THE REASON With proper storage, expect polish to last up to one year. "Start examining it after nine months for color fading or separation," says cosmetic chemist Ni'Kita Wilson. "That's when you can tell that it won't go on as smoothly."
THE REASON When mascara dries out or gets clumpy, it becomes a breeding ground for bacteria, says optometrist Jennifer Giermek of N.Y.C.'s Naked Eye Optical. Discard immediately if you get an eye infection. "Reusing the brush can cause recurrences," she says.
THE REASON Sunscreen is often labeled with an expiration date, but feel free to slather it on up to a year after opening, as long as it hasn't "separated, changed in smell, or become discolored," says Dr. Graf. "Trust your eyes and nose."
THE REASON "If the cream is in a jar, your fingers can introduce bacteria, so toss it after six to nine months, 12 months if it's in a pump," says Wilson. "Be sure to keep alpha-hydroxy creams tightly sealed or they can get more concentrated and cause irritation."