Two main classes of antibiotics have been shown to produce photosensitivity in patients; both are responsible for phototoxic side effects. Tetracyclines, used to treat bacterial infections like acne, typhus, chlamydia and conjunctivitis, and fluoroquinolones, antibiotics used to treat massive infections like MRSA, strep throat and mononucleosis, produce phototoxic effects.
Because they're phototoxic, both tetracyclines and fluoroquinolones absorb radiation from the sun and generate free radicals that can harm skin cells and tissue. Usually, this damage presents as very bad sunburn in the patient. Tetracyclines have the additional potential for two other phototoxic conditions. They can lead to pseudoporphyria, a condition where painful blisters and open sores develop and can leave skin with pigmented spots. There's also a chance that a lichenoid reaction can develop, which causes the appearance of tiny red bumps. The best way to prevent phototoxic conditions from antibiotics is to keep skin from direct sunlight while taking them.