Causes of Diarrhea: Common Triggers, Warning Signs and Tests

Warning signs come before identifying the cause

Contact a clinician urgently for bloody or black, tarry stool, pus, severe abdominal or rectal pain, frequent vomiting, high fever, or inability to replace lost fluid. Adults should also seek advice for diarrhea lasting more than two days or six or more loose stools in a day. Get help earlier if you are pregnant, over 65, immunocompromised, or currently taking antibiotics. These thresholds are reasons to act, not a requirement to wait when you already feel seriously ill. [2]

Children can lose fluid quickly, especially newborns and infants. Seek medical advice right away for an infant with diarrhea, a premature baby, or a child with another medical condition. Any fever in an infant, diarrhea lasting more than a day in a child, repeated vomiting, refusal to drink, or signs of dehydration also need prompt attention. A child who cannot drink enough or is not improving with rehydration needs assessment. Do not use an adult’s experience or medicine plan to judge what is safe for a baby. [4]

Some accompanying symptoms need emergency care even before dehydration develops: sudden severe abdominal pain, vomiting blood, vomit resembling coffee grounds, or green vomit. Green or yellow-green vomit in a child also requires immediate assessment. These findings should not be treated as an ordinary stomach bug at home. If vomiting repeatedly prevents you from keeping fluid down, obtain urgent help even if the stool itself contains no blood. The practical issue is both the possible cause and whether the body can keep up with fluid losses. [5]

What counts as diarrhea, and why duration matters

Diarrhea usually means passing loose, watery stools three or more times a day, or more often than your personal normal. Consistency matters as well as frequency. NIDDK’s duration categories distinguish acute episodes, which usually last under a week, from persistent symptoms lasting more than two but fewer than four weeks. Diarrhea lasting four weeks or longer is classified as chronic, whether continuous or intermittent. These labels organize the investigation; they do not override the earlier advice about when to seek care. Repeated episodes can also affect nutrition, so mention unintended weight changes alongside bathroom frequency. [1]

Bowel leakage and diarrhea can overlap, but they are not interchangeable descriptions. Loose stool is harder to hold, while impaired muscles or nerves can allow leakage even when stool consistency is otherwise normal. Some people feel a powerful urge and cannot reach the toilet; others notice soiling without feeling stool pass. Tell the clinician which experience you have, including small leaks that seem embarrassing or unrelated. Frequent leakage or symptoms that interfere with everyday life deserve help. This distinction can change the assessment from one focused only on stool production to one that also examines bowel control. [42]

Infections are common causes of sudden diarrhea

Viruses and household outbreaks

Norovirus can cause abrupt vomiting, watery diarrhea, nausea and abdominal discomfort. Symptoms generally begin 12 to 48 hours after exposure, and many people improve within one to three days. It spreads through close contact, contaminated food or drink, and contaminated surfaces. Illness among household members, classmates or coworkers is therefore relevant even when everyone ate different meals. The nickname “stomach flu” is misleading because norovirus is unrelated to influenza. Antibiotics do not treat it. A typically short course still needs attention to hydration, particularly when vomiting and diarrhea occur repeatedly together. [6]

Rotavirus particularly affects infants and young children, although adults can also become ill. Watery diarrhea and vomiting may last several days and cause substantial dehydration. Infection spreads when virus from stool reaches another person’s mouth, including through hands and shared objects. Vaccination reduces a child’s risk of rotavirus disease and severe illness, but it does not make every future diarrheal episode impossible. Discuss routine vaccination with the child’s healthcare team. A vaccinated child with poor intake, fewer wet diapers or unusual sleepiness still needs assessment according to their current symptoms. [7]

Foodborne bacteria and toxins

“Food poisoning” describes several different illnesses rather than one organism. Some begin within hours, while others take days. Vomiting, cramps, fever or diarrhea may dominate, depending on the cause. Food that made one person ill may also affect others, but symptom timing and severity can differ. Write down shared meals and when each person first felt unwell instead of selecting a culprit from memory alone. During pregnancy, contact your clinician if suspected foodborne illness includes fever or flu-like symptoms, because some infections that seem relatively mild can still affect the pregnancy. [8]

