I saw a photograph of the results of someone being exposed to strong sunlight for a few hours while on a certain medication. They burned _really_ badly, because the meds caused them to become extra photosensitive. I have heard of this effect before for various meds, but never considered how it works. Do we know how this happens?
[Note, I have seen there is another question about the medications themselves being sensitive to light. I am asking about medications that cause the _user_ to become extra photosensitive, rather than the meds themselves.]