Ingrown toenails can have many causes: trimming toenails too short, shoe pressure, injury, fungus infection, heredity, or poor foot structure. Ingrown toenails occur when the corners or sides of the toenail dig into the skin, often causing infection. Ingrown toenails start out hard, swollen, and tender. Left untreated, they may become sore, red, and infected and the skin may start to grow over the ingrown toenail. Antibiotics are sometimes prescribed if an infection is present. In some cases, surgical removal of part or the entire ingrown toenail may be necessary; in most instances this involves a minor in-office nail procedure.