This came from another forum....they said it was a Good Housekeeping article... interesting...I never thought about some actives getting stronger as you use them.

“Some ingredients [such as vitamin C, retinol, and hydroquinone] degrade even more rapidly if they’re left in direct sunlight or exposed to air,” says Tina Alster, M.D., a Washington, D.C., dermatologist. Less frequently — but more alarmingly — certain products can actually become more potent over time, says Boston dermatologist Ranella Hirsch, M.D. The reason: Active ingredients like retinol and glycolic acid become more concentrated as their bases degrade, separate, or evaporate. And when proportions change, your skin may get irritated. To prevent problems, store cosmetics properly, discontinue use after six months, and look for products that come in a pump, which helps keep air out. Another option that’s starting to hit shelves: special jars that dispense creams through a tiny hole or slit when you press the top (an internal “floor” rises with each push).