When it comes to the leftovers in your fridge, there’s usually no doubt when they’ve spoiled and need to be thrown out. It’s not always so obvious with sunscreen.

“Sunscreen does expire and, when it does, the chemicals that block the sun deteriorate and break down over time,” said Matthew E. Joseph, DO, a family medicine physician at Geisinger. “If your sunscreen is expired, it won’t matter how much you apply, it won’t protect your skin from a sunburn or cellular damage from the sun’s ultraviolet rays.”

The best way to tell if your sunscreen is still effective is by checking the container for an expiration date. The Food and Drug Administration (FDA) requires that all sunscreens be effective for at least three years – if the expiration date on the bottle indicates your sunscreen isn’t effective anymore, throw it out.

If you buy a new bottle of sunscreen and it doesn’t have an expiration date on the packaging, write the date you purchased it on the bottle.

“If a bottle from last summer doesn’t have an expiration date on it, err on the side of caution and buy a new bottle,” Dr. Joseph said.

The expiration date isn’t the only thing that could indicate your sunscreen won’t do its job anymore.

“If a bottle of last summer’s sunscreen has been sitting in your car’s glove compartment or another hot place, toss it. Sunscreen’s active ingredients can break down in the heat,” Dr. Joseph said.

If your leftover sunscreen has obvious changes in color, consistency and smell, even if it hasn’t expired, throw it out.

However, you really shouldn’t have leftover sunscreen lying around anyway.

“If you use sunscreen frequently and generously, one bottle shouldn’t last from one summer to the next,” Dr. Joseph said. One application should be 1 ounce, which is the size of a shot glass – that’s how much you generally need to cover all exposed skin.

“That means that if you have a 4-ounce bottle of sunscreen, you’ll use a fourth of it in just one application,” Dr. Joseph said. In fact, an 8-ounce bottle should barely keep you covered through a long summer weekend spent outdoors.

If you need to buy new sunscreen for your summertime outdoor activities, consider getting a broad spectrum lotion with an SPF of at least 30.

“Broad spectrum sunscreen means that it protects your skin from all of the sun’s UV rays,” Dr. Joseph said. “Apply it about 15 to 30 minutes before going outside and make sure to reapply every two hours, or more frequently if you’re swimming or sweating.”