Salmonella infection commonly causes diarrhea and cramps, sometimes with fever, nausea or vomiting. Stool may contain blood or mucus. The usual interval from infection to symptoms ranges from six hours to six days, with illness often lasting four to seven days. Some people have longer-lasting bowel symptoms, and infection can occasionally spread beyond the intestine. A familiar food exposure is not enough to diagnose Salmonella. Persistent illness, dehydration or blood in stool needs medical assessment, and new symptoms outside the gut should be mentioned rather than assumed to be part of routine recovery. [9]

Campylobacter is another bacterial cause, with diarrhea that may be bloody, fever and abdominal cramps. Symptoms often begin two to five days after exposure. Most people recover within about a week, but some develop later problems such as reactive joint symptoms or irritable bowel syndrome. Rarely, an immune complication called Guillain-Barré syndrome causes progressive weakness or tingling, often starting in the legs. New weakness following a diarrheal illness needs urgent medical attention. Do not interpret improving stool frequency as proof that an unrelated-looking neurological symptom can safely be ignored. [10]

Treatment depends on the particular bacterium, which is one reason leftover antibiotics are a poor response to undiagnosed diarrhea. With Shiga toxin-producing E. coli, antibiotics can increase the risk of hemolytic uremic syndrome, a serious complication. Medicines that slow diarrhea should also be avoided in that infection, and should not be used with bloody diarrhea or high fever without medical guidance. Most people with E. coli illness recover without antibiotics. Recognizing that a bacterial infection is possible does not establish that an antibiotic will help; an appropriate assessment comes first. [11]

Parasites and longer-lasting infection

Giardia can cause watery diarrhea, gas, cramps and greasy, unpleasant-smelling stools that may float. Symptoms usually start one to two weeks after infection and commonly persist for two to six weeks. They may disappear temporarily and then return. Weight loss or problems absorbing nutrients can occur, and illness may last longer in someone with weakened immunity. Giardia can spread through people, water, food and contaminated objects. Include exposures from earlier weeks when describing the history. A recurring course after apparent improvement should not automatically be labeled a new food intolerance. [12]

Cryptosporidium, often called Crypto, can produce frequent watery diarrhea lasting days to weeks, with cramps, nausea, weight loss or dehydration. Symptoms generally begin two to ten days after infection. People with healthy immune systems usually recover, although symptoms may fluctuate before resolving. In people with substantial immune suppression, infection can be prolonged and cause serious problems with nutrition and strength. Tell the clinician about transplant medicines, cancer treatment or another condition affecting immunity. Those details change the urgency and investigation; a typical recovery time for a healthy adult is not a safe timetable for everyone. [13]

Travelers’ diarrhea can occur in any destination, although unsafe food and water increase the risk. Prevention includes choosing freshly cooked hot food, using safe drinking water, and washing hands. If you become ill, replacing fluids matters more immediately than deciding whether a particular restaurant was responsible. Severe diarrhea after returning home warrants medical assessment, with the destination and travel dates included in the history. Antibiotics are reserved for situations where a clinician judges them useful. A medication supplied for a previous trip should not become an automatic treatment for every subsequent episode. [15]

Medicines, supplements and antibiotic-associated diarrhea

Review everything you take, including nonprescription products. Magnesium-containing antacids, laxatives, some cancer treatments, immune-suppressing medicines and several other drugs can cause diarrhea. Herbal “cleansing” teas may contain senna or another laxative, and supplements can also contribute. Note when each product was started and any dose or formulation changes. A medicine list is more useful than a general statement that nothing has changed, especially when occasional remedies are easy to forget. Bring packaging or a clear list to the appointment so the pharmacist or clinician can check ingredients and possible alternatives. [17]

