Photostability of drug substances - the more and more actual subject. The term "photostability" is used to describe how a compound responds to light exposure and includes not only degradation reactions but also other processes such as formation of radical, energy transfer and luminescence. It is well known that light can change the properties of different materials and products including drugs. Therefore photostability testing is an important aspect of any drug development project, and it is a requirement for manufacturing drug products. The information from such development studies is used to determine acceptable shelf life, proper storage conditions and suitable packaging. The rationale for evaluating drug photostability is that exposure to light can results in a loss of therapeutical activity of the product as well as light-induced side effects after administration to the patient by interaction with endogenous substances. The European Pharmacopoeia prescribes light protection for more then 250 medical drugs and adjuvants and the number of drugs susceptible to photolysis is steadily increasing.