A case of photoallergic dermatitis to a topically applied diphenhydramine preparation was observed, and the diagnosis was confirmed by photopatch testing. This diphenhydramine photoallergy appears to differ from most other forms of photoallergies in that it is elicited by ultraviolet light in the 290 to 320-nm range (termed UV-B) instead of by the more usual 320- to 400-nm range (termed UV-A). This may be related to the photochemical properties of diphenhydramine, which absorbs In the UV-B but not in the UV-A, the UV-B absorption being associated with a distinctive fluorescence. It is noted that the cases of UV-B photoallergy described to date have certain clinical features In common.
Emmett EA. Diphenhydramine Photoallergy. Arch Dermatol. 1974;110(2):249-252. doi:10.1001/archderm.1974.01630080047012