Be careful this Cinco de Mayo if you plan to enjoy a margarita.

Sipping on a margarita in the sunshine is a favorite summer pastime, especially during Cinco de Mayo celebrations.

But the health risk from these drinks goes beyond a gnarly hangover. Second-degree burns and itchy rashes can develop if a stray splash of lime juice lands on your skin.

It’s a condition called phytophotodermatitis, more commonly known as a “margarita burn.” The official name describes exactly why these burns occur.

“The term ‘phyto’ means plant, ‘photo’ refers to light, and ‘dermatitis’ is the inflammation of the skin,” said Dr. Keira Barr, a dual board-certified dermatologist and founder of Resilient Health Institute.

In other words, if lime juice drips on your skin when you’re whipping up a batch of margaritas at a summer barbecue and you stay out in the sun for a few hours, you might develop a nasty reaction that includes blisters, rashes, and burns. The reaction is worse on skin that’s wet or sweaty.

Margarita burns result when a chemical called furocoumarin reacts with sunlight. This chemical is found in limes and citrus fruits, along with celery, figs, fennel, and a number of other plants.

“This chemical can become activated by UVA rays. The furocoumarin is absorbed into the cells of the top layer of the skin, your epidermis, resulting in burning, redness, and blisters,” Barr said.

Margarita burns typically start as a rash that forms within 24 hours of exposure. The rash can then can grow into a cluster of painful blisters at the point of contact a day or two later. Once the swelling goes down, the blisters usually turn into dark patches or streaks (known as post-inflammatory pigmentation) that last for weeks or months.