What Causes Diarrhea? Triggers, Warning Signs, and When to Worry

Diarrhea is loose, watery stool happening three or more times in 24 hours. That's the clinical definition from the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). The cause matters because the duration tells you what kind you have, and the kind tells you whether to wait it out or pick up the phone.

Acute diarrhea is almost always an infection. Persistent diarrhea (over two weeks) and chronic diarrhea (over four weeks) are different problems entirely - usually medications, food intolerances, IBS, IBD, or something the colon shouldn't be doing. Below is the full breakdown, with the specific pathogens, drugs, and conditions involved.

The Three Durations and Why They Matter

Doctors don't lump all diarrhea together. The duration is the first thing they ask, because each category points to a different cause and a different workup. The NIDDK definitions:

• Acute diarrhea lasts less than two weeks and usually clears on its own. Infection is the default assumption.
• Persistent diarrhea lasts two to four weeks. By this point an infection is less likely and a workup is reasonable.
• Chronic diarrhea is four or more weeks of loose stools at least three times a day. This is rarely benign and needs evaluation. Chronic diarrhea affects up to 5% of the general population.

If you're new to thinking about stool by category, the Bristol Stool Chart guide covers the full 7-type scale - Types 6 and 7 (mushy and entirely liquid) are what doctors mean by diarrhea.

Acute Diarrhea: Almost Always an Infection

If you suddenly have loose stools and nothing in your routine has changed, you almost certainly caught something. The epidemiology is consistent: viral and bacterial gastroenteritis account for the overwhelming majority of acute cases in adults.

Viral causes

Norovirus is the leading cause of acute gastroenteritis in U.S. adults. It moves fast through dorms, cruise ships, nursing homes, and restaurants because the infectious dose is tiny (as few as 18 viral particles) and it survives on surfaces for days. Onset is 12-48 hours after exposure, recovery is typically 1-3 days, and there is no specific treatment beyond rehydration.

Rotavirus is the analogous problem in young children. Adenovirus, astrovirus, and sapovirus round out the common viral list. SARS-CoV-2 also causes diarrhea in a meaningful subset of cases - GI symptoms appear in 5-10% of COVID-19 patients.

Bacterial causes

Bacterial gastroenteritis tends to be more severe than viral, and is more likely to produce blood in stool, high fever, and pus on a stool sample. The big five, per the Cleveland Clinic:

• Salmonella - undercooked poultry, eggs, raw produce. Onset 6-72 hours.
• Campylobacter - the most common bacterial cause in the U.S., usually from raw or undercooked chicken.
• Escherichia coli (E. coli) - several strains. Shiga toxin-producing E. coli (STEC, including O157:H7) is the dangerous one. Linked to undercooked ground beef and contaminated produce.
• Shigella - very contagious, spreads through person-to-person contact and contaminated water.
• Clostridioides difficile (C. diff) - the classic post-antibiotic infection. Causes profuse, foul-smelling diarrhea and can be life-threatening.

Parasitic causes

Less common in developed countries but worth mentioning if symptoms persist past two weeks. Giardia lamblia is the classic culprit from contaminated stream water or daycare exposure. Cryptosporidium survives chlorinated pool water and is a frequent cause of recreational-water outbreaks. Entamoeba histolytica is more common in travelers returning from regions with poor sanitation.

Food poisoning

"Food poisoning" technically includes both bacterial infection and toxin-mediated illness. The fast-onset variety (1-6 hours after eating) is usually toxin-mediated - Staphylococcus aureus or Bacillus cereus toxins produced in food that sat at room temperature. These are nasty but short-lived. The 8-72 hour variety is typically bacterial infection (Salmonella, Campylobacter, E. coli).

Traveler's Diarrhea

If you got diarrhea on a trip abroad, it's almost certainly bacterial. Traveler's diarrhea (TD) is the most common travel-associated illness in the world. The incidence runs 30-70% within two weeks of arrival, with the highest rates in Africa, South Asia, and parts of Latin America.

Bacteria cause over 80% of TD cases. Enterotoxigenic E. coli (ETEC) alone is responsible for about 30%, followed by enteroaggregative E. coli, Campylobacter, Salmonella, and Shigella. About 3-10% of travelers develop persistent diarrhea (over two weeks) and roughly 4% develop chronic diarrhea (over four weeks) - often post-infectious IBS, which can last months.

Medication-Induced Diarrhea

This is the most under-recognized cause of chronic diarrhea, especially in older adults on multiple prescriptions. If your diarrhea started or worsened after beginning a new medication, the medication is the suspect until proven otherwise.

