Everyday Health

7 Reasons You Might Have Diarrhea

Originally published April 9, 2021

Last updated April 26, 2024

Reading Time: 5 minutes

From viruses to lactose intolerance, there are a number of causes that might be behind this unpleasant symptom.

Most people have experienced diarrhea at some point in their lives. It’s not fun to be running to the bathroom, wondering what happened to upset your intestines so much. Worse yet, diarrhea can sometimes have serious complications or even be chronic.

Here are some of the most common causes of diarrhea and when you should see a doctor about your symptoms.

1. Bacteria and viruses

Food that is contaminated with bacteria, such as E. coli, salmonella and listeria, can give you food poisoning, which often causes diarrhea. Viruses, such as norovirus and rotavirus, can give you viral gastroenteritis, which also causes diarrhea. (You may call this the stomach flu, but it’s not really a flu at all.) Traveler’s diarrhea occurs when people travel to a country where these infections are more common.

How do bacteria and viruses lead to diarrhea?

“Infectious bacteria or viruses commonly colonize the lining of your intestines and release toxins,” explains Matthew Jung, MD, an internal medicine physician at Keck Medicine of USC and clinical associate professor of medicine at the Keck School of Medicine of USC. “These toxins cause the lining of your intestines to secrete an excessive amount of electrolytes and fluid, which results in diarrhea.”

Bacteria and viruses can also cause inflammation along the lining of your intestines, which also results in diarrhea, according to Dr. Jung.

“The diarrhea can sometimes be bloody,” he adds.

2. COVID-19

Although most people associate COVID-19 with fever and respiratory symptoms, it can also cause gastrointestinal disturbances. But what’s the link between diarrhea and COVID-19?

“Research indicates that the SARS-CoV-2 virus — the virus that causes COVID-19 — has an affinity for cell receptors that are present both in the lungs as well as parts of the gastrointestinal tract,” Dr. Jung says. “Inflammation in the lungs can produce the typical respiratory symptoms we have come to expect from a COVID-19 infection, and inflammation of the gastrointestinal tract is likely the cause of the diarrhea that some people with COVID-19 experience.”

3. Food allergies or intolerance

Both food allergies and food intolerances can cause diarrhea.

“Food allergies are the result of the immune system’s excessive response to normal food,” Dr. Jung says. “A food intolerance, on the other hand, is generally the result of a gastrointestinal error in the way that food is processed.”

Lactose intolerance is a common food intolerance.

“Normally, an enzyme called lactase breaks down the sugar lactose so that it can be absorbed by the small intestine,” Dr. Jung says. “But when there is a decreased production of lactase or when it is not working effectively, an excessive amount of lactose can enter the large intestine, where it gets broken down by the bacteria that lives there. Unfortunately, this bacteria breaks the lactose down in such a way that hydrogen gas is created, which results in bloating and diarrhea symptoms.”

An inability to absorb other carbohydrates can also cause diarrhea. These intolerances commonly include sugars like fructose, which is found in fruits or is added to juices and other sweet things, as well as sugar alcohols used in artificial sweeteners, such as sorbitol, mannitol and xylitol.

4. Autoimmune conditions

Crohn’s disease and ulcerative colitis are related autoimmune conditions that affect the gastrointestinal tract and can cause chronic diarrhea.

“Crohn’s disease and ulcerative colitis are inflammatory bowel diseases (IBD) that cause inflammation in certain parts of the gastrointestinal tract,” Dr. Jung says. “This inflammation can interfere with the absorption of nutrients and carbohydrates and can cause diarrhea.”

IBD requires management by a doctor.

Celiac disease is another autoimmune condition that can lead to chronic diarrhea. When someone with celiac disease consumes the protein gluten, it can trigger inflammation in the small intestine, eventually damaging it.

5. Irritable bowel syndrome (IBS)

IBS is a chronic condition that causes diarrhea, bloating and constipation.

“The exact mechanism by which IBS causes diarrhea is uncertain,” Dr. Jung says. “It’s considered a syndrome, which means it is a collection of symptoms that commonly occur together, instead of a particular disease with a specific cause.”

According to Dr. Jung, IBS likely results from several factors, and the cause of one person’s IBS may not be the same as another person’s. He notes that the factors can range from how well food moves through the gastrointestinal system to intestinal inflammation, infection or even genetics.

6. Medication side effects

Taking certain medications can sometimes result in unwanted side effects, like diarrhea. Antibiotics, for example, can disrupt the balance of bacteria in your intestines, which can cause diarrhea. Even more seriously, this imbalance can lead to a greater likelihood of an overload of the bacteria, Clostridioides difficile, or C. diff.

Antacids are another example of a medication that can cause diarrhea. Antacids containing magnesium, in particular, have a laxative effect.

If you start taking any medication and then have diarrhea, call your doctor.

7. Parasites

Parasitic infection in the intestines can lead to diarrhea — and worse, sometimes these organisms stick around for a while, leading to chronic symptoms. This is uncommon in developed countries, like the United States, but people can pick up this type of infection while traveling to places where they are more common.

Parasites that commonly cause diarrhea include Cryptosporidium, often called crypto, and Giardia duodenalis, or Giardia.

When to see a doctor

Most of the time, diarrhea from acute infections goes away on its own.

“The most important thing in these situations is to try to stay hydrated and monitor for worsening or persistence of symptoms,” Dr. Jung says. “For mild cases, beverages like fruit juices or sports drinks may be sufficient to keep you hydrated.”

However, if you experience any of the following with diarrhea, Dr. Jung recommends letting your doctor know:

  • Fever
  • Bloody or black diarrhea
  • Not being able to stay hydrated
  • 6 or more unformed stools within 24 hours
  • Severe abdominal pain
  • Vomiting
  • Symptoms that last longer than 7 days

He notes that it’s also best to contact your doctor if:

  • You are over 70 years old
  • You are pregnant
  • You have a significant medical condition
  • Your immune system is suppressed

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Tina Donvito
Tina Donvito is a freelance writer covering health, culture, travel and parenting.