Through their repeated back-and-forth between faces, fingers and brushes, cosmetic products can pick up bacteria and disintegrate over time - not exactly the pretty image associated with makeup.
In fact, you shouldn't keep any cosmetics around for more than a year, and most should be pitched well before that.
Although makeup doesn't come with expiration dates, there are well-established rules of thumb. Keeping a product past its prime puts you at risk for infection, said Dr. Shari Hicks-Graham, a board-certified dermatologist at Downtown Dermatology.
"The tempting thing to do is to buy something that we love and then to keep it because we love it. And we love to share it with our friends," Hicks-Graham said. "Those are the two worst things you can do."
The products to be most wary about, Hicks-Graham said, are those used around the eyes - open, exposed areas that are vulnerable to infection.
If a contaminated beauty product gets into the eye, it can cause anything from a red rash around the area to a more serious infection or dermatitis.
Hicks-Graham doesn't see too many cases in which makeup is to blame for an infection, but when a woman comes in with an eye irritation, she usually tells her to discard her eye makeup and start fresh.
"In a susceptible person, a person who has sensitive skin or a person who has eczema, it can be a very big hazard," she added.
Wash your hands before applying makeup, Hicks-Graham stressed, and don't share it with friends, no matter how tempting.
The FDA doesn't require beauty products to be marked with expiration dates, though. So what's a girl who wants to keep herself pretty and healthy to do?
Keep track of how long something should last by placing a small sticker or a piece of masking tape on each item when you open it, marking down its disposal date based on the list below.
Also, take note of the quantity of product in a tube, bottle or tray when you're shopping. If it's more than you think you would use in the allotted period of time, check for smaller - and hopefully cheaper - options.Disposal Dates
Some signs your makeup's gone bad are obvious: discoloration, the separation of ingredients, a change in texture or smell. If you notice any of these, discard the product immediately. Otherwise, consider these expiration dates suggested by former model and current L'Oreal spokeswoman Dayle Haddon in her book The Five Principles of Ageless Living.
Liquid foundation: 3-6 months
Cream foundation: 4-6 months
Concealer: 6-8 months
Powder: 1 year
Pressed powder, blush: 3 months
Eye shadow, mascara: 3 months
Lipstick, gloss: 1 year
Eye/lip pencil: 1 year
Facial cleanser: 6 months
Moisturizer: 6 months
Facial toner: 1 year