Causes of Diarrhea: 18 Critical Triggers to Know

Diarrhea is one of the most common digestive symptoms, but it is not one disease.

It can happen after a short stomach infection, a new medicine, a food your gut cannot digest, or a long-term condition that affects how the intestine absorbs water and nutrients.

Understanding the causes of diarrhea starts with one simple question:

How long has it been happening?

Most acute cases last only a few days.

Diarrhea that continues for weeks is a unique problem and needs a different list of causes.

What Does Diarrhea Actually Mean?

Diarrhea usually means passing loose or watery stools three or more times in one day, or more often than is normal for you.

Other symptoms may include:

• Urgency
• Abdominal cramps
• Bloating
• Nausea
• Vomiting
• Fever
• Mucus
• Loss of bowel control

Blood, black stool, major pain, dehydration, or unexplained weight loss can point to a more serious problem.

For broader digestive nutrition guidance, visit the NextFitLife Gut Health and Digestion Hub.

Acute, Persistent, and Chronic Diarrhea

Duration gives doctors an important clue.

A three day stomach bug and three months of watery stool should not be investigated in the same way.

18 Critical Causes of Diarrhea

1. Viral Gastroenteritis

Viral gastroenteritis is one of the most common causes of sudden diarrhea.

Common viruses include:

• Norovirus
• Rotavirus
• Adenovirus
• Astrovirus

Symptoms may include watery stool, cramps, nausea, vomiting, and sometimes fever.

Many viral infections improve without antibiotics because antibiotics do not treat viruses.

The main short term concern is dehydration.

2. Bacterial Food Poisoning

Food poisoning can begin after contaminated food or drink.

Bacteria linked with diarrhea include:

• Salmonella
• Campylobacter
• Shigella
• Some forms of Escherichia coli
• Clostridium perfringens
• Staphylococcus aureus

Some infections cause mainly watery diarrhea.

Others can cause fever, severe cramps, mucus, or bloody stool.

Not every bacterial infection needs an antibiotic.

In some infections, the wrong antibiotic can cause harm.

3. Parasites and Traveler's Diarrhea

Contaminated food or water can carry parasites.

Giardia is especially important in persistent watery diarrhea.

Other travel related parasites can include:

• Cryptosporidium
• Cyclospora
• Entamoeba histolytica

Travel history matters because certain organisms become much more likely after exposure to unsafe food or water.

If diarrhea lasts more than two weeks after travel, parasitic infection deserves more attention.

For a practical travel example, see the NextFitLife Bali Belly Guide.

4. Clostridioides Difficile

C. difficile, often shortened to C. diff, is an important cause of diarrhea during or after antibiotic use.

Antibiotics can disturb the normal gut bacteria that usually help keep C. diff under control.

C. diff can then multiply and release toxins.

Symptoms may include:

• Watery diarrhea
• Abdominal pain
• Fever
• Nausea
• Loss of appetite

Not every case of diarrhea after an antibiotic is C. diff.

But new diarrhea during or after antibiotics should be discussed with a healthcare professional when it is significant or persistent.

5. Other Medicine Side Effects

Many medicines can cause diarrhea without causing an infection.

Examples include some:

• Antibiotics
• Antacids containing magnesium
• Cancer medicines
• Metformin
• Laxatives
• Some acid reducing medicines
• Liquid medicines containing sugar alcohols

Do not stop a prescribed medicine on your own.

Ask whether the timing of symptoms fits a medicine effect and whether another option is available.

6. Lactose Intolerance

Lactose intolerance happens when the small intestine does not make enough lactase to digest lactose well.

Lactose is the natural sugar in milk.

Symptoms can include:

• Diarrhea
• Gas
• Bloating
• Cramping
• Nausea

Lactose intolerance can also be temporary after a severe stomach infection because the intestinal lining may need time to recover.

This does not mean every person with diarrhea should permanently remove dairy.

7. Fructose, Sucrose, and Sugar Alcohol Intolerance

Some carbohydrates are poorly absorbed in certain people.

They stay in the intestine and draw water into the bowel.

This is one example of osmotic diarrhea.

Possible triggers include:

• Large amounts of fructose
• Sorbitol
• Mannitol
• Xylitol
• Some sugar free sweets
• Some liquid medicines

If diarrhea stops when the poorly absorbed substance is removed, that gives an important clue.

8. Food Allergy

A true food allergy is different from a food intolerance.

Food allergy involves the immune system.

Possible trigger foods include milk, egg, soy, wheat, seafood, and other foods.

Diarrhea may occur with vomiting, rash, swelling, breathing symptoms, or other allergic signs.

Difficulty breathing, throat swelling, or fainting after food is an emergency.

