What should you know about fine black lines on nails?

A narrow dark line may be a splinter haemorrhage after minor trauma, pigment produced by the nail matrix, or a change related to medication or inflammation. Several nails showing similar lines can point toward a broader process, while one persistent band needs closer inspection. Colour alone cannot identify the cause.

A clinician will consider the line's width, shade, position, borders, whether it reaches the surrounding skin, and whether the nail is becoming distorted. A benign explanation is possible, but a persistent or evolving streak should not be diagnosed from a photograph alone.

Take a clear photograph with the date and compare it over time. Avoid picking, biting, harsh nail products, or cutting deeply into the nail. Note recent knocks, manicures, new medicines, and whether other nails are affected. Book a dermatology assessment if the mark does not grow out, is changing, or has no clear injury.

Arrange prompt medical review for a widening or irregular band, pigment extending onto the skin beside the nail, bleeding, pain, nail splitting, or a new streak without an obvious cause. Seek urgent care if the nail change follows a severe crush injury with intense pain or signs of infection such as spreading redness, warmth, or pus.

Ask whether the pattern fits trauma, pigment, inflammation, or another nail disorder; whether dermoscopy is useful; how to monitor the mark; and which changes should trigger an earlier appointment.