At some point, almost everyone who wears makeup will use an expired product. Even if you aim to be diligent, makeup expiration dates aren’t always obvious. And since there’s little regulation in this area, it can be hard to know when your favorite mascara has become a playground for bacteria. Although cosmetics in the US are not required by law to have expiration dates, most makeup products do carry them. You can usually find a simple date printed on the original packaging or on the bottom of the bottle. You might also see a small icon that looks like a makeup pot with the lid tipped up. Inside the icon, there will be a number and then an “M,” which stands for months. If you see “6M,” that means the product is set to expire six months after it’s opened. This is always going to be the easiest, most straightforward way of knowing whether it’s time to toss your lip gloss.The risk level for a product depends on its texture and where it’s applied, said Amreen Bashir, a lecturer in biomedical science at Aston University who has studied bacterial contamination across makeup categories.And because makeup often expires after it’s opened, rather than after the date of purchase, it is also important to know when you actually opened something. To stay vigilant, get in the habit of using a permanent marker to write “opened on” dates on tubes and bottles. If you’d rather not mar the aesthetic of your pretty bottles, jot the dates on a small sticky note tucked inside your makeup drawer, or make a dedicated list in your phone to track the various lifespans of your products.