9. Irritable Bowel Syndrome With Diarrhea

IBS D is a disorder of gut brain interaction.

It can cause:

• Repeated abdominal pain
• Loose stool
• Urgency
• Bloating
• Changes in bowel frequency

IBS does not mean the symptoms are imaginary.

It means the gut and nervous system are not coordinating normally even when there is no destructive inflammation such as Crohn's disease.

Doctors usually consider warning signs and exclude important alternative causes before settling on IBS.

10. Functional Diarrhea

Functional diarrhea can cause chronic loose or watery stools without the repeated abdominal pain needed for an IBS diagnosis.

It is another disorder of gut brain interaction.

The diagnosis should not be used as a shortcut when a person has unexplained weight loss, blood, anemia, fever, nighttime symptoms, or another red flag.

11. Inflammatory Bowel Disease

Inflammatory bowel disease, or IBD, includes Crohn's disease and ulcerative colitis.

Unlike IBS, IBD causes real intestinal inflammation.

Possible symptoms include:

• Persistent diarrhea
• Blood in stool
• Abdominal pain
• Urgency
• Weight loss
• Fatigue
• Fever during active disease

Fecal calprotectin or lactoferrin can help doctors decide whether intestinal inflammation deserves further investigation.

12. Celiac Disease

Celiac disease is an immune reaction to gluten in people with the condition.

It can damage the lining of the small intestine and interfere with nutrient absorption.

Symptoms may include:

• Chronic diarrhea
• Greasy stool
• Bloating
• Weight loss
• Iron deficiency
• Fatigue

Do not start a strict gluten free diet before testing if celiac disease is being considered.

Removing gluten too early can make diagnostic testing less accurate.

13. Microscopic Colitis

Microscopic colitis causes chronic watery diarrhea.

The colon can look normal during colonoscopy.

The diagnosis is made by examining small tissue samples under a microscope.

This is one reason a normal looking colon does not always end the investigation of chronic watery diarrhea.

14. Bile Acid Diarrhea

Bile acid diarrhea happens when too much bile acid reaches the colon.

Bile acids can pull water into the bowel and speed stool movement.

It may happen after:

• Gallbladder removal
• Disease involving the end of the small intestine
• Certain intestinal surgeries
• Other problems with bile acid absorption

Some people labeled with IBS D may actually have bile acid diarrhea.

15. Small Intestinal Bacterial Overgrowth

Small intestinal bacterial overgrowth, or SIBO, means unusually high amounts or abnormal types of bacteria are present in the small intestine.

Possible symptoms include:

• Diarrhea
• Gas
• Bloating
• Abdominal discomfort
• Weight loss in severe cases

A hydrogen or methane breath test may be used in selected patients.

SIBO is not the answer to every case of bloating and diarrhea.

16. Pancreatic Insufficiency and Chronic Pancreatitis

The pancreas makes enzymes needed to digest food.

When the pancreas does not release enough enzymes, a condition called exocrine pancreatic insufficiency can develop.

The stool may become:

• Loose
• Greasy
• Oily
• Pale
• Foul smelling
• Difficult to flush

Weight loss and vitamin deficiency may occur too.

Chronic pancreatitis is one important cause.

For more detail, read the NextFitLife Pancreatitis Diet Guide.

17. Digestive Surgery and Dumping Syndrome

Some people develop chronic diarrhea after surgery involving the stomach, small intestine, colon, gallbladder, liver, or pancreas.

Food can move too quickly through the digestive tract after some stomach operations.

This is known as dumping syndrome.

Gallbladder removal can also lead to bile acid related diarrhea in some people.

Persistent diarrhea after surgery deserves a cause based review rather than indefinite use of over the counter medicine.

18. Endocrine, Metabolic, and Nerve Disorders

Some health conditions change how fast the intestine moves or how it handles fluid.

Examples include:

• Hyperthyroidism
• Diabetes related autonomic neuropathy
• Some adrenal disorders
• Rare hormone producing tumors

These causes are less common than infection or food intolerance, but they matter when diarrhea is chronic or unexplained.

For the diabetes connection, see Diabetes and Stomach Issues.

Why Does Diarrhea Become Watery?

Doctors sometimes classify diarrhea by what the intestine is doing.

Osmotic Diarrhea

Osmotic diarrhea happens when poorly absorbed substances pull water into the bowel.

Examples include lactose intolerance and excess sugar alcohols.

It often improves when the trigger is not eaten.

Secretory Diarrhea

Secretory diarrhea happens when the intestine releases more fluid or salt than it absorbs.

It may continue even when someone is not eating.

Bile acid diarrhea and some infections can act through secretory mechanisms.

Inflammatory Diarrhea

Inflammatory diarrhea occurs when the intestinal lining is inflamed or injured.

