Allergy to sulfa-containing medications is very common; these are typically antibiotics. However, not all antibiotics are pills. Doctors will sometimes prescribe antibiotic eye drops or ointments for 'pink eye'. Several eye drop preparations contain sulfa antibiotics, and it may not occur to the patient to answer that they have any drug allergies when they are getting an eye drop prescription.
For more information on the Allergen of the Month feature, see the overview, and check out the previous post on soaps and detergents.