Toenail fungus (onychomycosis) is caused by a fungal infection in the toenail. As the infection becomes more severe, the toenail will thicken. The fungus which is causing the condition is located throughout the toenail. Topical products which are sold to treat toenail fungus are often unable to penatrate the toenail deep enough to eliminate the fungus which is causing the infection. By taking oral antifungal medications, the medicine is able to be incorporated into the toenail itself, and over time, allows the toenail to grow out normally. Topical medicatins for toenail fungus are best reserved for mild cases only. More information on fungal toenails can be found by clicking here.