Metformin is a familiar example: diarrhea, nausea and abdominal discomfort are recognized side effects. Symptoms may settle as the body adjusts, but troublesome or persistent effects deserve a discussion with the prescriber or pharmacist. An acute illness with vomiting, diarrhea or fever can also create a dehydration risk for someone taking metformin. Follow any personal sick-day instructions already provided and obtain advice if you are unsure. Do not independently experiment with stopping, restarting or changing prescribed treatment. The decision needs to account for both the bowel symptoms and the condition the medicine is treating. [16]

C. diff is a bacterium that can cause serious inflammation of the colon, most often during or soon after antibiotic use. Antibiotics can disrupt protective gut microbes, allowing C. diff to cause illness. Diarrhea may accompany fever, abdominal tenderness or loss of appetite. Recent hospitalization, older age and weakened immunity increase risk, but infection can occur without these factors. Contact a healthcare professional if diarrhea develops during or after antibiotics. They may arrange a stool test and review the original prescription. C. diff treatment uses specifically selected medicines; it is not a reason to self-treat with another antibiotic. [14]

Difficulty digesting food and ingredients

Lactose and food intolerance

Lactose intolerance occurs when the small intestine does not produce enough lactase to digest the sugar in milk. Undigested lactose reaches the colon, where it contributes to fluid and gas. Diarrhea may occur with bloating, cramps and rumbling after lactose-containing food. The amount matters, so someone may tolerate one portion but struggle with another. Lactose digestion can also worsen after an intestinal infection or injury, including from celiac disease. Secondary intolerance may improve when the underlying problem is treated. This mechanism differs from an immune allergy to milk protein. [18]

A clinician may investigate suspected lactose intolerance through the symptom history, a directed dietary trial or a hydrogen breath test. During testing, a measured lactose drink is followed by breath samples and symptom recording. Other conditions can resemble lactose intolerance, including IBS, inflammatory bowel disease and small intestinal bacterial overgrowth. One uncomfortable evening after dairy does not distinguish them. If symptoms persist despite a sensible trial, review the original assumption rather than progressively removing more foods. Our separate guide to diarrhea that repeatedly follows meals discusses that timing-focused question. [19]

For broader suspected food intolerance, a food-and-symptom diary and a supervised elimination followed by reintroduction can help answer a specific question. Commercial home intolerance panels are not a reliable substitute; some have limited evidence of accuracy and encourage unnecessary restrictions. A dietitian can help preserve nutrition, especially for children. Do not mistake sudden lip, tongue or throat swelling, breathing difficulty, or collapse for simple intolerance: these may indicate a serious allergic reaction and need emergency help. Avoid deliberate home re-exposure to a food suspected of causing that kind of reaction. [20]

IBS and changes in gut–brain interaction

Irritable bowel syndrome involves recurrent abdominal pain associated with changes in bowel movements. Diarrhea-predominant IBS can cause loose stools, while other patterns involve constipation or both. Bloating and a feeling of incomplete emptying are common. Gut–brain interactions influence bowel movement and sensitivity, and symptoms may fluctuate with several factors, including prior infection. This does not make the symptoms imaginary. IBS itself does not damage the digestive tract, but its effects on daily activities can be substantial. The pattern over time is more informative than whether one stressful day coincided with a loose stool. [21]

IBS is assessed through a characteristic history, examination and selected investigations when appropriate. Anemia, bleeding and unexplained weight loss point toward checking for other explanations. A family history of celiac disease, inflammatory bowel disease or colorectal cancer also matters. There is no need for everyone to undergo every possible test, yet reassurance should be grounded in the actual symptom pattern. Ask what supports the working diagnosis and which changes should trigger another review. Having a previous IBS diagnosis should not prevent reassessment when the bowel pattern or associated symptoms meaningfully change. [22]

