Photolytic degradation is the degradation of photodegradable molecules caused by the absorption of photons, particularly those wavelengths found in sunlight, such as infrared radiation, visible light, and ultraviolet light. Some molecules get activated by the heat of light which increases the reaction rate.

Drugs which get chemically degraded in the presence of light are photosensitive or photo-labile drugs, e.g., riboflavin, tetracycline, and chlorpromazine. Colour fading or colour development are a type of photochemical degradation.

Some other examples of photochemical reactions are:

1. Ergosterol converts into vitamin D when exposed to UV. light, and
2. Chlorophyll absorbs visible light and undergoes photosynthesis forming carbohydrates and oxygen.

Photodegradation involves combined action of sunlight and air. It involves the process of oxidation (loss of electrons and increase in oxidation state) and hydrolysis (split of chemical bonds by adding water).

Furosemide, acetazolamide, and cyanocobalamin are the examples of some phototoxic drugs.

1. Packaging of drugs in amber coloured bottles.
2. Packaging of drugs in cardboard outers.
3. Packaging of drugs with aluminium foil overwraps.

Drugs can be stabilised against photolytic degradation as follows:

1. Temperature: The drugs should be stored at appropriate temperatures so that thermal acceleration of decomposition is avoided.
2. Light: Light sensitive drugs should be stored in amber coloured bottles.
3. Humidity: The drugs should be packed in glass and plastic materials to prevent the exposure of drugs to high humid conditions.
4. Oxygen: While packing the oxygen-sensitive drugs, the oxygen content of the solution should be kept less and a very little headspace should be left in the bottle above the products.