You'd never take an expired Rx, but do you check to see if makeup is past its prime? Formulas change as they age and may foster bacteria that could cause infection if applied over a cut. Here are the typical product life spans and when it's time to toss.

1. LIPSTICK

Lasts: Up to 2 years

Lipstick is one of the stablest cosmetics because it's oil-based (bacteria breed more easily in water). But after a year or two, natural oils like those from avocado, sunflower or safflower can become rancid. You won't have any negative health effects, but the product won't go on smooth and may taste strange.

Longer-lasting: Lipsticks without natural oils — YSL Rouge Volupt Shine ($34; yslbeautyus.com), for example — are more durable.

2. FOUNDATION

Lasts: 6 months to 1 year

You may see streakiness and changes in color or texture after 6 months, as the liquid formula breaks down and the pigment separates from the rest of the product (called demulsification). If you keep it longer (more than 2 years), germs could build up as the preservatives become less effective.

Longer lasting: Solid foundations, such as Revlon Nearly Naked Powder ($10; Revlon.com), won't demulsify.

3. SUNSCREEN

Lasts: 6 months to 1 year

Over time, sunscreen can destabilize and lose SPF power. Plus, the active ingredients may become less evenly distributed in the bottle, which means you'll get spottier coverage when you apply.

Longer lasting: Sprays, like Neutrogena Beach Defense ($10.50; neutrogena.com), have less contact with air, so they'll last longer than SPF in tubes.