You should set ground rules for specific products that would be unhygienic to share, like products that you ingest (lipstick) or could cause an eye infection (mascara).
Of course, confronting your roommate can be awkward, especially if they're a close friend. "It's important to nicely talk about it with your roommate as soon as you notice," said Myka. "It's equally important not to come off [as] accusing or confrontational, and more just factual when you speak to them."
Myka advises you mind your tone and carefully explain your reasoning. Looking for a good excuse? Myka has some suggestions: Perhaps your beauty products are pricey, or they are tailored for your specific skin type. Or you only shop for products once a month and don't want to run out. "Whatever it is, you should explain in a polite way that there are some things you're willing to share, however XYZ beauty products you want to keep to being just used by you," said Myka.
It's best to be straightforward. "Say something along the lines of 'I got these special products that are just specific to my hair or skin, and so I wanted to point them out so nobody else uses them. These other ones are communal and I'm happy to share if you ever need to use.'"