.

Treatment for Functional Diarrhea

Treatment for functional diarrhea typically aims to reduce symptoms through dietary changes. For example, this may include eliminating food triggers and slowly increasing fiber intake. If stress appears to be a possible contributing factor, stress-management strategies might be helpful.

Your healthcare provider may give you a prescription or recommendation for one of the following medications to reduce your diarrhea:

5 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  2. Saha L. Irritable bowel syndrome: Pathogenesis, diagnosis, treatment, and evidence-based medicine. World J Gastroenterol. 2014;20(22):6759-73. doi:10.3748/wjg.v20.i22.6759

  3. Bul V, Sleesman B, Boulay B. Celiac disease presenting as profound diarrhea and weight loss - a celiac crisis. Am J Case Rep. 2016;17:559-61. doi:10.12659/AJCR.898004

  4. Deng Y, Misselwitz B, Dai N, Fox M. Lactose intolerance in adults: Biological mechanism and dietary management. Nutrients. 2015;7(9):8020-35. doi:10.3390/nu7095380

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Additional Reading

By Barbara Bolen, PhD
Barbara Bolen, PhD, is a licensed clinical psychologist and health coach. She has written multiple books focused on living with irritable bowel syndrome.