The photosensitizing capacity of crude coal tar, methoxsalen, and chlorpromazine was found to be strongly influenced by the choice of vehicle. When coal tar was incorporated in an optimal base, occlusion had little influence on its phototoxicity. Although no single base was found to provide optimal absorption for these compounds, emulsion-type creams were generally superior to petrolatum. Polyethylene glycol was a uniformly poor vehicle and tended to neutralize the activity of all these substances.
Kays H. Kaidbey, Albert M. Kligman. Topical PhotosensitizersInfluence of Vehicles on Penetration. Arch Dermatol. 1974;110(6):868–870. doi:10.1001/archderm.1974.01630120018004