Herbs That Increase Sensitivity to Sun

I would like information on taking herbs that make your skin more sensitive to the sun, i.e., exposure to the sun will result in a rash.

Herbs can cause increased sensitivity to the sun in two main ways. One is contact photodermatitis. This occurs when the skin comes in contact with an offending plant, the skin is then exposed to sun, and a rash or other reaction develops. I have experienced this effect myself after coming in contact with the herbs rue (Ruta graveolens) and cow parsnip (Heracleum spp.). Interestingly, it only occurred when working in the garden, not in the greenhouse, probably because greenhouse plastic screens out the sun's ultraviolet rays which are known to activate the rash effect. For a list of plants that cause contact photodermatitis, see www.lni.wa.gov/sharp/derm/phytoderm.pdf.
The other way is to ingest a herb that causes systemic photodermatitis. St. John's wort is known to do this in some individuals, and there are others. Some herbs such as laceflower (Ammi majus) are known to contain photoactive molecules such as the psoralens that can reach the skin and react with the ultraviolet light of the sun. In fact, this process has been put to good use medically to treat vitilago, a disease that causes white patches to develop in the skin. Chemical analogues of psoralens are being used today to treat other diseases, even cancer, in a process where the patient is made sensitive to light by injecting the chemical and then light is aimed at the cancer to kill the growth. Because this can even be done to internal cancers there is much promise that this process can target cancers better than radiation, surgery, or chemotherapy.
Unfortunately, we do not have a list of herbs that cause the systemic type of photodermatitis. Besides St. John's wort and laceflower, there are a few others. Celeriac, or root celery, and other members of the carrot family, such as parsley and fennel, have apparently caused systemic photodermatitis in a few individuals, but the reports we have seen as controversal. These plants contain the chemicals that could cause photodermatitis, but the levels seem to be too low. Certainly, we can say with confidence that the vast majority of herbs do not cause systemic photodermatitis.

Thank you for your reply it is very informative. Some of the herbs I take are echinacea, milk thistle, cascara sagrada, red clover, garlic, sometimes eyebright and ginseng and kava kava sometimes, not all of these are every day. Could any of them cause this?

Anything can cause an allergic reaction or increased sensitivity to something like sunlight in some individuals. But as far as I know none of these herbs have the known specific effect of increasing sensitivity to sunlight.