If your pills are left lying in the sun, they can become ineffective. Chemical changes, and more specifically oxidation or reduction reactions, may take place due to high temperatures. These reactions, which can happen fairly quickly, may cause dangerous side effects.
Do not leave your medicines in a car, in a beach bag or anywhere in the sun for long periods of time. Store your medications below 25 degrees Celsius, unless instructed to do otherwise. Also don't store your medication in the refrigerator, unless directed to do so. Only certain medications, like some types of probiotic supplements, insulin and antibiotics, should be stored at temperatures below room temperature, usually at 2-8 degrees Celsius.