Photosensitizers that are readily incorporated into the external tissues can increase the rate at which reactive oxygen species are generated upon exposure to UV light (such as UV-containing sunlight). Some photosensitizing agents, such as St. John's Wort, appear to increase the incidence of inflammatory skin conditions in animals and have been observed to slightly reduce the minimum tanning dose in humans. 
Some examples of photosensitizing medications (both investigatory and approved for human use) are:
* St. John's Wort
* 9-me-bc
* Doxepin
* Amoxapine
* Ethinyl estradiol