Photostability is the process of exposing product to lighting conditions that simulate real-world environments. This can include direct sunlight, sunlight through a window, or fluorescent office lighting. The purpose of this testing is to demonstrate that unacceptable change to drug substances and products has not occurred. The testing can also be used to examine the lightfastness of inks used in packaging.
The importance of this stability study can be seen in the way products are stored. For example, pills are often kept in transparent or semi-transparent containers that may be exposed to sunlight when stored. Photostability can be used to assess the length of time these products can be exposed to direct or indirect sunlight and still be safe and effective. For pharmaceuticals and combination products, the primary guidance comes from ICH Q1B. ICH recommends multiple levels of test and check from raw materials to primary packaging to secondary packaging.
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