All sunscreens have an expiration date printed on the bottle. Typically they last a year or two, and most are good for three years. The safest bet, naturally, is to ditch old product and replace it. But in a pinch, here’s a secret: Sunscreen is still good for six months beyond its expiration date, says scientist Karen Burke, M.D., a dermatologist and research scientist in private practice in New York and on the dermatology faculty at Mt. Sinai Medical Center. In that period, it’s better than nothing, but after that, it’s time to suck it up and buy a new bottle.
Every sunscreen formula is different, which makes it impossible to generalize how long it’ll take for them to break down. Plus, other factors contribute to the degradation of the product, which is somewhat unstable by nature (especially when you’re talking about chemical, or organic, sunscreens versus physical blockers). UV light and heat are top culprits, which is why you shouldn’t store your sunscreen in the car or bring every bottle you own to the beach at once.
Sunscreens are tested in the particular bottles in which their sold, because the formulas can react with the material the bottle is made of, altering the chemistry. And over time, the plastic bottle itself can degrade, meaning that it's possible when you use old sunscreen you might actually be spreading plastic particles onto your skin. If you squeeze some out and it seems to have separated or looks or smells funny, play it safe and toss the whole bottle.
The truth is, expiration shouldn’t be an issue if you’re using the amount of sunscreen required to ensure protection. You should apply one ounce at a time to cover all your exposed parts. And, you should be reapplying every hour and a half that you're outside. That means your sunscreen really shouldn’t last from year to year anyway.