Loose, watery stool can be due to contaminated food, chronic diseases, or even the medications you take. Find out what could be causing your symptoms–and how to stay healthy.

By Amanda Gardner
Updated August 06, 2018

There are about 179 million cases of diarrhea in the U.S. every year, and there are many, many reasons why a bout of loose, watery stool might happen to you.

But most causes of diarrhea can be broken down into two main categories: those that are infectious and those that aren't.

Infectious causes of diarrhea include bacteria, viruses, and other microbes, which are often spread through contaminated food. Diarrhea caused by an infection usually comes on quickly and goes away in a few days, says Lawrence Schiller, MD, past president of the American College of Gastroenterology. “Most people’s immune systems will clear it quickly.”

Noninfectious causes of diarrhea range from side effects of medications to symptoms of a chronic disease. The good news is that regardless of the cause, most types of diarrhea can be treated or cured.

Here are some of the more frequent causes of this common, unpleasant malady–plus, how to protect yourself.

Food contamination

Food contamination is one of the main ways bacteria, viruses, and parasites make their way into your system. In addition to diarrhea, many people have vomiting and stomach cramps as well.

Bacteria including E. coli, Salmonella, Campylobacter, Shigella, and Listeria have traditionally lurked in eggs, raw and undercooked meats and shellfish, unpasteurized milk, and raw vegetables. They can also be found in foods that have not been refrigerated well enough.

But these diarrhea causes are increasingly being found in other foods as well. “Nowadays, if you look at food recalls, there’s a whole variety of things, even crackers,” says Dr. Schiller, who is also program director of the gastroenterology fellowship at Baylor College of Medicine in Houston. “It has to do with the industrialization of the food supply because a small amount of contamination can reach so many people.”

Make sure all produce is carefully washed before you eat it. Cook all meat, poultry, and fish thoroughly, and wash your hands and any surfaces that come into contact with food.

Viral infections

Viruses are another major culprit in causing diarrhea. Fortunately, these stomach bugs usually work their way out of your system in a few days. You can get infected through contaminated food or even from visiting farms or touching animals.

Norovirus is probably the most common of the diarrhea-causing viruses. There have been several high-profile outbreaks on cruise ships.

“It spreads quickly [from person to person], so you hear about it on ships a lot,” says Sean Drake, MD, a general internist at Henry Ford Health System in Sterling Heights, Michigan.

Norovirus can also be a risk in other tight quarters such as nursing homes, hospitals, schools, and prisons.

Avoid viruses by washing your hands often, practicing smart food hygiene, and not sharing food or drinks with someone who is sick.


Foodborne illness is common enough at home. It’s even more of a risk when you’re traveling, especially to the developing world. In fact, it’s so common abroad that it has earned various nicknames, among them traveler’s diarrhea (not to be confused with vacation constipation) and Montezuma’s revenge.

Not only is your body less used to microbes found in other countries, hygiene practices may also be different abroad.

“Traveler’s diarrhea is a very common mechanism for acute diarrhea,” says Dr. Schiller, adding that people usually are infected by microbes found in food or water. “Usually, the advice is to be careful: If you can’t peel it, don’t eat it. No tap water, and no ice.”

Other precautions to take include:

  • Make sure food is well cooked.
  • Always drink beverages, including water, beer, and wine, right out of their bottles.
  • Don’t brush your teeth with tap water, and keep your mouth closed when you shower.
  • If you do use tap water, make sure it’s boiled for at least five minutes.
  • Make sure milk is pasteurized.
  • Don’t eat food from street vendors.
  • Before you travel, ask your doctor about prophylactic antibiotics or other medications that may prevent you from getting diarrhea.


Ironically enough, some of the medications used to treat bacterial diarrhea–namely, antibiotics–can actually also cause diarrhea. The problem is that antibiotics don’t always distinguish between “bad” bacteria (which may be causing an infection) and “good” bacteria (which are vital to gut health).

By changing your gut flora, antibiotics can also set the stage for infection with the Clostridium difficile bacterium, which can cause more chronic diarrhea.

Certain blood pressure medications, cancer drugs, antacids that contain magnesium, and nonsteroidal anti-inflammatory drugs (NSAIDs) can also cause GI problems, including diarrhea.

So can the diabetes drug metformin, says Dr. Drake. “That’s why we usually start at a lower dose and go up to reduce that risk.”

