What should you know about a horizontal dark line on a nail?

A mark that crosses the nail from side to side may reflect a temporary interruption in nail growth after illness, physical stress, or an injury to the nail. Dark colour can also come from dried blood beneath the nail, pigment made in the nail unit, or changes linked with inflammation. Nail fungus and nail psoriasis may alter colour and texture; related information is available under Nail Fungus, Nail Psoriasis, and Discoloured Nails.

A clinician may examine the nail, surrounding skin, and other nails. They may ask when the line appeared, whether it is moving toward the fingertip as the nail grows, and whether there was trauma or a new medicine.

Take a clear photograph in good light and note the date, affected nail, recent injury, illness, or treatment. Keep the nail trimmed, avoid picking beneath it, and protect it from repeated pressure. Do not scrape, bleach, or cover a changing mark with polish before an examination. Treatment depends on the cause; oral antifungal treatment is relevant only when a clinician confirms a fungal infection.

Arrange a dermatology assessment if the line is new, widening, uneven, painful, associated with nail lifting or bleeding, or not growing out with the nail. Seek prompt care after a crush injury if pain is severe, the nail is badly damaged, or the fingertip is numb or misshapen. A changing dark mark deserves examination because its appearance alone cannot identify the cause.

Could this be related to an injury, infection, inflammation, or pigment production? Does the line appear to be moving with nail growth? Would examination of the nail or a sample help clarify the cause? What changes should prompt an earlier review?