What should you know about dark horizontal lines on nails?

A groove or band crossing the nail may appear after illness, physical trauma, tight footwear, or irritation to the nail-producing area. Dark colour can also come from dried blood, pigment, or a nail infection. Nail fungus may cause colour and texture changes, while nail psoriasis can produce pits, lifting, or irregular growth. A clinician can examine the nail surface and the skin around it rather than judging the cause from colour alone.

Take a clear photograph and note when the line appeared, whether it is moving outward as the nail grows, and whether one or several nails are involved. Keep nails trimmed, avoid picking or aggressive filing, and protect them from repeated pressure. Do not start oral treatment without a diagnosis; testing may be needed before an antifungal medicine is chosen.

Arrange a dermatology assessment for a new dark band, a line that widens or changes, pain, swelling, bleeding, nail lifting, or colour extending onto nearby skin. Seek prompt care if several nails change suddenly or you feel unwell. A changing mark deserves review even when it causes no discomfort.

Could this be related to trauma, infection, psoriasis, or another nail condition? Does the line move with nail growth? Would a nail sample or examination of the surrounding skin help? Which changes should prompt an earlier review?