Certain medications can trigger a rash. You might be allergic, or it could be a side effect of the medication. Anti-inflammatory medications, antibiotics, and pain relievers like Tylenol can cause what doctors call “fixed drug eruptions.” These affect the genitals and can cause swollen, red plaques with a gray center. They can also cause blisters and dark areas that appear once the inflammation goes away. The type of reaction you have, how soon it shows up, and how severe it is depend on the medication.