When Isabel Tan, a beauty and fashion influencer, spent a holiday weekend in Montauk, New York, she thought she only needed to worry about applying sunscreen. But in the week following the trip, she noticed a dark spot had appeared on the left side of her mouth.

At first, Tan brushed it off, thinking it was a strange suntan or melasma that would fade. But just two days later, the spot had darkened even more, and her skin felt dry, flakey, and even a little painful.

Tan rushed to the dermatologist, who “immediately asked what I drank over the weekend,” the content creator told Bon Appétit. She said she had drunk cocktails with lime. Her dermatologist informed her she had phytophotodermatitis, a skin condition resulting from exposure to both sunlight and an irritant found in limes. “The lime wedge on the rim of the cup must have been rubbing against my lip.”

Tan’s TikTok video about her experience has garnered over a million views since she posted it early this month. On the Today Show, the celebrity dermatologist Dr. Pimple Popper has also recently explained what phytophotodermatitis is. While the condition is usually not severe, it is common. At the same time, “most have never heard of it,” Tan notices from the commenters on her video.

Have you heard of phytophotodermatitis? Casually called “margarita burn,” the skin reaction looks like blisters or dark spots. Read on to learn what margarita burn is, what causes it, how to avoid it, and how to treat it if you get it.

What causes phytophotodermatitis?

Margarita burn is caused by the psoralens, or skin irritants, that can be found in limes, as well as other citrus fruits and celery.

Psoralens are irritants that make your skin more sensitive to UV light, enabling stronger and faster reaction to UV light. “A burn that would typically happen in half an hour can happen in minutes, or a burn that would happen in an hour could happen in fifteen minutes,” Dr. Lindsey Bordone, a dermatologist and professor at Columbia University, says.

The margarita burn doesn’t just come from margaritas. Handling guacamole, mojitos, or chicken marinades in the sun might result in the skin reaction as well. Bordone says some of the worst cases she sees come from homemade lime popsicles.

Phytophotodermatitis can also result from celery, and Bordone’s heard of people getting it from making Bloody Marys or restocking celery stalks at grocery stores. But, she says, 99% of the cases she sees come from limes. She’s personally never seen a case from lemons, although people have been known to suffer from lemon juice burns.

You don’t need to be outside to get it, either. “The other thing that people don’t realize is that it can happen next to a window,” Bordone says. You can be squeezing limes in sunlight in your kitchen and still get a “really nasty” phytophotodermatitis.

How dangerous is phytophotodermatitis?

Bordone sees multiple cases of phytophotodermatitis every week. Most cases, much like Tan’s, begin with mild irritation or pigment on the hands or mouth. But in more severe cases, the marks can blister and turn into second-degree burns. Bordone says she’s even seen cases in the shape of a handprint on someone’s arm that was grabbed by people who previously touched limes.