Possible causes include:

• IBD
• Some invasive bacterial infections
• Other inflammatory intestinal disease

Blood, fever, severe cramps, or mucus can raise concern for inflammatory disease.

Fatty or Malabsorptive Diarrhea

Malabsorption means the gut is not absorbing nutrients normally.

Greasy, bulky, foul smelling stool can occur with:

• Celiac disease
• Pancreatic insufficiency
• Some small bowel disease

The stool pattern can therefore give useful clues.

How Do Doctors Find the Cause of Diarrhea?

Doctors usually start with the story.

Useful questions include:

• When did it start?
• How many times a day?
• Is the stool watery, bloody, greasy, or black?
• Is there fever?
• Is there vomiting?
• Has there been recent travel?
• Did anyone else become sick?
• Were antibiotics used recently?
• Did a new medicine start?
• Is weight falling?
• Does diarrhea wake you from sleep?
• Does a certain food trigger it?

The answers can narrow a very long list quickly.

When Are Stool Tests Useful?

Most short, mild diarrhea does not need a stool test.

Testing becomes more useful when there is:

• Blood or mucus
• High fever
• Severe abdominal tenderness
• Signs of sepsis
• Persistent diarrhea
• Recent antibiotic exposure
• Immune suppression
• A suspected outbreak
• Relevant international travel

A stool sample may be tested for bacteria, parasites, C. difficile, or other pathogens depending on the clinical picture.

What Tests Are Used for Chronic Diarrhea?

When diarrhea lasts four weeks or longer, the workup changes.

Tests may include:

• Blood tests
• Celiac disease testing
• Stool tests
• Fecal calprotectin or lactoferrin
• Giardia testing
• Testing for bile acid diarrhea
• Hydrogen breath testing
• Colonoscopy
• Upper endoscopy

AGA guidance specifically recommends testing for Giardia and celiac disease in chronic watery diarrhea and suggests considering testing for bile acid diarrhea.

Should Everyone With Chronic Diarrhea Be Tested for Parasites?

No.

If there has been no travel or immigration from a region where parasitic infection is more likely, routine broad parasite testing is often unnecessary.

Giardia is different.

AGA specifically recommends Giardia testing in chronic watery diarrhea because it is common enough and treatable.

How Important Is Dehydration?

Dehydration is one of the most important short term dangers of diarrhea.

Every loose stool carries away water and electrolytes.

Warning signs include:

• Strong thirst
• Dry mouth
• Dark urine
• Urinating less than usual
• Dizziness
• Weakness
• Sunken eyes

Babies, young children, older adults, pregnant people, and people with weakened immune systems can become ill faster.

What Is Oral Rehydration Solution?

Oral rehydration solution, or ORS, contains water, glucose, and electrolytes in measured amounts.

The glucose helps the intestine absorb sodium and water.

ORS can be especially useful when diarrhea is frequent or dehydration risk is high.

During travel, use safe water to mix packaged ORS exactly as directed.

Very sweet soda or juice is not the same as a proper oral rehydration solution and can sometimes make osmotic diarrhea worse when consumed in large amounts.

Should You Fast When You Have Diarrhea?

Usually no.

When your appetite returns, current NIDDK guidance says most people can return to their normal diet even if some diarrhea continues.

Long periods without food are not required for most mild cases.

Choose foods you can tolerate.

Examples may include:

• Rice
• Potatoes
• Oatmeal
• Bread or toast
• Soup
• Bananas
• Eggs
• Chicken
• Yogurt if lactose is tolerated

For more day to day food ideas, use the NextFitLife Gut Health and Digestion Hub.

Foods and Drinks That Can Temporarily Make Diarrhea Worse

During acute diarrhea, some people do better when they temporarily limit:

• Alcohol
• Large amounts of caffeine
• Very fatty meals
• Large amounts of added sugar
• Sugar alcohols
• Large servings of fruit juice
• Lactose if temporary lactose intolerance develops

This does not mean these foods caused the original illness.

They may simply make symptoms harder to control while the intestine is irritated.

Are Probiotics a Proven Treatment for Every Case?

No.

Probiotic products differ by species, strain, dose, and quality.

NIDDK notes that researchers are still studying probiotics for diarrhea and that many professional groups do not believe the evidence supports routine use for all cases.

A generic “probiotic” label is not the same as evidence for a specific product and condition.

Can Loperamide Help?

Loperamide can reduce stool frequency and urgency in some adults with uncomplicated watery diarrhea.

But it is not right for every situation.

Do not use it as a simple default when you have:

• Bloody diarrhea
• High fever
• Suspected invasive infection
• Possible Shiga toxin producing E. coli

Children need separate medical advice before using antidiarrheal medicines.

