After a day on the shores of Marbella, Spain, Isabella Skeel-Gerhardt stirred from a sleepy haze to an undeniable sensation. Something wasn’t right. “I asked my friend to come in and look at me, and I will never forget the expression on her face. She was completely shocked,” the Copenhagen-based content creator recalls. “When I saw myself, I didn’t know whether to laugh or cry. My lips were getting bigger and bigger.”

For nearly three hours, Skeel-Gerhardt’s lips continued to expand. “It felt like they were about to explode,” she shares, “I was afraid my lips might fall off.” Although the doctors at the hospital were stumped, they landed on one potential theory: Perhaps those five hours in the hot summer sun had affected her lip filler.

Skeel-Gerehardt was given an injection in her posterior and a seven-to-10-day round of antibiotics that quelled the swelling. Still, the experience and the language barrier left her with more questions than answers. The most pressing of which was, how did this possibly happen now? After all, it had been nearly 20 months since she’d received 0.7 millimeters of filler in her lips, the first and only time her lips were injected.

Despite the years of anecdotal evidence about the sun’s effect on fillers, a quick look into the research revealed clashing medical opinions, scarce medical data, and few definitive answers. So, does sun exposure really affect filler?

First, let’s review what we already know.

Hyaluronic acid is a naturally occurring substance already found in the skin that keeps it plump and hydrated. However, given its track record, it’s no surprise that many popular fillers are just injectable hyaluronic acid, according to the American Board of Cosmetic Surgery. These hyaluronic acid dermal fillers are injected into the dermis and treat a range of concerns, including wrinkles, thinning lips, fine lines around the mouth, loss of volume under the eyes and on the back of hands, and nasolabial folds, among others. While outcomes and longevity vary based on injection location and patient characteristics, the American Society for Dermatologic Surgery reports that results generally last between six and 12 months.