Drug Rashes Drug rashes are side effects of medications. Drugs cause rashes in several ways. Most drug rashes are allergic reactions to medications. Later exposure to the drug may trigger an allergic reaction. Usually within minutes, though sometimes hours or days later, the skin breaks out in a rash.
Drugs also produce rashes directly without involving an allergic reaction.
These drugs include certain antipsychotic medications, tetracycline, antibiotics that contain sulfa, chlorothiazide, and some artificial sweeteners. No rash appears when the drug is taken, but later exposure to the sun produces an area of red, sometimes itchy, skin or gray-blue discoloration. A dark red or purple rash that reappears at the same spot each time a particular drug is taken. The rash most often appears in the mouth or on the genitals. The blotches most often appear on the legs.