Does the Flu Cause Diarrhea? Yes, Here’s Why It Happens

Having the flu is never easy. Flu symptoms are notoriously tough to deal with and they can linger for 7 to 10 days. You may already be familiar with common flu symptoms like fever, fatigue, stuffy nose, and cough. But if you've also been dealing with stomachaches, diarrhea, and nausea, you might be wondering if the flu is to blame or if you've somehow come down with another illness at the same time.

It turns out the flu can cause diarrhea. If you're experiencing diarrhea along with flu-like symptoms, here's what you should know about flu-related diarrhea and how to treat it.

Is diarrhea a symptom of the flu?

Yes, diarrhea can be a symptom of the flu. But it's not very common. Children are more likely to develop symptoms like diarrhea, nausea, and vomiting when they're sick with the influenza virus, which is the virus that causes the flu.

This trend has been reported in medical literature for decades. Children who get sick with influenza B - one of the major strains of influenza virus - are more likely to develop vomiting and diarrhea than those who get sick with influenza A, the other major strain of the influenza virus.

In studies, about 10% to 15% of children with influenza B develop diarrhea and other gastrointestinal symptoms. While children sick with influenza A can also develop diarrhea, it's less common.

In adults, diarrhea and other gut symptoms are generally uncommon with the flu. But it can happen, especially in adults who are sick with influenza B and certain strains of influenza A. For example, during the 2009 H1N1 pandemic, 3% of adults reported diarrhea and other gastrointestinal symptoms. And cases were high again during the 2014-2015 flu season when H3N2, a strain of influenza A, dominated flu season.

Overall, most adults are unlikely to get diarrhea with the flu. But this can vary from year to year depending on the strains circulating each year. Children are more likely to develop diarrhea with the flu, especially if they're sick from an infection with influenza B.

Why does the flu cause diarrhea?

It's not clear why the flu causes diarrhea. The influenza virus directly attacks your upper respiratory tract. Sometimes, it attacks your lower respiratory tract as well. This explains why flu symptoms often include respiratory symptoms like cough, congestion, and a stuffy nose.

But there's no clear evidence that the influenza virus also attacks your gastrointestinal tract. So, the exact reason for flu-related diarrhea remains a mystery. Scientists have put forth a couple of possible explanations:

• Cytokine release: Cytokines are proteins that send signals to your immune system and trigger your inflammatory response. Your body releases cytokines when you're sick. They're partially responsible for flu symptoms like headaches, fatigue, muscle aches, and fever. It's possible that this cytokine release also triggers diarrhea.

• Flu medications: Oseltamivir (Tamiflu) and baloxavir marboxil (Xofluza) are FDA-approved oral medications used to treat influenza infections. But they can also cause side effects, including diarrhea.

• COVID-19 co-infection: You can get sick with influenza and COVID at the same time. This co-infection is so common that people have dubbed it "flurona." While COVID and flu share similar symptoms, diarrhea is a common symptom of COVID. So, if you know you have the flu but also have diarrhea, it's worth taking a COVID test to make sure you don't also have COVID.

How long can influenza diarrhea last?

It depends. Most flu symptoms last between 2 and 5 days. But symptoms can last longer, especially if you have a weakened immune system. Flu-related diarrhea usually goes away along with your other flu symptoms. In some cases, it may clear up earlier.

How is the stomach flu different from the flu?

Influenza, also called "the flu," is an illness caused by the influenza virus. In contrast, the term "stomach flu" refers to gastroenteritis - an infection of the gastrointestinal tract. It can be caused by several types of viruses, bacteria, or parasites.

These two conditions are caused by different pathogens, spread in different ways, and need different treatments. Despite their similar names, they're very different medical conditions.

But the symptoms of the stomach flu (gastroenteritis) and the flu (influenza) can overlap. This makes it hard to know which one you have if you're suffering from diarrhea and cold-like symptoms. Both illnesses can cause:

• Headaches

• Fatigue

• Stomachache

• Nausea

• Vomiting

• Diarrhea

How do you treat flu-related diarrhea?

It's important to take action if you have flu-related diarrhea. Flu-related diarrhea can quickly lead to dehydration, especially in young children and older adults. While it can be difficult to completely stop flu-related diarrhea, here are three steps you can take to manage it at home.

1. Drink plenty of fluids

It's important to drink plenty of fluids to avoid dehydration. Extra fluids will replace the fluids your body is losing from diarrhea and fever. The best drinks for diarrhea include:

• Water

• Electrolyte drinks (like Pedialyte, Gatorade, or homemade oral rehydration solution)

• Broth

• No-pulp fruit juice

Avoid alcohol and caffeine, which can make you pee more often and increase your risk of dehydration. Sugary beverages and milk can sometimes worsen diarrhea, so it's best to avoid these until you're recovered.

2. Choose light foods

You don't have to follow a special diet when you have flu-related diarrhea. But avoiding certain foods may be easier on your stomach.

Try to avoid fried, fatty, and spicy foods. Some people find that bland foods are easier to eat when they feel sick and don't make their diarrhea worse.

You might try the BRAT diet for a few days. The BRAT diet focuses on bland, easy-to-digest foods such as:

• White bread

• Plain pasta

• White rice

• Plain crackers

• Broth

• Applesauce

• Cooked cereals

While these foods might not upset your stomach, they're also not high in fiber or vitamins. So make sure to go back to your usual, varied diet once you start feeling better.

3. Try over-the-counter medications

An over-the-counter (OTC) probiotic supplement may help when you have diarrhea. Probiotics contain live bacteria and yeast that may help keep your gut healthy. While there's no evidence that probiotics can make diarrhea go away faster, some people feel better after taking them.

You can also try taking loperamide (Imodium), an OTC medication that slows the movement of stool through your intestines. Loperamide isn't recommended for infants or children.

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