Food intolerance

Food intolerance is different from food poisoning or foodborne illness, but it also can cause diarrhea. If you have a food intolerance, your body has trouble digesting certain foods. One of the most common forms of food intolerance is lactose intolerance, which is when you lack an enzyme necessary to process lactose, a sugar found in dairy products. If you drink milk or eat yogurt or cheese, you’ll likely end up with diarrhea.

“Lactose intolerance is less of an acute issue and more on the chronic side,” says Dr. Drake. “Usually people start to recognize a pattern. Every time they eat pizza or drink a glass of milk, they get all this crampy pain and diarrhea.”

You can also have an intolerance to fructose, a sugar in fruits and honey that’s sometimes added as a sweetener (look for high fructose corn syrup in the list of ingredients), and to sorbitol and mannitol, which are artificial sweeteners found in sugar-free products like gum and candy.

Some people get diarrhea from food allergies, although more common symptoms are swelling or itching, says Dr. Drake.

The best thing to do is avoid any foods that are causing you trouble.

Irritable bowel syndrome (IBS)

IBS is a gastrointestinal disorder that refers to a collection of symptoms often occurring together, including bloating, diarrhea, constipation, and general discomfort. Some people with IBS might have diarrhea more often, while others tend toward constipation–but more common still is to alternate between the two.

The causes of IBS remain mysterious but may be related to how the brain communicates with the gut and with changes in the balance of gut bacteria.

If you suspect IBS is causing your diarrhea, your doctor might suggest keeping a food diary and then tailoring your diet to avoid foods that trigger your symptoms. Certain medications can also help keep IBS at bay.

Celiac disease

Experts aren't entirely sure what causes this autoimmune disease, but symptoms flare when people with celiac eat gluten, a type of protein found in wheat, barley, and rye. Diarrhea is one of the possible symptoms. Others include bloating, fatigue, nausea, vomiting, constipation, gas, and weight loss. People with celiac disease have to follow a strict gluten-free diet; otherwise, celiac disease can lead to damage of the lining of the intestine, anemia, and difficulty absorbing all the nutrients you need.

Diarrhea and other celiac symptoms should go away when you adhere to a gluten-free diet. Luckily for the 1% of the population thought to have celiac disease, gluten-free foods are widely available.

Inflammatory bowel disease (IBD)

IBD is often confused with IBS, but it’s a different entity altogether. Actually, it’s two distinct entities, one being Crohn’s disease and the other ulcerative colitis. In both conditions, the digestive tract becomes inflamed, reducing its ability to absorb and deliver nutrients.

While Crohn’s can affect any part of the long GI tract, ulcerative colitis only targets the lining of the colon. Regardless, diarrhea is a symptom common to both. Experts aren’t sure what causes the two conditions, but it looks like genetics and the immune system are involved.

There’s no cure for either Crohn’s or ulcerative colitis, and no one surefire treatment for either. Instead, your doctor will likely ask you to try different diets and medications to find the right mix for you. In extreme cases, surgery may be called for.

Endocrine disorders

Endocrine disorders–those that mess with your hormones–are another common cause of diarrhea.

Diabetes is a big one. “Twenty to 25% of people with long-standing diabetes have chronic diarrhea, particularly if they don’t take good care of [their diabetes],” says Dr. Schiller.

An overactive thyroid (hyperthyroidism) is another common cause of diarrhea. “The thyroid gland sitting in the front of your neck helps control metabolism,” explains Dr. Drake. “[An overactive thyroid] can cause weight loss, tremors, and heart palpitations. It also stimulates the gastrointestinal tract to move too quickly.”

Addison’s disease (when your body doesn't produce enough cortisol and other hormones from your adrenal gland) can also cause chronic diarrhea.

The good news, says Dr. Drake, is that there are treatments for all of these conditions, and they should relieve not just the diarrhea, but other symptoms as well.

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Parasites are small organisms which, like viruses and bacteria, can infect food and water and end up in your system after you’ve eaten–or imbibed–the wrong thing. You’re more likely to get a parasite if you’re traveling. You may find them in raw or undercooked fish as well as underdone beef and pork (another reason not to eat raw foods abroad). Common infectious parasites are Cryptosporidium, Entamoeba histolytica, and Giardia lamblia.

Some infections cause worse symptoms than others, so always get medical help if you notice blood in your stool or if the diarrhea lasts a long time, regardless of what you think the cause might be, says Dr. Drake.