If you do, you may be exposing yourself to disease. Don't read on if you are eating breakfast - wait until later.
I have some rules about department store testers. I will use certain products, if applied with a clean applicator, and I know the artist and store's practices. However, I never use eye liner (inside lid), mascara, and lip product testers. They are breeding grounds for diseases that can enter through mucous membranes (eyes and lips) or broken skin. Testers can carry bacteria (e.g., staph, strep, and E. coli) and viruses that can cause eye infections, herpes cold sores, conjunctivitis (pink eye), ocular herpes (want a corneal transplant?), and even intestinal diseases. Why take that risk?
Test colors on your hand or wrist, choosing whatever patch of skin is closest in color to your face. The best place to test lipstick and lip gloss is right on the pad of your finger because the color is closer to your lips than your wrist or any other testing spot. I don't ever recommend that you put store testers right on your lips. Even if the sales rep tries to convince you that they have protected their testers with single-use tester wands or brushes, those same people go to lunch. What do they think happens when they are gone? Follow your own instincts. If that lipstick tester is available to the public, it has been exposed, and there is a risk associated with using it.
I have another rule of thumb: I won't buy drugstore lipsticks that aren't sealed. You just don't know who might have tested the product you are about to buy. Would you drink from a stranger's water bottle? Unlikely. There's no difference.
If you must try on eye and lip products in the store, try to minimize the risk. Ask the artist if they can open a new tester; make sure that brushes and lip products are cleaned with an alcohol sanitizing product after each use; and wash your face as soon as you get home.
Apply the same basic advice to makeup swaps. Not a good idea! Sorry for such a "downer," but I think it's important that makeup lovers weigh the risks against the rewards when shopping.
Photo courtesy of silentskydesign.com.