Diarrhea for Weeks: Causes and When to See a Doctor

What counts as chronic diarrhea?

Diarrhea is generally defined as three or more loose or watery stools per day. Acute diarrhea lasts less than two weeks and is usually caused by a viral or bacterial infection that resolves on its own. Persistent diarrhea lasts two to four weeks. Chronic diarrhea persists for more than four weeks and has a broader list of possible causes that warrant systematic evaluation 1Ref 1Lacy BE, Pimentel M, Brenner DM, Chey WD, Keefer LA, Long MD, Moshiree B (2021).ACG Clinical Guideline: Management of Irritable Bowel Syndrome.IBS-D as a common cause of chronic diarrhea, definition of chronic diarrhea, and role of fecal calprotectin in distinguishing IBS from IBD.

Frequency, consistency, timing (day vs. night), presence of blood or mucus, and associated symptoms like pain, weight loss, or bloating all help narrow the cause.

What are the most common causes of diarrhea that won't stop?

Irritable bowel syndrome (IBS-D) IBS is one of the most common causes of chronic loose stools, particularly in younger adults. It causes recurring abdominal pain that improves with defecation, along with changes in stool frequency and form. IBS does not cause inflammation or tissue damage and does not increase cancer risk — but it significantly affects quality of life 1Ref 1Lacy BE, Pimentel M, Brenner DM, Chey WD, Keefer LA, Long MD, Moshiree B (2021).ACG Clinical Guideline: Management of Irritable Bowel Syndrome.IBS-D as a common cause of chronic diarrhea, definition of chronic diarrhea, and role of fecal calprotectin in distinguishing IBS from IBD.

Food intolerances Lactose intolerance and fructose malabsorption are common and can cause daily loose stools that improve dramatically with dietary adjustment. Celiac disease — an immune reaction to gluten — causes diarrhea, weight loss, and nutritional deficiencies and is diagnosed by specific blood tests and small bowel biopsy 2Ref 2Rubio-Tapia A, Hill ID, Semrad C, Kelly CP, Greer KB, Limketkai BN, Lebwohl B (2023).American College of Gastroenterology Guidelines Update: Diagnosis and Management of Celiac Disease.Celiac disease as a cause of chronic diarrhea with diagnostic workup including tTG-IgA serology.

Inflammatory bowel disease (IBD) Crohn's disease and ulcerative colitis both cause chronic diarrhea, often with blood, mucus, abdominal pain, and fatigue. IBD is diagnosed with colonoscopy and biopsies. It requires ongoing specialist management [3, 4].

Bile acid malabsorption The small intestine normally reabsorbs bile acids; when it cannot (after surgery, ileal Crohn's disease, or idiopathically), bile acids reach the colon and cause secretory diarrhea. This is underdiagnosed and treatable.

Medication effects Many common medications cause diarrhea, including metformin, antibiotics (including post-antibiotic *C. diff*), proton pump inhibitors, magnesium-containing supplements, and certain blood pressure drugs.

Microscopic colitis A common but often overlooked cause of chronic watery diarrhea, especially in older adults. The colon looks normal on colonoscopy but biopsies show characteristic inflammation. Often associated with NSAID use.

Overactive thyroid, diabetes-related GI changes, and other systemic conditions Chronic diarrhea can reflect a systemic condition rather than a primary GI problem. Blood tests including thyroid function are often part of the initial workup.

When should diarrhea be evaluated urgently?

See a clinician soon — within days, not weeks — if you have:

• Blood in the stool (red or dark/tarry)
• Diarrhea that wakes you from sleep at night (a red flag that distinguishes organic disease from functional causes like IBS)
• Unintended weight loss
• Fever accompanying the diarrhea
• Signs of dehydration: dizziness, dark urine, dry mouth, or reduced urination
• Age 45 or older with new onset of diarrhea (colorectal cancer is in the differential)
• A close family history of colorectal cancer, IBD, or celiac disease

What testing is usually done?

A primary care clinician will often begin with:

• Blood tests: complete blood count, metabolic panel, thyroid function, celiac antibodies (tTG-IgA), inflammatory markers (CRP, ESR)
• Stool studies: cultures, ova and parasites, *Clostridioides difficile* testing, fecal calprotectin (a marker that helps distinguish IBS from IBD)

If initial tests are non-diagnostic or findings suggest IBD or another structural cause, referral to a gastroenterologist for colonoscopy or upper endoscopy with biopsy is the next step. A gastroenterologist is the right specialist for chronic diarrhea that has not responded to initial evaluation or treatment [1, 3].

Cludia can help you schedule with a primary care clinician as a first step, or connect you directly to gastroenterology if you already have a diagnosis.