If you have a habit of storing the pills in the bathroom, put a silica gel sachet with it. You can add a silica gel sachet to store the vitamins, tablets, and other medications in your bathroom to keep them dry.
The bathroom environment is highly humid, so the medications get exposed to high quantities of moisture every time you open the bottle for use. These pills start to dissolve prematurely when coming in contact with moisture. Adding a silica gel packet will absorb the moisture and prevent the damage.