Natural D-Hist is the ideal solution for allergy sufferers. Unlike other over-the-counter products which only mask symptoms - Natural D-Hist provides support to sinus and nasal passage tissues to help inhibit the body�s allergy defense mechanisms. Ortho Molecular's Natural D-Hist unique formula contains: Quercetin A powerful natural flavonoid, quercetin has also been shown to inhibit mast cell degranulation and its unfortunate consequences. Stinging Nettles Leaf Stinging Nettles components have been shown to block important enzymes within the arachidonic acid cascade as well as having proven their effectiveness in a single ingredient clinical trial. Bromelain A strong pineapple enzyme, bromelain has proven mucolytic properties. As such, bromelain supports normal mucosal tissue function and enhances the absorption of quercetin. N-Acetyl L-Cysteine (NAC) An amino acid that is also a natural mucolytic, NAC reduces the viscosity of mucus. Clinical studies have proven oral doses of NAC to be an excellent means to recharge cellular glutathione, a foundational antioxidant.