An ingrown toenail occurs when the side or corner of the nail penetrates the adjacent skin, most often on the big toe. The medical term is onychocryptosis.
If left untreated, infection or even an abscess can develop; rarely, bone infection (osteomyelitis) can occur.
For moderate cases: soak in warm water, apply antibacterial ointment, and gently lift the offending edge with a small cotton or gauze wick changed daily for about two weeks. Avoid tight footwear. If symptoms persist — or in severe cases — a podiatrist may remove the offending nail edge surgically.