Sulfa drugs can treat a range of health problems from eye infections to rheumatoid arthritis (RA). But if you get a rash or more serious reaction when you take one of these drugs, you may have a sulfa allergy. Sulfa drugs, also called sulfonamides, include antibiotics and other types of drugs. Allergies happen most often with antibiotics. Around 3% of people have some type of reaction to them. If you are allergic to one sulfonamide antibiotic, there is a risk that you might also react to other sulfonamide antibiotics. A simple blood plasma test is used to check sulfamethoxazole and sulfisoxazole levels in blood serum.