Cosmetics oxidize after you have opened the jar or removed the seal on the dispensng container. They pick up germs and in the end- lose their effectiveness and COULD be the harbinger of infections. I advise all my patients who use my special VITAMIN C CREAM that they use the contents DAILY (so they get their money's worth) and discard the jar after six months because it's efficay will probably start to decrease at that point. Another example would be MASCARA which comes in contact with your eye, skin and lashes frequently. It's a geat place for germs to grow- sort of a culture.LIPSTICK, TONER,even SKIN COLORANTS like ROUGE or powder and liquid EYE SHADOWS are great place for bacteria to grow.
I know that they are expensive, but,applicators like make up brushes deserve to be changed. As frequently as you can afford to. Oh yes, NEVER, loan beauty poducts or their applicators to your friend in a powder room or anyplace else for that matter. You are sharing germs! How do you know what is on your companion's face? 

I know that many folks in the entertainment business read this blog locally and I want to warn of the dangers of using shared brushes and make up for stage and TV. Do like I do, I consulted with a cosmetologist regarding what I need for my TV show and I bought what she suggested and I keep it in its own place before filming.