Active ingredients, photoinstable

Definition
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Active ingredients or excipients in extemporaneous preparations or finished medicinal products which change under the influence of UV radiation and may therefore be responsible for loss of active ingredient, reduction in quality and effectiveness and for the formation of phototoxic degradation products.

Classification
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Overview of photostable active ingredients and excipients used in external magisterial and finished medicinal products:

General information
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The photo-instability of active ingredients is a stability problem in magistral and finished drugs which should not be neglected. In principle, a distinction can be made between primary and secondary photodecomposition of active ingredients and excipients:

In primary photoreactions, active ingredient molecules absorb energy in the form of photons. The excited energy-rich molecules can release the absorbed energy by fluorescence, heat radiation, energy transfer or in the form of chemical energy by a chemical reaction, among other things, and thus influence the stability of a formulation.
In secondary or photosensitized photo-decompositions, non-active substance molecules, e.g. excipients or synthetic impurities, absorb energy and transfer it to drug molecules, which then decompose.