ABSTRACT
Daylight has destructive effects on many natural and manufactured products in common use. The photochemical reactions these products undergo are complex and depend on the chemical and physical structure of the materials exposed to irradiation. Many drug substances are white in color, and hence their degradation depends mostly on the amount of ultraviolet (UV) radiation absorbed by the material. For colored products, radiation of any wavelength that is absorbed may be effective. Sometimes photochemical reactions are difficult to identify or prevent. The reactions take place from the electronic excited state of a molecule and lead in many cases to high-energy products, such as radical ions and free radicals. These may eventually react to form final, stable products through thermal reactions. In some cases, the photochemical degradation products may be the same products formed in the thermal reaction of the ground state (dark reaction); this similarity is not generally the rule but coincidental. Some drug substances or excipients have a sensitizing effect and may initiate reactions of products in which they are present.
