Each time you open and close a vitamin bottle in the bathroom you add at bit of humidity and moisture. Right after a shower, the humidity in the room could be close to 100%. This high moisture content degrades the effectiveness of vitamin supplements. You can lose almost all the vitamin C in a week. Once the chemical change has occurred, it can’t be reversed.
Store vitamin supplements in a cool, dry place. Bathrooms and kitchens have the most moisture. Your bedroom or desk drawer is a drier spot.