Check the ingredients
When it comes to longevity, all ingredients are not created equally. "If your product contains fairly unstable compounds, like vitamin C or hyaluronic acid, use it up faster, because you need to also add the months that it has spent in the warehouse and in stores, plus oxygen that gets in after opening," says Teren. If any of these serums start to change color or smell funky, it's time to toss 'em.
Opt for oil
Skincare products can fall into either the "oil-based" or "water-based" category, and the former tends to be superior when it comes to sticking around for the long haul. "Oil-based products may potentially last longer because there is lesser risk of bacterial contamination," says Teren, who notes that you should try to use anything water-based as quickly as possible because that's where bacteria breeds.
Buy (and use!) to last
It's hard to pin down a firm "expiration date" for a particular product, because there are so many factors that come into play "If your product is packaged in an opaque jar, it can definitely last longer," says Julia Teren of Thesis Beauty. If you're using something in a pot or jar, use a stick or spoon (instead of your fingers) to scoop it out to avoid spreading bacteria. If you are dunking your filthy paws into a product regularly, it's best to use it up STAT.