When the toenails are exposed to a warm moist environment or some type of trauma occurs to the toenails, a fungus can develop on or under the toenails. This is known as onychomycosis. Fungal toenails may become thick and turn yellow, gray, brown, or even black. The nail may become brittle, loose, crack or become ingrown. There are several non-prescription topical treatments available. The antifungal products available at DrTodds.com are products that Dr. Todd has used in practice and has found effective. To help prevent fungal growth, keep toenails dry and clean, change socks often and use an antifungal foot spray on feet as well as in shoes and socks.