Ultraviolet light, chlorpromazine

Our conductrimetric work [207] with three hydrochloride psychotropes—chlorpromazine, trifluoperazine, and methotrimeprazine—also showed that these formed charge-transfer complexes with acetylcholine, 6-hydroxydopamine, and noradrenaline which were not highly associated. Two different types of interaction occurred, one in which a single product was formed, and a second which involved two almost simultaneous reactions. In the former case the product was stable to ultraviolet light, while in the latter case the more slowly formed product was strongly UV sensitive. The reactions in solution were first order. 

Chlorpromazine most often causes photosensitivity reactions (incidence around 3%), which may result from formation of a cytotoxic by-product after exposure to ultraviolet light. Patients should be advised to avoid prolonged exposure to strong indoor light, to wear protective clothing, and to use a combined para-aminobenzoic acid plus benzophenone sunscreen when exposure to strong sunlight is unavoidable. 

The phenothiazine chlorpromazine (CLP) is rapidly decomposed under the action of ultraviolet light, the decomposition being accompanied by discoloration of the solutions (Scheme 4.10). 

Cahn MM, Levy EJ (1957) Ultraviolet light factor in chlorpromazine dermatitis. Arch Dermatol 75 38-40... 

---