By Wu Zhang, Year 12
Medicines have saved countless lives and are constantly innovated to continue to save and improve human health, yet many are subject to the same problem: they are highly sensitive to light. Exposure to sunlight or artificial light (including UV and visible light) can cause certain drugs to degrade, reducing their effectiveness and sometimes forming harmful by-products. This process, known as photodegradation, is a major concern in pharmaceutical chemistry because it directly affects patient safety, drug efficacy, and the shelf life of medicines. For this reason, understanding photostability is essential in ensuring that drugs remain safe and reliable from manufacture to consumption.
Photostability refers to a drug’s ability to maintain its chemical structure when exposed to light, generally concerning sunlight and artificial (LED-lights), because x-rays are too uncommon in an everyday setting, whilst infra-red lights have too little energy to have a significant effect. When a medicine lacks photostability, light energy can initiate chemical reactions that alter its molecular composition. These changes may result in a reduction in therapeutic activity, the formation of toxic degradation products, or inaccurate dosing. Regulatory authorities such as the International Council for Harmonisation, therefore, require photostability testing for all new drug substances and products before they can be approved for public use.
Light triggers drug degradation by providing energy in the form of photons. When a drug molecule absorbs ultraviolet or visible light, electrons within the molecule are promoted to higher energy levels, creating an excited and unstable state. In this state, chemical bonds are more likely to break or react with surrounding molecules. One common mechanism is photolysis, in which absorbed light causes direct bond cleavage, particularly in weaker bonds such as carbon–nitrogen or carbon–chlorine bonds. Another important mechanism is photo-oxidation, where excited drug molecules form free radicals that react with oxygen, producing oxidised degradation products that may continue forming even after light exposure has ended.
Not all medicines are equally affected by light, as photodegradation depends strongly on molecular structure. Drugs that contain chromophores, which are parts of molecules that absorb light, are particularly vulnerable. Conjugated double bonds and aromatic ring systems allow electrons to be excited more easily, increasing reactivity. For example, nifedipine, a calcium channel blocker, contains a conjugated system that absorbs ultraviolet light and degrades rapidly. Riboflavin, also known as vitamin B₂, is highly photosensitive due to its aromatic ring structure, while chlorpromazine undergoes photo-oxidation that can produce potentially toxic by-products.
Photodegradation has serious implications for patient safety and treatment outcomes. A degraded drug may lose its therapeutic effect, leading to treatment failure, while some degradation products may be toxic even if the original drug is safe. Partial degradation can also result in patients receiving a lower dose of the active ingredient than intended, increasing the risk of inconsistent or unpredictable effects. These problems are especially concerning because light exposure can vary significantly during storage, transport, and everyday use, particularly when medicines are stored in clear containers or exposed to sunlight.
To prevent light-induced degradation, pharmaceutical chemists apply several strategies during drug development. Protective packaging, such as amber glass bottles and opaque blister packs, is commonly used to block ultraviolet radiation. Formulation techniques may include the addition of antioxidants or stabilising excipients to reduce free radical formation, while in some cases chemists modify the molecular structure of a drug to reduce its ability to absorb light without affecting its therapeutic function. These approaches are supported by regulatory photostability testing, which simulates real-world light exposure to ensure medicines remain stable throughout their shelf life.
In conclusion, photodegradation of medicines illustrates the close relationship between chemistry and healthcare. Through processes such as electronic excitation, bond cleavage, and oxidation, light can transform beneficial drugs into less effective or potentially harmful substances. By understanding these chemical mechanisms, scientists can design formulations and packaging that protect medicines from light exposure, ensuring that patients receive safe, effective, and reliable treatments.
Works Cited
EMA. “ICH Q1B Photostability Testing of New Active Substances and Medicinal Products – Scientific Guideline European Medicines Agency.” European Medicines Agency, 17 Sept. 2018, www.ema.europa.eu/en/ich-q1b-photostability-testing-new-active-substances-medicinal-products-scientific-guideline.
Felmeister, Alvin, and Clarence A. Discher. “Photodegradation of Chlorpromazine Hydrochloride.” Journal of Pharmaceutical Sciences, vol. 53, no. 7, Elsevier BV, July 1964, pp. 756–62, https://doi.org/10.1002/jps.2600530712. Accessed 14 Mar. 2024.
Grundy, John S., et al. “Photostability Determination of Commercially Available Nifedipine Oral Dosage Formulations.” Journal of Pharmaceutical and Biomedical Analysis, vol. 12, no. 12, Dec. 1994, pp. 1529–35, https://doi.org/10.1016/0731-7085(94)00100-6.
“Pharma – Hoenle.” Hoenle, 6 Aug. 2025, www.hoenle.com/products-solutions/applications/photolysis/pharma/. Accessed 16 Jan. 2026.