Antibiotics

Antibiotic-associated diarrhea (AAD) occurs in 5-25% of patients on antibiotics, depending on the drug. Broad-spectrum agents like amoxicillin-clavulanate, clindamycin, and the fluoroquinolones are the worst offenders. The mechanism: antibiotics wipe out beneficial gut bacteria, allowing opportunistic organisms (especially C. difficile) to overgrow. Severe or bloody diarrhea during or shortly after antibiotic use needs a stool test for C. diff.

Metformin

Metformin is the first-line diabetes drug and one of the most commonly prescribed medications in the world. Diarrhea is its most common side effect, affecting roughly 20% of patients. Most cases happen early and resolve within weeks, but a smaller subset develops late-onset chronic diarrhea years into treatment - often misdiagnosed as IBS for years. Metformin alters bile acid reabsorption and speeds gut motility. Extended-release formulations cause fewer GI side effects.

GLP-1 drugs (Ozempic, Wegovy, Mounjaro, Zepbound)

GLP-1 receptor agonists slow gastric emptying, which causes nausea early on but can also cause diarrhea and constipation. We covered the full digestive profile of these drugs in the Ozempic digestive side effects guide. The short version: diarrhea typically peaks during dose escalation and improves over weeks.

Magnesium supplements and laxatives

Magnesium oxide, citrate, and hydroxide pull water into the intestine osmotically - that's why they work for constipation. Take too much and you get diarrhea, predictably. Same mechanism for sorbitol, mannitol, and lactulose. Sugar-free gum and candy contain enough sorbitol to cause "sugar-free diarrhea" if you chew through a pack.

Other common offenders

• Proton pump inhibitors (PPIs) like omeprazole increase the risk of bacterial overgrowth and C. diff infection by reducing stomach acid.
• NSAIDs (ibuprofen, naproxen) can cause diarrhea through direct mucosal irritation.
• SSRIs, particularly sertraline and fluoxetine, cause diarrhea in some patients due to serotonin's role in gut motility.
• Chemotherapy agents, especially irinotecan and 5-fluorouracil, are notorious for treatment-limiting diarrhea.
• Statins and certain blood pressure drugs (notably olmesartan, which can cause sprue-like enteropathy) round out the list.

Food Intolerances and Sensitivities

If diarrhea reliably follows specific foods, you're not "stress-pooping" - you have an intolerance. The big three:

Lactose intolerance

About 65% of the global adult population has reduced ability to digest lactose after infancy, per MedlinePlus. Prevalence varies enormously by ancestry - around 5% in Northern Europeans, over 90% in East Asians. Undigested lactose ferments in the colon, producing gas, bloating, and osmotic diarrhea 30 minutes to two hours after dairy. Lactose intolerance is present in around 45% of IBS patients vs 17% of healthy adults, which is why many IBS "triggers" are actually just lactose.

FODMAPs

Fermentable carbohydrates - fructans (wheat, onion, garlic), galacto-oligosaccharides (legumes), polyols (stone fruits, sugar alcohols), excess fructose (high-fructose corn syrup, honey) - pull water into the gut and ferment rapidly. They are a leading dietary trigger in IBS. Our low FODMAP diet guide walks through the elimination and reintroduction protocol.

Celiac disease and non-celiac gluten sensitivity

Celiac is an autoimmune reaction to gluten that damages the small intestinal lining. It affects about 1% of the U.S. population, and chronic diarrhea is the textbook presentation - though many adults present with atypical symptoms (fatigue, anemia, infertility). One review found celiac accounted for over half of adult cases of chronic diarrhea referred to GI clinics. If you have chronic diarrhea, get the celiac panel before going gluten-free; the test only works while you're still eating gluten.

IBS, IBD, and Other Chronic Conditions

Irritable bowel syndrome - diarrhea predominant (IBS-D)

IBS-D is defined by recurrent abdominal pain with loose stools (Bristol Types 6-7) more than 25% of the time, lasting at least three months. There is no biomarker. Diagnosis is clinical, using the Rome IV criteria. The condition is real - it involves visceral hypersensitivity, altered gut motility, and a measurable signature in the microbiome - but it's a diagnosis of exclusion. Doctors look for celiac, IBD, microscopic colitis, and bile acid diarrhea first. The signs of IBS guide covers diagnosis in detail.

Inflammatory bowel disease (IBD)

Ulcerative colitis and Crohn's disease are autoimmune conditions that cause inflammation of the GI tract. Diarrhea is the dominant symptom, often with visible blood (ulcerative colitis especially), weight loss, fevers, and nighttime awakening to defecate. Nocturnal diarrhea is a red flag that almost never accompanies IBS. If your diarrhea wakes you from sleep, get evaluated.

