"There are several sulfonamide-based groups of drugs. The original antibacterial sulfonamides (sometimes called simply sulfa drugs) are synthetic antimicrobial agents that contain the sulfonamide group. Some sulfonamides are also devoid of antibacterial activity, e.g., the anticonvulsant sultiame. The sulfonylureas (main article: sulfonylureas) and thiazide diuretics (main article thiazide) are newer drug groups based on the antibacterial sulfonamides.

Sulfa allergies are common, hence medications containing sulfonamides are prescribed carefully.It is important to make a distinction between sulfa drugs and other sulfur-containing drugs and additives, such as sulfates and sulfites, which are chemically unrelated to the sulfonamide group, and do not cause the same hypersensitivity reactions seen in the sulfonamides. . . .

Side effects
Sulfonamides have the potential to cause a variety of untoward reactions, including urinary tract disorders, haemopoietic disorders, porphyria and hypersensitivity reactions. When used in large dose, it may develop a strong allergic reaction. . . .

Adverse reactions
Approximately 3% of the general population have adverse reactions when treated with sulfonamide antimicrobials."

http://en.wikipedia.org/wiki/Sulfa_drugs

Cherie