We're probably all guilty of it: holding on to a makeup bag full of products of uncertain vintage. But how bad is it, really, to use old or expired cosmetics?

Pretty bad, actually.

“At best, the product may smear or not give even coverage. At worst, you could give yourself an eye infection, or bacterial infection of the skin, such as staph,” says dermatologist Dr Ava Shamban. Yikes!

We spoke with makeup artists, dermatologists, and a cosmetic chemist (yup, that’s a thing!) to find out what else might happen to your skin if you use expired makeup. Plus, how to tell when it's time to chuck a product and how to organise your makeup stash so you can actually keep track of how long you've been using each thing.

How long does makeup last?

All cosmetics come with an expiration date (check the packaging or the bottom of the product) that indicates their shelf life. But these expiration dates are really just for unopened products, says David Pollock, a beauty chemist.

“Consider the date of when you open and start using a product the countdown for when you should toss it,” advises cosmetics brand founder Julie Fredrickson.

The type of product, and how it's applied, both help determine how long it can last. “Balms or lip gloss with a dipable wand should be replaced every three months. If you don’t have an active cold or cold sore during use, lipsticks can be used to completion,” says dermatologist and cosmetic surgeon Dr Melanie Palm. Any eye products - mascaras, eyeliners and eyeshadows - should be disposed of after three months of use, she adds. (Pro tip: Snap a photo on your phone the first time you use a product so you can refer back to the image as time goes by and remind yourself how long you’ve been using it. You can even set a reminder on your calendar to toss a product on a certain date.)

Other makeup can last a bit longer. “Foundation, especially if it is in a pump bottle, can last up to two years,” says Shamban. Face powders, blush and other products applied with a brush should be discarded after 6 months, says Shamban, even if you’re diligently washing the brushes. After around 12 months, Fredrickson recommends putting BB creams, concealers, and foundation out to pasture. That’s around when the ingredients break down and separate in the packaging. “At that point, you don’t want to attempt to mix it back up - it isn’t salad dressing, after all!” she says.

A product's consistency is one way to know when a product is on its way out. The scent is another. "Old mascara has an unpleasant fishy odour," says makeup artist Joyce Connor. And, in general, anything that smells rancid or sour should not be kept. "If [a foundation or concealer] is malodourous, dispose of it immediately," Palm says.

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