Natural curcumin was evaluated as a potential photosensitizer for oral applications. The photocytotoxicity of curcumin on salivary gland acinar cells (SM 10-12) was investigated in five aqueous preparations consisting of 5% DMSO, non-ionic micelles, cyclodextrin, liposomes, or a hydrophilic polymer. The difference in phototoxic effects between natural curcumin and synthetic curcumin was examined. Cytotoxicity in SM 10-12 cells exposed to curcumin in the concentration range 0.4-13.5 microM was investigated by MTT test, a fluorescence-staining microscopic test, and by Western immunoblotting techniques. The potential formation of a photoreaction product, hydrogen peroxide, was evaluated by a fluorescence assay. The light source was a halogen lamp used in the dental clinic, emitting mainly in the blue part of the spectrum. The phototoxic effect on SM 10-12 cells was dependent on curcumin concentration, the light dose and the type of preparation. Natural and synthetic curcumin induced phototoxicity to the same extent. Significant effects on the cells were obtained at low curcumin concentrations (< or =0.5 microM) and at a low light dose (< or =6 J cm(-2)), after 3 h incubation. Neither the activation of caspases-3, -7, -8 or -9, nor the formation of hydrogen peroxide could be detected in cells exposed to curcumin and light. The liposome preparation was the most efficient vehicle for curcumin to induce cell death. The phototoxic effect induced by curcumin is highly dependent on the type of preparation. Curcumin might be a potential photosensitizer in the treatment of oral lesions and cancers provided careful selection of the vehicle.