As the first warm weather weekends slowly trickle in, you dig in your cabinet for last summer’s bottle of sunscreen to slather on any newly exposed skin. But wait. Does sunscreen expire? You bet it does. Here’s how to tell if your SPF is off.First, let’s go over some basics of a solid sunscreen routine. Number one: You’re wearing it year-round. Number two: You’re applying it everywhere — even your eyelids. Number three: You’re wearing a high SPF sunscreen for optimal protection. And finally, you’re using sunscreen that’s still fresh.”Sunscreen absolutely expires and should never be used past its expiration date,” Shari Marchbein, a board-certified dermatologist in New York City, tells Allure. “Like food, sunscreen can go bad and the ingredients can spoil, leading to a watery consistency,” she explains. “They also become less effective, which means a significant increase in the potential for sunburns, sun damage, brown spots, and the risk for skin cancer development.”Why you shouldn’t use expired sunscreen There are two main issues with expired sunscreen. One, it won’t protect you. “‘Expired’ means that the product should no longer be expected to achieve the SPF rating stated on the container,” Perry Romanowski, an independent cosmetic chemist, tells Allure. That goes for both chemical and mineral sunscreens.Chemical sunscreens that contain ingredients such as oxybenzone, avobenzone, and homosalate “can oxidize and become less effective,” Romanowski explains. Mineral sunscreens — those with zinc oxide or titanium dioxide on the label — don’t have that problem, but they do still degrade. “This would include emulsion separation, graininess, preservative breakdown, color, and odor changes,” Romanowski says. “So while the sunscreen ingredient still works, it may not spread properly on the skin to get the required film that gives the protection.”That leads us to the second problem with expired sunscreen. Even if the active SPF ingredient is still technically good, changes in the formula over time can make it both ineffective and problematic for skin. “Handling of the sunscreen container with dirty hands, or frequent opening and closing can expose the sunscreen to bacteria,” Erin Gilbert, a board-certified dermatologist in New York City, tells Allure. As the bacteria grows in the tube, it can cause breakouts, she says.When to replace your sunscreenMost sunscreens are designed to last three years, says Gilbert. So, the first thing to do is check for an expiration date on the container (though not all brands have one, Gilbert warns).