Functional diarrhea is another gut–brain interaction disorder. It may be considered when chronic diarrhea is not explained by another cause after appropriate assessment. The distinction matters because repeated loose stools do not automatically establish IBS, whose characteristic pattern includes recurrent abdominal pain. [2]

Celiac disease and inflammatory bowel conditions

Celiac disease

In people with celiac disease, eating gluten sets off an immune reaction that injures the small-intestinal lining. Symptoms can include ongoing diarrhea, bulky or greasy stools, bloating and abdominal discomfort. Some people instead have constipation, fatigue or symptoms outside the digestive system. Children may have poor growth or weight gain because nutrients are not being absorbed properly. Symptoms can vary greatly, so an immediate reaction after every wheat-containing meal is not required for the condition to be considered. Report persistent tiredness, mouth symptoms or a relevant rash alongside bowel changes. A person with few digestive symptoms may still have an important underlying condition that merits testing. [23]

Crohn’s disease and ulcerative colitis

Crohn’s disease can cause diarrhea, cramping and weight loss, sometimes with fever, anemia or marked fatigue. Eye discomfort, joint pain and skin changes may occur too. The symptom pattern varies with the location and severity of inflammation, which helps explain why another person’s experience may look different. Researchers describe interacting immune, genetic, environmental and microbial factors rather than a single food as the explanation. Mention a family history and symptoms outside the bowel. A list confined to stool frequency may miss useful clues about the broader inflammatory process. [25]

Ulcerative colitis often causes diarrhea with blood, mucus or pus, abdominal cramps and an urgent need to pass stool. Some people feel a persistent urge even when the bowel is empty, called tenesmus. Symptoms may settle during remission and return during a flare, so a better week does not necessarily explain away a recurring problem. More severe illness may involve fever, fatigue, nausea and weight loss. New bloody diarrhea needs prompt medical assessment, whether or not inflammatory bowel disease has already been diagnosed. A symptom list cannot distinguish a flare from every other possible cause. [26]

Microscopic colitis

Microscopic colitis is an important cause of chronic watery diarrhea that usually is not bloody. Episodes can occur at night and may involve urgency, incontinence, fatigue or weight loss. Onset can be sudden or gradual, with periods of improvement followed by relapse. Some medicines are associated with increased risk, although taking one does not prove it caused the condition. Bring a full medication list rather than stopping a suspected drug yourself. The absence of blood is not enough to classify ongoing watery diarrhea as harmless, particularly when sleep or normal activities are repeatedly disrupted. [27]

Diagnosis of microscopic colitis requires examining small tissue samples from the colon under a microscope. The bowel lining often looks normal during colonoscopy, so appearance alone does not exclude it. A clinician may also use blood and stool tests to investigate conditions with overlapping symptoms. If colonoscopy is proposed for ongoing watery diarrhea, ask what it is intended to check and whether biopsies are relevant. This is an example of why a test’s details matter: “the colon looked normal” and “appropriate tissue samples were examined” answer different questions. Not every brief episode needs this procedure. [28]

Bile acids, pancreatic problems and previous surgery

Bile acid diarrhea

Bile acids help digest fat and are normally reabsorbed near the end of the small intestine. If excess bile acids reach the colon, they draw in water and can cause ongoing diarrhea and urgency. This may occur with disease or surgery affecting the terminal ileum, after gallbladder removal, or alongside some other digestive conditions. It can also occur without an obvious underlying cause. Tell the clinician about operations even if they were not recent. Bile acid diarrhea has specific management options, so recurring urgency should not simply be accepted as an unavoidable consequence of surgery. [39]

Pancreatic enzyme problems

The pancreas normally supplies enzymes that help break down fat, protein and carbohydrate. Exocrine pancreatic insufficiency, or EPI, means these enzymes cannot do that job adequately in the small intestine. Too few may be produced, too few may reach the intestine, or they may not mix with food effectively. Poor digestion can then lead to malabsorption and nutritional deficiencies. EPI often develops gradually and is more likely in people with significant pancreatic disease or certain operations. It is a specific medical condition, not a label that can be inferred from occasional bloating. [43]

