Can Allergies Cause Diarrhea?

You’ve got diarrhea. Again. And you’re wondering why. Is it something you ate, the start of a stomach flu or something more serious?

As you reach for yet another tissue for your runny nose, your thoughts turn to allergies. Is it normal to have diarrhea with allergies?

Allergist Katherine Weller, MD, explains which allergies can cause diarrhea, which can’t and when to get help.

Can seasonal allergies cause diarrhea?

Seasonal allergies don’t typically cause diarrhea. They usually only impact areas that are directly exposed to the trigger allergen, like your:

• Eyes
• Nose
• Throat
• Skin

But what about pollen-food allergy syndrome, also known as oral allergy syndrome? Could that cause diarrhea?

It’s possible, but pretty unlikely.

“Pollen-food allergy syndrome is a cross-reaction between food proteins and plant proteins,” Dr. Weller explains. If you’re allergic to birch pollen, for example, your body may react to the proteins in foods like apples because they resemble the proteins in birch pollen.

These reactions can include symptoms like mouth itching or lip swelling — but typically not diarrhea.

That’s because the proteins involved in pollen-food allergy syndrome often break down in your stomach before they can be absorbed into your system.

“It’s much more common to have local symptoms where the food touches, like on your skin, lips and tongue,” she notes. “It’s rare for pollen-food allergy syndrome to cause diarrhea or any other kind of full-body reaction.”

If you’re dealing with diarrhea at the same time that your seasonal allergies are hitting, it’s understandable to blame pollen or oral allergy syndrome. But if you don’t have any other symptoms, something else is probably going on.

“People with allergies and atopic diseases like asthma or eczema are a little bit more likely to have GI conditions like irritable bowel syndrome,” Dr. Weller says. That’s why your primary care provider may send you to a gastroenterologist, not an allergist, to narrow down the cause.

Can food allergies cause diarrhea?

Food allergies can cause diarrhea, but they rarely do so in isolation. If you have a food allergy — not an intolerance, sensitivity or GI disorder like celiac disease — you’re probably dealing with other symptoms, too, like:

• Hives
• Itching
• Swelling
• Nausea

“When diarrhea’s caused by a food allergy or an allergic-type condition, it affects your mast cells, your allergy cells, all over your body,” Dr. Weller explains. “So, we’d expect to see a full-body reaction.”

To understand why, it helps to know how allergies work.

At its core, an allergy is a case of mistaken identity. Your immune system thinks that cat hair, mango peel or ragweed pollen is a germ that can make you sick. So, it launches a full-on freakout. Your eyes get red and itchy, your nose starts running and you sneeze up a storm trying to get that non-existent germ out of your system.

Your gut sits on the sidelines for seasonal allergies, like mold or pollen, which typically affect your nose, throat, skin and eyes.

But if you eat food you’re allergic to, your gut will enter the allergic reaction fray. And your gut motility — the muscle contractions that move food through your digestive system — may change, leading to diarrhea.

When to see a doctor

Knowing how allergies and diarrhea are connected is important. But it doesn’t mean you know what to do about it. Here’s a quick guide to the type of care you need — and how quickly you may need it.

See your primary care provider for severe diarrhea that:

• Continues beyond two or three days
• Keeps coming back
• Has traces of blood in it

“Your primary care doctor will help tease out what’s going on and decide whether an allergist or a gastroenterologist is your best next step,” Dr. Weller says. “They can even start the process of figuring out a potential cause by getting blood and stool samples while you wait to see a specialist.”

Go to urgent or express care if your diarrhea is causing symptoms of dehydration, like dry mouth, or if you aren’t peeing as much as you usually would. A provider will check you to see if you need treatment, like IV fluids.

If there’s blood clearly visible in your toilet bowl, Dr. Weller says that’s another good reason to seek same-day care.

Call 911 or local emergency services if, in addition to diarrhea, you’re displaying symptoms of anaphylaxis, like trouble breathing, hives or swelling of your lips, tongue or throat. Anaphylaxis is a life-threatening allergic reaction that requires immediate treatment.

Dr. Weller also urges you to call an ambulance for diarrhea accompanied by:

• Signs of severe dehydration, like confusion, a racing heart or dark-colored pee
• Extreme abdominal pain with or without vomiting
• A fever over 102 degrees Fahrenheit (38.9 degrees Celsius) that isn’t getting better
• Large amounts of blood, be it bright red, brown or black

Allergies, dehydration, blood loss and severe infections can cause fainting. Do not try to drive yourself to the hospital.

Final thoughts

The relationship between allergies and diarrhea is complex. But the bottom line is simple: If allergies or diarrhea are impacting your quality of life, they deserve attention, whether they’re connected or not.