Microscopic colitis

An underdiagnosed cause of chronic watery diarrhea, especially in women over 60. The colon looks normal on colonoscopy - the inflammation only shows up under a microscope, which is why biopsies are essential. Often triggered by PPIs, NSAIDs, or SSRIs.

Bile acid diarrhea

About one in three patients labeled with IBS-D actually have bile acid malabsorption. Bile acids that should be reabsorbed in the ileum instead spill into the colon and act as a powerful laxative. Common after gallbladder removal, with Crohn's disease, or idiopathic. Often responds dramatically to bile acid sequestrants like cholestyramine.

SIBO

Small intestinal bacterial overgrowth causes diarrhea, bloating, and malabsorption when bacteria proliferate where they shouldn't - the small intestine. We covered the difference between SIBO and IBS in the SIBO symptoms guide.

Hyperthyroidism, diabetic enteropathy, pancreatic insufficiency

Endocrine and pancreatic problems frequently masquerade as GI conditions. An overactive thyroid speeds everything up, including bowel transit. Diabetic autonomic neuropathy causes alternating diarrhea and constipation. Pancreatic exocrine insufficiency produces fatty, foul-smelling diarrhea that floats. These all need workup, not anti-diarrheal medication.

Stress and the Gut-Brain Axis

Acute stress can trigger diarrhea through the vagus nerve and corticotropin-releasing factor (CRF), which speeds colonic transit and increases mucus secretion. This is real biology, not "just nerves" - the wiring between the limbic system and the enteric nervous system is dense and bidirectional. We unpacked the mechanism in the gut-brain connection guide. For people whose primary issue is anxiety-driven GI symptoms, the nervous stomach guide covers practical approaches.

Dehydration: The Real Danger

Most diarrhea won't kill you. Dehydration from diarrhea absolutely can - it's the leading cause of diarrhea-related death globally. Per Johns Hopkins Medicine, watch for:

• Dark yellow or amber urine, or urinating less than usual
• Dry mouth, lips, and tongue
• Sunken eyes or cheeks
• Dizziness or lightheadedness when standing
• Rapid heartbeat or rapid breathing
• Headache and unusual fatigue
• In older adults: confusion or disorientation

Plain water alone is suboptimal for replacing what you're losing. Oral rehydration solutions (ORS) - or a homemade equivalent with salt, sugar, and water in the right ratio - replace sodium, potassium, and glucose together, which the gut absorbs more efficiently. Sports drinks contain too much sugar and too little sodium to be optimal but are better than water.

When to See a Doctor

Most acute diarrhea in healthy adults resolves in 1-3 days and needs nothing more than fluids and patience. But these signs warrant medical attention, per the Mayo Clinic and NIDDK:

• Diarrhea lasting more than 2 days in adults without improvement
• Blood in stool, or black tarry stools (a sign of upper GI bleeding)
• Severe abdominal or rectal pain
• Fever above 102 degrees Fahrenheit (39 degrees Celsius)
• Six or more loose stools in 24 hours
• Signs of dehydration (see above)
• Recent antibiotic use with worsening diarrhea (rule out C. diff)
• Recent international travel
• Nocturnal diarrhea waking you from sleep
• Unexplained weight loss with diarrhea
• Diarrhea in an immunocompromised person, infant, or older adult - the threshold to call is lower

The when to see a gastroenterologist guide covers what specialist visits look like, what tests to expect, and what to bring with you.

What Actually Helps

Treatment depends on the cause, but a few principles cover most acute cases:

• Rehydrate aggressively with oral rehydration solution, broth, or electrolyte drinks.
• Eat when you can. The old BRAT diet (bananas, rice, applesauce, toast) is not actively wrong but it's restrictive. Most current guidelines recommend resuming a normal diet as tolerated. Avoid alcohol, caffeine, and very fatty or sugary foods until you're back to baseline.
• Loperamide (Imodium) is safe for adults with watery, non-bloody diarrhea and no fever. Avoid it if you suspect bacterial dysentery, C. diff, or IBD - slowing the gut traps the pathogen and can make things much worse.
• Bismuth subsalicylate (Pepto-Bismol) reduces both diarrhea and nausea, with the side effect of harmless black tongue and black stools.
• Antibiotics are the wrong move for most acute diarrhea. They're indicated for specific bacterial infections (typhoid, severe traveler's diarrhea, C. diff with appropriate agents) and useless for viral causes.

For chronic or recurrent diarrhea, treatment follows the diagnosis. The right move there is to start tracking - food, symptoms, frequency, Bristol score - and bring the log to a clinician. Most chronic GI conditions are diagnosed by pattern.