Clues that may lead a clinician to consider EPI include loose, greasy, bad-smelling stools, weight loss, excess gas and abdominal discomfort. Relevant history includes pancreatitis, cystic fibrosis, pancreatic surgery or surgery on the upper digestive tract. These clues overlap with other causes of malabsorption, so stool appearance is a starting point rather than a diagnosis. Describe how weight and appetite have changed and any established pancreatic condition. That context helps determine whether an investigation of pancreatic function belongs in the plan instead of repeatedly treating diarrhea as an isolated symptom. [31]

Short bowel and dumping syndrome

Short bowel syndrome occurs when the small intestine is shortened or damaged enough that it cannot absorb sufficient nutrients to maintain health. It most often follows surgery removing part of the small bowel. Diarrhea may accompany fatigue and weight loss, with needs that extend beyond simple symptom control. Care can include individualized nutrition support, fluid and electrolyte replacement, medicines and sometimes further surgery. Tell new clinicians exactly which operation you had, if known. An established short-bowel care plan should guide food and fluid choices rather than a generic internet diet intended for a brief stomach infection. [30]

Dumping syndrome is different: stomach contents move too rapidly into the small intestine, most often after stomach or esophageal surgery. Early symptoms can appear within 30 minutes of eating and include diarrhea, cramping, fullness, sweating, lightheadedness or a racing heartbeat. Later symptoms, one to three hours after food, relate to low blood glucose and can include shakiness or weakness. Describe the timing of each symptom separately. Knowing that bowel urgency happens early but faintness comes later can help the surgical or digestive team assess the pattern and develop an appropriate management plan. [29]

Small intestinal bacterial overgrowth, commonly shortened to SIBO, means excess bacteria are present in the small intestine. It can occur when anatomy or bowel movement allows bacteria to accumulate, including after some operations or with certain diseases. Diarrhea, gas, bloating and discomfort may occur, sometimes with weight loss or fatty stools. Evaluation depends on the history and may include breath testing or other investigations. These symptoms are not specific enough to confirm SIBO at home. Treatment also needs to consider why overgrowth occurred, rather than relying on repeated unsupervised antibiotic courses. [44]

Other explanations a clinician may consider

Hormonal and nerve-related conditions

An overactive thyroid can speed up many body functions and cause more frequent bowel movements. Additional clues include weight loss despite appetite, heat intolerance, sweating, tremor or a rapid heartbeat. A change in bowel frequency does not by itself establish thyroid disease, and frequent formed stools are not the same as watery diarrhea. Mention the accompanying symptoms and any thyroid medicines or supplements. Diagnosis requires clinical assessment and appropriate tests. The useful question is whether the bowel change belongs to a broader body-wide pattern, rather than assuming every cause must be located inside the intestine. [32]

Diabetes-related autonomic neuropathy can affect the nerves controlling digestion. Possible symptoms include nighttime diarrhea, alternating diarrhea and constipation, and loss of bowel control. Other nerve-related symptoms may involve blood pressure, bladder function or sweating. Diabetes does not automatically explain a new bowel problem: clinicians may need to exclude other causes before attributing it to neuropathy. Discuss symptoms with the diabetes team and ask before using nonprescription digestive medicines. An organized description of timing and accompanying problems is more helpful than assuming the symptom is simply an inevitable part of living with diabetes. [45]

Overflow constipation and bowel cancer symptoms

Liquid stool can leak around a buildup of hard stool, a problem called fecal impaction. This can look like diarrhea even though constipation is central to the problem. Mention recent hard stools, straining, difficulty emptying or a longstanding constipation pattern. The treatment is different from simply slowing bowel movements, and professional assessment may be needed to decide how to clear the retained stool safely. Do not attempt manual removal yourself. Apparent diarrhea after a period of constipation is a reason to explain the full sequence, not to omit the earlier constipation as irrelevant. [34]

