English drugs have side effects because they affect the body in more than one way. A medicines primary role is to address a certain concern, but when it does this it also can interfere with other areas of the body and hence cause reactions. These reactions, or side effects, are because every body is different, and the same drug acts differently on every individual. The drugs are tested by the drug companies and although the treatments are well evaluated sometimes side effects occur because the human body is complicated and responds in different ways. Largely, the chemical preservatives as used in the drug preparation act as very dangerous chemical substances to the body.