Sealed, Dry, Dark, and Cool.
Oxygen is bad, water is bad, light is bad, and high temperature is bad.
Think of medicine like you think of milk, just with a different time frame.
Milk in the jug loses some of the vitamins due to the light compared to the cardboard 1/2 gallon size. Milk lasts longer in the refrigerator than on the countertop. If you put the cap back on, the milk lasts longer too.
And this is the most important: Milk is not good one day and bad the next (black or white), it goes through a continuum from good to bad, just like medicine, just like all other chemicals, just like food in storage, just like shoes as you wear them down, just like our bodies wear out.