A persistent change in bowel habits can sometimes be a symptom of bowel cancer, particularly alongside blood, unexplained weight loss, abdominal symptoms or tiredness related to anemia. These symptoms also occur in many noncancerous conditions, so they do not mean that cancer is the likely diagnosis. They do mean the change should be assessed instead of repeatedly attributed to diet without review. Seek urgent help for black or dark-red stool or bloody diarrhea, and emergency help for heavy or continuous rectal bleeding. Early evaluation is useful even when the eventual explanation is less serious. [33]

How clinicians investigate ongoing diarrhea

An assessment begins with the duration, stool pattern, medicines, diet, travel and medical or family history, followed by examination. Testing is chosen to answer particular questions. Stool samples may look for infection, blood or other signs of disease; blood tests can investigate causes and complications. Breath tests or endoscopy may be appropriate for selected problems. The aim is not to order every test at once. Ask what the leading possibilities are, what each test can establish, and what the next step will be if the initial results do not explain continuing symptoms. [35]

What helps while the cause is being clarified

Replace fluid and electrolytes lost through diarrhea. Oral rehydration solutions are designed for this purpose, and a pharmacist or clinician can advise on an appropriate product. People with kidney disease, diabetes, weak immunity or other significant conditions should check what fluid and solution plan is suitable for them. Severe dehydration may require hospital treatment. Medicines that reduce stool frequency do not replace hydration or assessment. Do not self-treat bloody diarrhea or diarrhea with high fever using loperamide. Do not take it for severe diarrhea after antibiotics, an inflammatory bowel disease flare, constipation or a swollen abdomen. Obtain clinician-directed medicine advice for children and during pregnancy. [36,46]

For a short episode, prolonged fasting or a highly restricted diet is generally unnecessary. Resume your usual food as appetite allows, adjusting foods that clearly worsen symptoms. Alcohol, caffeine, very fatty foods, large amounts of simple sugars and sugar alcohols can aggravate diarrhea. Some adults temporarily struggle with lactose after an infection. Children should continue their usual age-appropriate food, and infants should continue breast milk or formula. Keep formula at its usual strength. Longer-term restriction should answer a clear clinical question and protect nutrition, rather than becoming an expanding list of foods to fear. [37,5]

Probiotics are not interchangeable products or a universal solution. Research findings differ by organism, formulation and clinical situation, and the best choice is often uncertain. Safety also matters: people who are seriously ill or immunocompromised may face greater risks, and severe infections have been reported in premature infants given probiotics. Discuss supplements with a healthcare professional, particularly for vulnerable people. A product marketed for “gut balance” should not delay assessment of bleeding, dehydration, weight loss or persistent diarrhea. Treating a confirmed cause and meeting fluid and nutritional needs remain central to a useful care plan. [40]

Reduce infection risk without overlooking noninfectious causes

When an infectious illness is possible, wash hands thoroughly with soap and water after the toilet and before food handling. Hand sanitizer alone does not work well against norovirus. Avoid preparing food for others or providing hands-on care while ill and for at least 48 hours after symptoms stop. Clean areas contaminated by stool or vomit promptly with a suitable disinfectant, following its label, and handle soiled laundry without shaking it. Continue good hygiene after recovery because virus can still be shed. Follow any stricter local workplace or public-health requirements. [38]

Food safety reduces another important route of infection. Keep raw meat and its juices separate from ready-to-eat foods, clean preparation surfaces, use a thermometer to check appropriate cooking temperatures, and refrigerate perishable food promptly. Wash produce and choose pasteurized products where appropriate. These habits reduce exposure; they cannot prevent medication side effects, celiac disease or other noninfectious explanations discussed here. If diarrhea repeatedly returns despite sensible precautions, shift attention from trying to make every meal “safer” to obtaining a structured evaluation of the recurring pattern and any associated symptoms. [41]