Like food in the fridge, skincare and makeup products expire. However, most of us don’t know how to tell if a product is past its prime, and we continue to use our favorites way longer than we should.

Although it may seem harmless to use an old beauty or skincare product, it can have negative health effects. When opened, makeup and other products are exposed to oxygen, which over time can encourage bacterial growth and contamination. This can lead to irritation and infections. A study that examined the makeup routines of women found 70% used expired eye makeup, 67% of which was contaminated with bacteria.

The U.S. Food and Drug Administration only requires formal expiration labels on products that are labeled as over-the-counter drugs, like sunscreen and anti-acne products. However, most beauty and skincare products carry a Period After Opening, or PAO, symbol. This tiny symbol printed on the back of a bottle is usually the letter M followed by a number, which indicates how many months after opening a product should be thrown away.

There are guidelines you can follow to tell if it’s time to toss a product. Eye makeup like mascara should be replaced every three months to avoid the risk of an eye infection. Foundations and concealers can last between six months to a year if using clean applicators. Cleansers and moisturizers often can last a year, but versions that include active ingredients like hyaluronic acid or retinol will lose potency over time.

In general, if a product has changed color or smell, or if the consistency has changed, it’s time to say goodbye. Use these guidelines to make sure you’re using your favorite products safely.