Did you know makeup has a shelf life? It's true! It's not good to keep a makeup bag full of products you've had for years. Your makeup will actually expire after a certain length of time. And although makeup does contain preservatives similar to those in food, it's still possible for bacteria like Staphylococcus to grow and cause nasty infections. But don't worry! Keeping track of how long you've been using a product and being vigilant about hygiene and disinfection can prevent bacteria growth and prolong the shelf life of your favorite products.
Here are some general estimated guidelines about the shelf life of different products and when it's best to throw them out. They do vary from source to source, since the FDA doesn't require cosmetic manufacturers to provide actual expiration dates (the phrase "up to X years/months" is key here). And please, if your makeup changes color, consistency, or smell, this is a sign the preservatives have broken down and it should be discarded immediately.
Lipstick lasts the longest at anywhere from 2 - 4 years, according to one apparently lax source; another expert recommends throwing them out after 12 months. Regardless, storing them in the refrigerator prolongs the shelf life.
Eyeliner pencils, lip liners, and eyeshadows will last up to 3 years.
Powder, concealer, and nail polish are good for up to 12 months.
Mascara should be discarded after 4 months, always. If you're going to take just one bit of advice in this entry, let it be this one! Mascara is the absolute worst offender in terms of bacterial growth and since it's being applied in such close proximity to your eyes, using ancient mascara can be kind of dangerous. It's doubtful you'll go blind or anything, but the risk of infection definitely increases. And you should never "pump" your mascara wand in and out of the tube. The only thing this accomplishes is pushing air and bacteria into the tube, which dries out and contaminates your mascara.
Liquid foundation lasts up to 12 months. If the texture thins or thickens, or if the ingredients settle or separate, it's time to throw it out. Liquid foundation in a tube helps prevent bacteria from getting in; wide mouthed jars expose the foundation to air which, again, dries out the product and exposes it to more bacteria.
Worried that you won't be able to keep track of all these expirations? Fear not! A company called Beauty Alert sells handy stickers to label your cosmetics with the date you open them. Also printed on the stickers are different expiration times: 3, 6, 12, and 18+ months to remind you when to throw the old product out. Find them online at Beauty Alert. Photo below courtesy of Beauty Alert's website.