As the Mayo Clinic describes, a sulfa allergy most often refers to an adverse reaction to an antibiotic called sulfamethoxazole which is present in antibiotics that contain sulfonamides. Some of these antibiotics are Septra and Bactrim. Sulfisoxazole, present in Pediazole, should also be avoided.
In addition to antibiotics, there are many other medications that contain sulfonamides. These medications include some diuretics, oral diabetes medications, and some nonsteroidal anti-inflammatories. Check with your doctor to make sure these are safe.
Skin reactions are the most common reactions to sulfa medications, but the liver, kidneys, lungs, and blood also may be affected.
Note: Sulfa allergies are not to be confused with sulfite allergies, which involve allergies to common preservatives found in both foods and medications.