Black Line on Nail: What It Means, Causes, and When to See a Doctor

Last reviewed: 2026-08-06

Quick answer

A black vertical line on a nail is called linear melanonychia, and most of the time it is harmless. It happens when pigment-producing cells in the nail add extra color, and it is especially common in people with medium to dark skin. Most lines are nothing to worry about, but see a doctor if the line is wider than 3 mm, keeps growing or darkening, or pigment spreads onto the skin around the nail.

On this page

• What a black line looks like
• Common causes
• Is it serious?
• What your body might be telling you
• What you can do
• FAQ

What it looks like

A black line on the nail usually appears as a single vertical band that starts near the cuticle and runs toward the tip as the nail grows. The color can be light brown, dark brown, gray, or black. Because the line is part of the nail plate itself, it grows forward with the nail, so its position along the nail gives a rough idea of when the pigment formed.

Variations worth knowing:

• A thin, pale band is often simple pigment variation.
• A broad stripe is worth measuring, because width matters for triage.
• Several nails with similar lines often points to a harmless cause such as natural pigment or a medication.
• A line where pigment also spreads onto the skin beside the nail is the pattern that needs prompt evaluation.

Common causes

Most black lines have a benign explanation. Causes, from most to least common:

Common

• Natural pigment variation (ethnic-type melanonychia). This is the most common cause, especially in people with medium to dark skin. It is common in people of Caribbean descent by age 50, and around 20% of people of Japanese descent have benign melanonychia.
• Repeated minor trauma. Nail biting, picking at the cuticle, or friction from tight shoes can stimulate pigment production.
• A bruise under the nail (subungual hematoma). Usually follows a clear injury and grows out with the nail over time.

Less common

• A mole (nevus) in the nail matrix, the tissue that grows the nail.
• Medications. Some chemotherapy drugs, antimalarials, and HIV treatments can darken nails.
• Fungal or bacterial infection of the nail.
• Pregnancy, when hormone changes temporarily increase pigment.

Rare

• Hormone or immune conditions such as Addison’s disease, thyroid problems, lupus, or scleroderma.
• Nutritional deficiency. Vitamin B12 deficiency is a recognized but uncommon cause of dark nail pigmentation.
• Subungual melanoma, a rare nail cancer. It is the main reason new or changing lines deserve a professional look.

Is it serious?

Usually benign, monitor at home. A stable band that has looked the same for years, several nails with matching pigment, a known ethnic-type pattern, or a line that appeared after an injury and is growing out with the nail.

Worth a dermatologist look, not an emergency. A new line on a single nail in an adult, especially one that keeps widening or darkening over months.

See a doctor, ideally a dermatologist, if:

• The band is wider than 3 mm (about an eighth of an inch).
• The width, color, or border changes over time.
• Pigment spreads onto the cuticle or the skin next to the nail (called the Hutchinson sign).
• The nail itself is damaged: splitting, thickening, lifting, bleeding, or a bump under the nail.
• A new band appears on the thumb, index finger, or big toe in someone 50 or older.
• You have a personal or family history of melanoma.

A dermatologist can examine the nail with a dermoscope and, if anything looks suspicious, take a small biopsy. That is the only way to be certain, and it is a quick, routine procedure.

What your body might be telling you

Straight answer: most black lines on nails are not a vitamin deficiency, and no supplement will erase one. If you are hoping the fix is nutritional, the honest answer is usually no.

• Vitamin B12 is a recognized but uncommon cause of dark nail pigmentation, and it rarely appears alone. Fatigue, tingling in the hands or feet, or pale skin usually come with it. Do not self-supplement on the strength of a nail line alone; a blood test is the way to check.
• Iron, zinc, and calcium have no established link to black lines. Dark nail pigment comes from melanin activity in the nail matrix, not from mineral levels. (The old “calcium deficiency” story belongs to white spots on nails, and the evidence does not support it there either.)

For most people the real story is pigmentation, trauma, medication, or rarely a mole or melanoma. Nutrition is the least likely explanation for most cases.

What you can do

• If the line is new or changing, see a dermatologist. Nail pigmentation is their specialty, and a biopsy is the only certain answer.
• Take a photo next to a ruler or coin, dated, so you can track width and color honestly over months.
• Protect the nail from repeated trauma: avoid picking or biting, and wear properly fitted shoes.
• Do not try to bleach, file, or scrape the pigment off. It will not work, and it can damage the nail.
• Do not start B12 or any other supplement based on a nail line alone. If you are concerned about a deficiency, talk to a doctor about testing first.
• Be patient with tracking. Nails grow slowly, and meaningful change happens over months, not days.

FAQ

Is a black line on my nail cancer? Most are not. Melanoma under the nail is rare. But it is exactly why dermatologists take new or changing bands seriously. Only a professional examination, with a biopsy if needed, can tell the difference, so a new band is worth one visit.

When should I worry about a black line on my nail? Worry flags are: a band wider than 3 mm, growth or darkening over time, pigment spreading onto the skin around the nail, damage to the nail itself, a new band on the thumb, index finger, or big toe at age 50 or older, or a personal or family history of melanoma.

Can trauma cause a black line on a nail? Yes. Repeated friction, nail biting, or a bruise can all leave pigment behind. A bruised toenail from running or stubbing is a different situation, usually harmless, and covered in black toenail from injury. A line that appears without any injury and keeps changing deserves a check.

What deficiency causes black lines on nails? Usually none. Vitamin B12 deficiency is a recognized but uncommon cause, and it typically comes with other symptoms like fatigue or tingling. Do not guess; a blood test is the reliable way to find out.

Do black lines on nails go away? If the cause is temporary, such as a bruise, a medication, or pregnancy, the band grows out with the nail over months. Ethnic-type pigment is usually permanent, which is normal and harmless. Melanoma does not go away, which is why change matters.

Explore more: this guide is part of the complete library, what your nails say about your health.

Sources

• Cleveland Clinic: Melanonychia, What It Is, Causes and Treatment. https://my.clevelandclinic.org/health/diseases/melanonychia
• DermNet NZ: Melanoma of the nail unit. https://dermnetnz.org/topics/melanoma-of-the-nail-unit
• StatPearls (NCBI/NIH): Subungual Melanoma. https://www.ncbi.nlm.nih.gov/books/NBK482480/

Disclaimer

This article is for information only and is not medical advice. Always consult a qualified healthcare provider about your health.

Reviewed by the VI Editorial Team. Last reviewed: August 6, 2026