Why does sunlight affect benzoyl peroxide and doxycycline? does the uv close pores? or does it chemically alter what it was originally supposed to do?

Photosensitizing: Doxycycline is a great drug but one of it's side effects is photosensitivity( meaning the skin becomes sensitive to sunlight). So if you take this medicine make sure you wear your sunscreen and reapply on any exposed areas of skin or you will likely get burned

Not exactly known: Doxy, like other tetracyclines are known to increase sun sensitivity-- but hte exact mechanisms are not known. Benzoyl has a great deal of effects on the skin, drying, anti-inflammatory and cell growth effects which all contribute to the sensitivity.