Medications are chemical structures which can be sensitive to heat and/or cold, or to changes in light or humidity. Therefore, storage conditions are important.
For example, indinavir becomes less active when it is exposed to moisture in the air. It should always be kept in its original container with a desiccant (a product which helps to keep the indinavir dry).
Sometimes, the coating of the drug can be sensitive to temperature.
For example, if ritonavir capsules are left at room temperature, they will melt. Therefore, it is important to always keep ritonavir capsules in the refrigerator. There are special cold packs available if you are going to be somewhere where you are unable to store your ritonavir in a refrigerator. You can also ask about using ritonavir solution, which may be kept at room temperature for one month.