At first, Nancy Clanton thought it was nerves. Or maybe it was the oranges? Whatever the culprit of the bright red welts covering her body after running in a track meet, Clanton – then a junior high student in Long Beach, California – hoped the reaction was a fluke.

It wasn’t. The itchy, clustered bumps continued to sprout up regularly – after required high school swimming classes, for example, and while playing on the school’s tennis team. “I don’t remember being teased,” Clanton says, “but I do remember the questions and stares.”

Decades later, doctors identified one cause of Clanton’s hives: exercise. “I felt relief having this problem acknowledged, as I never believed it to be stress-induced,” says Clanton, now a 66-year-old retired teacher in Fountain Valley, California, who also has a similar reaction to cold temperatures.

[See: Pharmacist-Recommended Cough, Cold and Allergy Medicines.]

About 11 percent of the population is estimated to have cholinergic urticaria, or hives triggered by heat, exercise, sweat or all of the above, says Dr. Bruce Robinson, a dermatologist in New York City. The hives can also appear in reaction to a hot bath, sauna or even when eating hot or spicy foods. “It all goes hand in hand,” Robinson says. “It’s not just an exercise situation.”

In rarer cases, exercise can even trigger anaphylaxis, a potentially deadly allergic reaction. “Exercise is good,” says James Peterson, a sports medicine consultant and fellow of the American Academy of Sports Medicine, but “in this case, exercise can kill.”

Diagnosing Exercise ‘Allergies’

On its face, diagnosing cholinergic urticaria is simple: If you break out in tiny red “halos” when you exercise, sweat or otherwise heat up, you very likely have it, says Dr. John Wolf, a professor and chair of Baylor College of Medicine’s department of dermatology. If patients don’t bring in a photo of their exercise-induced hives, he’ll ask them to run up and down the stairs or do pushups – and watch what happens. Typically, the hives appear within 30 minutes of exercising and go away not long after patients stop and cool off. “It’s a fairly easy diagnosis,” says Wolf, noting that people with other medical conditions like asthma, hay fever and allergies to medications seem to be more prone to developing it.

[See: 8 Surprising Facts About Asthma and Seasonal Allergies.]

Diagnosing a type of exercise-induced anaphylaxis, called food-dependent exercise-induced anaphylaxis, is more complicated, since the reaction only occurs when someone eats a particular food and then exercises vigorously soon after, says Dr. Janna Tuck, an allergist in Cape Girardeau, Missouri, and spokeswoman for the American College of Allergy, Asthma and Immunology. Neither the food nor the exercise alone is a problem; an athlete, for example, could enjoy a granola bar on his rest day without a problem, but could eat that same granola bar before running and go into anaphylactic shock.

“You have to have a really good story” about when the reactions occur to get an accurate diagnosis, says Tuck, noting that some factors like alcohol, non-steroidal anti-inflammatory drugs like ibuprofen and, for women, being premenstrual can exacerbate the reactions. While wheat, shellfish, peanuts and corn are most commonly implicated in food-dependent exercise-induced anaphylaxis, there are also reports of it being caused by hummus, milk, meats and more.

In all cases of exercise “allergies,” many people never get an accurate diagnosis because they don’t seek medical help, perhaps because their symptoms aren’t consistent or they clear up soon after stopping exercise, Wolf says. As Tuck puts it: “People figure out if they just stop what they’re doing, they get better.” Still, she adds, it’s important for people with exercise-related anaphylactic symptoms to see a board-certified allergist, since some conditions can be fatal and treatments are available.

Prescription: Exercise (With) Caution

When Clanton got to college, doctors prescribed Benadryl for her breakouts, which they assumed were stress-related. But the drowsiness it caused was rarely worth it. “I recall a few times hiking in Redwoods in Northern California having to take it, but then ended up missing a hike the next day due to sleepiness,” she says. Instead, she gritted through the discomfort. She even completed a half marathon and climbed Half Dome, a challenging hike in Yosemite National Park, in her 40s. “I have never let my hives stop me,” Clanton says, although they have made her “quite miserable.”

It wasn’t until Clanton was around 50 that her doctor referred her to an allergist, who recommended she test out a half-dose of a newer, non-drowsy antihistamine. “[I] was quite delighted to have the first relief in years,” remembers Clanton, who now typically controls her hives by taking a pill like Zyrtec before bed.

Outside of antihistamines – which some people take preventively and others only in response to a reaction – clinicians recommend carrying an Epipen and always exercising with someone who knows how to use it, if you’re at risk of exercise-related anaphylactic shock. It can also help to exercise in a cool gym rather than in the summer heat, but “that alone won’t do it,” Wolf says, since your body temperature will still rise. Importantly, if you feel a reaction coming on (say, your skin begins to itch, burn or tingle), stop what you’re doing and cool off as quickly as possible, be it by standing in front of an air conditioner, using cold compresses to ease the hives’ discomfort or both. “You can’t tough this out and if you try to … it could be life-threatening,” Peterson says.

[See: 7 Signs You Should Stop Exercising Immediately.]

The rest of a treatment plan typically comes down to trial and error, says Tuck, adding that some people may be able to run a 5K but not a marathon, or horseback ride but not play tennis. “It’s not a black-or-white answer,” she says. That is, with one exception: Swearing off exercise is not an option, she and other experts agree. “Since exercising is so important for us … you don’t want them to not be able to exercise,” Wolf says. “What you have to try to do is work with them to help control the symptoms when they exercise.”