Natural Factors C Extra + Quercetin Bioflavonoids is an immune-support formula that combines the antioxidant benefits of vitamin C and the bioflavonoid quercetin in a synergistic 2:1 ratio. Vitamin C helps in collagen formation to maintain healthy bones, cartilage, teeth, and gums, while quercetin enhances the activity of vitamin C in the body. Quercetin is currently being studied for additional health benefits