An allergic reaction to sulfonamide antibiotics (often referred to as sulfa or sulfur antibiotics), such as sulfamethoxazole, is one of the most commonly reported allergic responses to medications. Though not as prevalent as reactions to antibiotics, other sulfonamide-containing medications can also trigger an allergic response.  Much research has been done to determine the exact mechanism of the allergic responses to sulfonamide medications and the general conclusion is that the sulfur portion is not the cause[1].  Organic Sulfur lacks any functional groups or metabolites thought to be responsible for the allergic reactions produced by sulfonamide medications. Furthermore Organic Sulfur has been shown to be one of the least toxic substances in biology[2].  If you have an allergy to a sulfonamide medication, consult your healthcare provider before taking Organic Sulfur. [1]Brackett CC. Sulfonamide allergy and cross-reactivity. Curr Allergy Asthma Rep. 2007 Apr;7(1):41-8. [2]Parcell S. Sulfur in human nutrition and applications in medicine. Altern Med Rev. 2002 Feb;7(1):22-44.