The intrinsic photostability characteristics of new drug substances and products
should be evaluated to demonstrate that, as appropriate, light exposure does not
result in unacceptable change. Normally, photostability testing is carried out on a
single batch of material selected as described under Selection of Batches in the Parent
Guideline. Under some circumstances these studies should be repeated if certain
variations and changes are made to the product (e.g., formulation, packaging).
Whether these studies should be repeated depends on the photostability
characteristics determined at the time of initial filing and the type of variation and/or
change made.