Sulfonamides are a type of antibacterial medication that is most commonly delivered orally or topically for burns. By thwarting bacteria's synthesis of folate, sulfonamides eventually starve bacterial colonies and prevent the spread of bacterial infections [source: Merck].
But sulfonamides also have the side effect of causing photoallergic reactions in patients as well. Remember that photoallergies occur when drugs are converted into haptens. This type of molecule creates an immune response in the body, leading to an allergic reaction that can spread to areas of the skin that haven't been exposed to sunlight.
Since sulfonamides absorb sunlight and turns that sunlight into haptens, users should stay out of sunlight during sulfonamide regimens. Typically, the photoallergic reaction takes the form of a rash, a deep itch that isn't satisfied by scratching (called pruritus) and blisters [source: Merck].
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It's super weird that a bruise changes from deep purple to green to yellow during the healing process. HowStuffWorks looks into this strange process.