"I'd swap used powder products if I know who they're coming from, because you can sanitize them. If you had a makeup artist do your makeup, she didn't buy the makeup to use on you only." - Darlene on TotalBeauty.com

Experts say: According to James Hammer, cosmetic chemist and founder of Mix Solutions in Uxbridge, Mass., these users are on the right track: Powder-based products, both loose and pressed -- think blushes, eyeshadows, compacts and finishing powders -- are low risk in the grand scheme of things because they're dry, which makes it hard for bacteria to multiply and spread (FYI, they prefer warm, moist environments). Plus, these products are usually applied to the cheeks and forehead, not infection-prone spots like the eyes and mouth.

But you're not entirely off the hook. Oils (and bacteria!) from your face, or your fellow swapper's face, are easily transferred to powder products, especially if you use one brush for multiple products, or are guilty of double dipping -- and who isn't? (You can't get a complete smoky eye in one swipe, after all.)

The verdict: Maybe. Powdery products are low risk, but proceed with caution -- bacteria may be present, and the risk goes up if you add water, such as with an eyeshadow or cake eyeliner, and apply near more sensitive areas like your waterline.

"I think swapping some things [like nail polish] could be a good idea, and it's actually cleaner than nail salon polishes! Swapping things like mascara and eyeliner however ... you must have a death wish." - Tammy on TotalBeauty.com

Experts say: NYC dermatologist Dr. Ariel Ostad says previously used and shared oil-based and water-based eye products rank as the most dangerous, hands down. The most common makeup-sharing related infection he sees? Conjunctivitis, a.k.a. pink eye. "Not only is it extremely contagious, but people don't always know they're brewing it," Ostad told me. (Suddenly, I'm giving the side eye to everyone within my 10-foot proximity.) Oh, and there's also blepharitis, the inflammation of the super-sensitive eyelid. But that could be the least of your worries when swapping eye products.

Not to get all "you're gonna go blind" panicky, but Dr. Edward Kondrot, the director of the Healing The Eye & Wellness Center in Dade City, Fla., says if even the smallest particle of infected makeup gets lodged inside your eye next the cornea, it can be "devastating." Devastating as in ... you're gonna go blind. It's rare, but it happens. "Pseudomonas, an infectious bacteria, is of most concern since it grows in a moist environment and can lead to a serious corneal ulcer," says Dr. Kondrot. How does a pesky case of pinkeye jump to losing your sight for good? "A closed eye acts as a bacterial incubator," says Kondrot. A comforting thought as you drift off to sleep tonight.

The verdict: No. These are high risk, avoid at all costs.

"I'd for sure swap nail polishes. I have at least 100 from my online China Glaze buying binges, half I've never even tried, and the others I've used like once!" - Tamara on TotalBeauty.com

Experts say: Although nail polish -- which is both liquid-y and constantly being dipped into -- has all the characteristics of a "do not share" product, Hammer says it gets the closest to a green light (which is a good thing, since every mani-pedi we get utilizes communal polish). "Nail polishes are made of traditional solvent-based formulations and are typically not a problem because the solvents are quite toxic to bacteria," Hammer says. Makes sense: Many humans can't stand to smell the harsh fumes for long -- think about trying to swim in a vat of "Ballet Slippers" all day.