You wouldn't pour yourself a glass of milk from last week's expired carton. But what about that years-old pricey perfume you only spritz on for special occasions? 

Turns out, it doesn't keep forever. Like groceries, fragrances, moisturizers, and makeup only last so long -- and what you put on your skin is almost as important as what you eat.

The moment you open a new tube of mascara or bottle of lotion, the clock starts ticking, says Anne Marie Fine, an Arizona-based naturopathic physician and skin specialist. 

That goes for drugstore finds as well as high-end products, conventional as well as organic. Though you might save money by using items bit by bit over time, an old product can become ineffective or even unsafe. 

"An expired moisturizer, for example, might irritate your skin," says Fine. "But it could further harm you, especially if it contains certain ingredients that eventually break down into carcinogens."

Because we don't yet know all the effects of using potentially spoiled beauty goods on our skin and body, it's best to not take chances. With Fine's help, we've outlined the average shelf life of eight key products. To make things easier, put a Sharpie in your makeup bag. When you open a new item for the first time, write the date on the package -- so you'll never have to guess when its time is up.

What's PAO?
Have you noticed this logo on your products? It's the Period After Opening (PAO) icon, designed to help consumers keep tabs on freshness. The number tells how many months the item should last after you crack the seal. While mandatory in Europe on goods with a shelf life of 30 months or more, the icon's not yet required in the United States. Many domestic brands use it anyway.

What to Toss and When
Mascara, liquid liner
Lasts: Three to four months
Toss it sooner if: You get an infection, such as pink eye, or it dries out.

Insider info: Replace often. "Bacteria gets transferred easily from your eyes into the container via the brush," says Fine. If you do contract an eye infection, toss all eye makeup and start fresh after it has cleared up.

Powders: eye shadow, blush, face powder
Lasts: Up to two years

Toss it sooner if: It gets suspiciously crumbly or shiny.

Insider info: "You can literally see powder going bad," says Fine. "When it glistens or disintegrates, that's a sign the moisture has evaporated from the cakes."