Sulfa drugs include erythromycin, sulfisoxazole, sulfamethoxazole, and sulfasalazine. These are drugs composed of many elements creating large chain molecules which are used as antibiotics. It is highly unlikely that anyone would be allergic to MSM or sulfur, a naturally occurring substance in the body. Sulfites are preservatives, antioxidants and browning agents used in foods. Ingestion of these is associated with adverse reactions such as asthma attacks, nausea, and diarrhea.
Sulfates are a salt of sulfuric acid.
Sulfuric Acid is a heavy, corrosive, oily acid used in making fertilizers, chemicals and petroleum products. MSM is rated as one of the least toxic substances in biology, similar in toxcity to water. If you take more MSM than you need, it simply passes through the body and is excreted in the urine. MSM can't hurt you. MSM has been tested as a food ingredient by Dr. Stanley Jacob without any reports of allergic reaction.