Finding an excuse to skip the gym just got easier. It turns out you can actually be allergic to exercise.

According to researchers, exercise-induced anaphylaxis is when a reaction to an allergen only happens when you are working out. However, it’s a rare condition. The anaphylaxis, which was first described in 1979, affects 50 in every 100,000 people, according to Popular Science.

For most people who struggle with the condition, the reaction occurs when exercise and a certain type of food (or in some cases medication) are combined. In other instances, women on their menstrual cycle have been affected due to higher levels of estrogen, which binds to the cells involved in the allergic reaction. However, little is known about what actually triggers the reaction.

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“There are a variety of things that it might be,” allergist Maria Castells told the science publication. “And for a proportion, it’s nothing, really, just the exercise itself.”

There are several theories about this crazy connection between exercise and an allergic reaction. One working theory is that the increased blood flow caused by working out might push immune cells around the body. Another is that proteins in your gut may change the way they interact with the foods in your stomach during exercise, causing the allergic reaction.

And it can happen outside the gym, as almost all types of exercise, including dancing and biking, have caused the anaphylaxis. Luckily, once diagnosed, the condition is treated the same way you would any other allergy — by avoiding the allergen and carrying an EpiPen.

So next time you friend invites you to the gym, don’t feel guilty about sitting it out. The last time we checked, the couch wasn’t blamed for an allergic reaction.