Dark streaks in the nail (melanonychia)

What it is

Longitudinal melanonychia is a brown or black band running from the cuticle to the tip of a nail. It is caused by pigment produced by cells in the nail matrix.

How it works

A brown or black band runs along the nail when pigment cells at the nail root become active. In darker skin many people have harmless bands on several nails. Rarely, a band is caused by melanoma of the nail unit.

Why it matters on darker skin

Multiple even bands on several nails are very common and usually normal in people with darker skin. However, nail melanoma, though rare, is a type of skin cancer that is proportionally more common in darker skin and is often found late.

Causes and triggers

Normal ethnic variation, trauma, medicines, pregnancy, infections — and rarely melanoma.

What helps — and why it works

Take a photograph each month to monitor any band. Keep nails short and protect them from repeated trauma. Photograph and monitor: changes over time are the key warning sign. Expected timeline: check monthly. See a doctor for warning signs: early assessment of a suspicious band can save lives. Expected timeline: prompt.

What to avoid and why

Do not cover a new streak with polish or acrylics, or ignore it hoping it fades.

Common mistakes

• Assuming a single new dark band in an adult is harmless without checking.
• Covering a band with gel polish so changes are missed.

Everyday nail habits

Healthy nails depend on protecting the nail matrix — the growing area hidden under the cuticle — and on limiting cycles of wetting and drying. Wear gloves for washing and cleaning, keep nails short and filed in one direction with a fine file, and moisturise hands, nails and cuticles after washing and at night. Leave the cuticle in place: it is a seal that keeps water and germs away from the matrix, and cutting it is a common cause of infection. Fingernails grow about three millimetres a month and toenails about one, so a new fingernail takes four to six months and a toenail up to eighteen months; any treatment needs at least that long to judge. Choose salons that sterilise metal tools between clients, or bring your own. Take breaks between gel or acrylic sets, and never peel coatings off. Because polish can hide important changes, look at bare nails regularly, particularly for a new dark band, a lifted nail or a change in shape.

Products and treatments

Many nail products make claims with little evidence. Moisturisers containing urea, lactic acid or petrolatum have the most support for dry, brittle nails and cracked skin around them. Formaldehyde-based hardeners can make nails more brittle over time and can cause allergy, so we do not recommend them. Antifungal nail lacquers such as amorolfine or ciclopirox can help mild fungal infection limited to the tip of one or two nails, but only after the diagnosis is confirmed — many thick or discoloured nails are not fungal at all. Biotin supplements are widely sold; evidence is weak and high doses can interfere with some blood tests. If you have diabetes or poor circulation, foot and toenail problems deserve early professional care.

Questions to ask your doctor or pharmacist

Useful questions include: What do you think is causing this? Is it likely to come back, and how can I prevent it? Which products should I stop? How long before I should expect improvement, and when should I come back if it isn't better? Are there side effects to watch for? Is this treatment safe if I am pregnant, breastfeeding or for my child? Writing your questions down, bringing your products (or photos of their labels) and taking dated photos of the problem make an appointment much more useful.