Bosley Hair Thickening Fibers Dark Brown is a hair loss concealer. It conceals thinning and sparse areas of hair using Keratin fibers. The hair fibers adhere to existing natural hair giving it a thickened and full appearance. It is waterproof, sweat proof, and will not rub off for long-lasting wearability. Can also be used as a root re-growth cover-up for extended in-between visit to colorist. Washes off easy with shampoo.