What should you know about dark longitudinal lines on nails?

A lengthwise dark band is called longitudinal melanonychia. It may arise from pigment-producing cells, repeated trauma, medication effects, or a benign mole in the nail unit. Nail fungus can alter colour and texture, while nail psoriasis may produce pitting, lifting, or debris as well as colour change. The pattern matters: note whether the band is on one nail or several, whether it is narrow or widening, and whether the colour reaches the surrounding skin. A photograph can help compare changes over time, but it cannot replace examination.

Keep nails short and avoid picking, biting, aggressive filing, or repeated pressure from manicures and footwear. Record when you first noticed the line and take clear photographs in consistent light. Do not cover a changing band with polish before an appointment if you can avoid it. A dermatologist may inspect the nail, skin, and surrounding nail fold, and may take a sample when infection is suspected. Oral antifungal treatment is appropriate only when a clinician confirms a fungal cause and checks whether it is suitable for you.

Arrange a prompt dermatology review for a new dark band on one nail, a band that widens or becomes irregular, bleeding, pain, nail splitting, or pigment extending onto the nearby skin. Seek earlier assessment if the change is progressing or you cannot recall an injury. Several stable bands may have a different explanation, but they still deserve review when their pattern changes.

Ask whether the band looks like pigment, trauma, fungus, psoriasis, or another nail-unit change. Ask if you need dermoscopy, a nail sample, or monitoring photographs, and whether any medicine, manicure habit, or work-related exposure could be contributing.