Why Can Antibiotics Be the Wrong Treatment?

Most acute watery diarrhea is not automatically a bacterial infection that needs antibiotics.

Viruses do not respond to antibiotics.

Unnecessary antibiotics can cause side effects, promote resistance, and increase C. difficile risk.

There is another important exception.

Antibiotics are generally avoided in suspected or confirmed Shiga toxin producing E. coli because treatment can increase the risk of hemolytic uremic syndrome.

This is one reason bloody diarrhea should not be treated blindly at home with leftover antibiotics.

What About Traveler's Diarrhea?

Most mild traveler's diarrhea can be managed with fluids and symptom care.

The CDC Yellow Book 2026 advises that antibiotics are generally not recommended for mild traveler's diarrhea.

They may be considered for more severe illness.

All dysentery, meaning blood in the stool from an invasive infection, is treated as severe traveler's diarrhea.

Persistent diarrhea after travel raises more concern for parasites such as Giardia.

Can COVID Cause Diarrhea?

Yes.

COVID can include diarrhea, nausea, vomiting, and abdominal discomfort in some people.

But diarrhea during COVID is not automatically caused by COVID itself.

Food poisoning, another virus, antibiotics, medicine effects, IBS, and other causes can happen at the same time.

For a dedicated guide, read How Long Does Diarrhea Last With COVID?.

Can Stress Cause Diarrhea?

Stress can change gut movement and worsen urgency in some people.

It is especially relevant in disorders of gut brain interaction such as IBS.

But stress should not be used as an easy explanation for new bloody diarrhea, fever, weight loss, dehydration, or persistent nighttime symptoms.

Those signs deserve a medical review.

What If Diarrhea Comes With Gas and Loud Stomach Sounds?

Gas and gurgling often occur when fluid, food, and air move through the intestines quickly.

Possible causes include infection, food intolerance, IBS, large meals, sugar alcohols, medicine effects, and other bowel changes.

Read the NextFitLife Stomach Pain, Gurgling, Gas, and Diarrhea Guide for a focused symptom breakdown.

When Should an Adult See a Doctor?

NIDDK recommends prompt medical advice when adults have diarrhea with concerning features such as:

• Diarrhea lasting more than 2 days
• Six or more loose stools in a day
• High fever
• Frequent vomiting
• Severe abdominal or rectal pain
• Blood or pus in the stool
• Black tar like stool
• Signs of dehydration
• Mental status changes

Pregnant people, adults over age 65, people taking antibiotics, and people with weakened immune systems may need help sooner.

When Is Diarrhea an Emergency?

Seek urgent medical care if diarrhea occurs with:

• Severe dehydration
• Fainting
• New confusion
• Severe constant abdominal pain
• Large amounts of blood in stool
• Black tar like stool
• Repeated vomiting with inability to keep fluids down
• Signs of shock
• Severe weakness

Infants and very young children can dehydrate much faster than healthy adults.

Key Principles for Understanding Diarrhea

Duration, possible causes, hydration and warning signs guide appropriate care.

• Diarrhea should be considered by duration. Acute and chronic diarrhea have very different cause lists.
• Infection is not the only explanation. IBS, IBD, celiac disease, microscopic colitis, bile acid diarrhea, pancreatic disease, and medicines all matter.
• Fasting is not routine treatment. Most people can return to their normal diet when appetite returns.
• Antibiotics are not automatic treatment. Most acute watery diarrhea does not need them.
• Some E. coli infections should not be treated with antibiotics. Shiga toxin producing strains are an important exception because of HUS risk.
• Probiotics should not be presented as a universal cure. Evidence depends on the strain, product, and condition.
• Loperamide is not appropriate for every case. Blood, fever, and certain infections change the safety picture.
• Chronic watery diarrhea deserves targeted testing. Giardia, celiac disease, inflammatory markers, and bile acid diarrhea may need evaluation.
• Greasy diarrhea points toward malabsorption. Celiac disease and pancreatic insufficiency belong on the differential diagnosis.
• Hydration is the first priority. Severe fluid and electrolyte loss can become more dangerous than the diarrhea itself.

Conclusion

The causes of diarrhea range from a short viral illness to food poisoning, medication effects, food intolerance, IBS, inflammatory disease, celiac disease, pancreatic problems, and other chronic conditions.

Start with the timeline. A few days of mild watery stool is very different from diarrhea that lasts four weeks, wakes you at night, causes weight loss, or contains blood. Replace lost fluids, eat when you can, and seek medical care when warning signs appear or the problem does not improve.

References and Sources

The medical information in this article was checked against current digestive health, infectious disease, and public health guidance. Full URLs are included below for direct review.