If you’ve ever joked about being allergic to exercise or used the line “Yes, the treadmill/elliptical/stairclimber will literally kill me,” this might make you stop laughing: Exercise-induced anaphylaxis (EIA) is real, and it actually can be life-threatening.

Before you freak out (or rejoice, depending on how much you hate the gym), the chances of you having EIA are slim. The condition, which was first noted in a 1979 study, affects just 50 in every 100,000 people.Exercise-induced anaphylactic reaction to shellfish. Maulitz RM, Pratt DS, Schocket AL. The Journal of allergy and clinical immunology, 1979, Aug.;63(6):0091-6749. Epidemiology of anaphylaxis. Tang ML, Osborne N, Allen K. Current opinion in allergy and clinical immunology, 2010, Jan.;9(4):1473-6322.

Anaphylaxis is a severe allergic reaction that causes the allergy cells in your body to release a bunch of histamines, which typically help your body get rid of allergens. When someone goes into anaphylactic shock, their bloodstream is flooded with inflammatory cells that overwhelm the body, bringing on itching, nausea, and making it hard to breathe, explains Purvi Parikh, M.D., an allergist and immunologist with the Allergy & Asthma Network.

Exercise-induced anaphylaxis occurs when a specific allergy—most commonly to food—is combined with exercise. “With EIA, people find that within two to three hours of eating a certain food, they’ll develop an allergic reaction,” Parikh says. “It’s typically foods they could eat normally with no reaction, but working out with those foods in their system is what triggers the reaction.”

Unfortunately, the science behind this is murky; it’s a rare enough condition that not much research has been done. But, according to Parikh, one of the thoughts behind it is that “exercising increases blood flow, and increased blood flow can heighten an allergic reaction”—even one that hasn’t previously shown up.

The symptoms for EIA vary, according to Gerardo Miranda-Comas, M.D., assistant professor of rehabilitation medicine at the Icahn School of Medicine at Mount Sinai. “They could be as simple as flushing, fatigue, or itchiness, or more serious, like respiratory distress, throat closure, hypertension, or cardiovascular collapse,” he says.

Uh, but aren’t flushing and fatigue typical side effects from the gym? The difference between a tough workout and an allergic reaction like EIA comes with the combination of symptoms. “Over 90 percent of allergic reactions have some type of skin manifestation,” Parikh says. “So if you have anything on your skin—a rash, hives— plus involvement from another organ system—trouble breathing, severe abdominal cramps—that’s a tell-tale sign that this is an allergic reaction versus any other problem that may be going on.”

And EIA isn’t just likely to show up during super-intense workouts where you’re pushing your body to its limits. “Any intensity of fitness can bring on an episode,” Miranda-Comas says. “It’s more associated with aerobic exercise, more cardio, but even light stuff like strength training or yoga can trigger it.”

If you’re at the gym and experiencing a mix of symptoms, both doctors recommend stopping immediately—a reaction can get worse the more you push yourself, and some of these reactions can be life-threatening. “There aren’t really medications you can take prior to working out to prevent this,” Miranda-Comas says, so if you notice symptoms, head to a doctor ASAP.

“A board-certified allergist can identify which foods you’re sensitive to—wheat, celery, and shellfish are the most common, but EIA can happen with any food,” Parikh says. And a sports medicine doctor like Miranda-Comas can help reproduce the symptoms in a controlled environment (with medication on hand) to see if exercise is really at the root of the allergic reaction. “If someone does suffer from EIA, we’ll usually prescribe them an Epi-Pen, antihistamines, and an inhaler if needed,” Parikh says. “And once we know the allergy, we recommend they don’t eat anything with that ingredient three to four hours before exercising.”

So, while an exercise allergy is a real thing, it’s rare. And while it can be life-threatening, it shouldn’t stop you from going to the gym. “Seek health care and evaluation, and take care of it if needed,” Miranda-Comas says. “But don’t use it as an excuse just because you don’t want to exercise.”