If you down your vitamins with a little bit of tap water, chances are high you store them in your bathroom or kitchen. Unfortunately, a new study indicates that the watery elements of those two rooms make them the worst places to keep these pills. High humidity and high temperatures, often found in both the kitchen and bathroom, can degrade the potency and decrease the shelf life of certain supplements — even if the bottle caps are screwed on tight. Opening and closing the bottles allows humidity to creep inside, and that moisture can dissolve ingredients in the vitamins, decreasing their quality. The chemical changes are irreversible. It doesn't make your supplement unsafe, just less potent.
Check your vitamins and if you notice irregular brown spots, darkening, or signs of moisture in the container, you may want to replace them. And when you do, store your bottles away from the bathroom and kitchen. I keep mine on my desk at work. Where do you store yours?