A black or brown line running up a fingernail or toenail has a medical name, melanonychia, and most of the time the cause is harmless. It can be natural pigment, an old knock, a medicine, a fungus or a harmless mole in the nail root. A small number of lines need a dermatologist, and the signs that separate them are easy to learn.
If the line is on only one nail, is new, or is changing, skip ahead to when to see a doctor. Otherwise, here is what is likely behind it. You can also photograph it in the app and track whether it changes. A photo log is for monitoring, not a verdict.
- •A dark stripe running from cuticle to tip is called melanonychia. Most often it is natural pigment, an injury, a medicine, a fungus or a harmless mole in the nail root.
- •Count the nails first. Several nails with thin, even, steady lines lean harmless. One new band on one nail in an adult is the pattern worth a doctor's look.
- •Photograph the line today and again in a few weeks. Change is the most useful clue, and a dermatologist can check the nail with a dermatoscope.
What a dark line on a nail actually is
The nail grows out from the nail matrix, a patch of tissue under the skin at the base. Cells in the matrix can put pigment into the nail as it forms, and that pigment travels outward with the nail, which is why the stripe runs in a straight line from base to tip. Doctors call it longitudinal melanonychia when it runs lengthwise.
Not every dark line is melanin. A nail expert review lists pigment from the body (melanin) and from outside it, including blood, bacteria and fungus. So the same-looking stripe can have quite different causes.
Common causes of a black line on a nail
- Natural pigment. Common in people with darker skin tones, often on several nails at once. The bands are usually thin, even and stable for years.
- Injury or pressure. A knock, tight shoes, repeated pressure or a nail habit can leave a band, and a bruise under the nail can look dark red, brown or black. A bruise grows out with the nail, so the dark patch moves toward the tip. Thin dark lines running lengthwise, called splinter hemorrhages, often follow a small injury too.
- Medicines. Some medicines can darken nails, including some chemotherapy drugs and other prescription medicines. Do not stop a medicine on your own. Tell your doctor or pharmacist about the nail change.
- Fungus or bacteria. Some fungi make dark pigment and can stain a nail. It is an uncommon form of nail fungus. Read early signs of nail fungus if the nail is also thick, crumbly or lifting.
- A benign mole in the nail root. A nail matrix nevus is a harmless collection of pigment cells that makes a steady band, often present since childhood or for years.
- Stains and dirt. Dye, dark polish or dirt trapped under the edge of the nail can imitate a line. These sit on the surface or at the tip and wipe or trim away.
When several nails carry similar thin bands and nothing is changing, that pattern leans strongly toward harmless. Children can have bands that look worrying and turn out to be benign, and doctors usually follow those closely rather than rushing to a procedure.
How to tell the causes apart
Five questions narrow it down:
- How many nails? Several nails with similar lines suggest natural pigment or a medicine. One nail suggests an injury, a nail root mole or something to check.
- How wide and how sharp? A thin, even stripe with clean edges is reassuring. A broad, blurry band is not.
- How many colors? One steady shade is reassuring. Several shades of brown and black in one band is a reason to have it looked at.
- Is it moving or changing? A bruise grows toward the tip. A band that keeps coming from the base, or gets wider or darker, is made by the nail root and should be tracked.
- What is around the nail? Pigment on the skin of the cuticle or fingertip next to the band should be checked. Some harmless things can mimic it, so a doctor looks at the whole picture.
Pair this with the nail color chart if you are not sure whether the color is a line, a spot or a bruise.
What a doctor checks
A dermatologist usually starts with questions: when you first saw it, any injury, your medicines, your family history, and whether other nails are involved. Then they look with a dermatoscope, a small lens that magnifies the band and shows its pattern. From there, a doctor may:
- Take photos and measurements to watch it over time
- Test nail clippings for fungus or other causes
- Take a small sample from the nail root when one nail has a new band in an adult and nothing explains it. A sample examined under a microscope is how doctors confirm or rule out nail melanoma
Many people leave with a harmless answer and a plan to recheck in a few months.
The cancer question, kept short
Nail melanoma is rare. When it happens, it most often first shows as a dark band, which is why doctors take a new single band in an adult seriously. Features that raise the level of concern:
- One nail, new in adult life
- A band that gets wider or darker over weeks and months
- A band about 3 mm wide or more, or one with blurred, uneven edges
- Several shades of brown and black in one band
- Pigment spreading onto the skin around the nail, called Hutchinson's sign, though harmless things can imitate it
- A nail that splits, forms a lump or bleeds at the base
One of these does not mean cancer. It means a dermatologist should look rather than wait. For the full set of nail melanoma warning signs, see CheckMole's guide to nail melanoma signs. It is written by our sister site that focuses on moles and skin cancer.
What to do this week
- Take a clear photo of the line in daylight, with the finger flat, and note the date.
- Look at all ten fingernails and toenails. Count which carry lines.
- Think back: a knock, new shoes, a new medicine, a nail habit.
- Take another photo in a few weeks. A band that looks the same, thin and steady, is reassuring.
- If any warning feature above applies, or you simply cannot explain a new line on one nail, book a dermatologist. Bring both photos.
If you are pregnant, breastfeeding or take regular medicines, mention that at the visit, since it changes which causes and tests make sense.
When to see a doctor
Make a dermatology appointment if the line is:
- New in adulthood and on a single finger, thumb or toe
- Getting wider, darker or more irregular over time
- Spreading pigment onto the skin around the nail
- Paired with nail splitting, a lump or bleeding at the base
- A line you cannot explain and cannot stop worrying about
Pain, redness and swelling around a darkened nail after an injury are a reason to get checked sooner. More signs are in when to worry about your nails.
Frequently asked questions
Is a black line on my nail always cancer?+
No. Most dark lines on nails are harmless: natural pigment, an old injury, a medicine, a fungus or a benign mole in the nail root. Nail melanoma is rare, but a new single band in an adult should be checked.
Why do I have a black line on only one fingernail?+
A single line often follows a knock or repeated pressure on that nail, or a small benign mole in the nail root. A new band on one nail in an adult, with no injury you can point to, is worth a dermatologist visit.
Will a black line on a nail go away?+
A line from a bruise or a medicine grows out with the nail over months, and toenails are slower than fingernails. A band that keeps being made at the base is pigment from the nail root and does not fade on its own.
Is a dark line on the nails normal in darker skin?+
Yes, it is common. Several nails often carry thin, even bands that stay the same for years. A band that is wider, darker, blurred or on just one nail still deserves a check, whatever your skin tone.
Can a fungus cause a black line on a nail?+
Sometimes. A few fungi make dark pigment and can stain the nail, though this is uncommon. Nail clippings tested at a clinic can show it, so ask a doctor if the nail is also thick or crumbly.
Should I get a black line on a toenail checked?+
Toenail lines are often from shoe pressure or a bruise, and the big toe is the nail that gets hit most. If it has no clear cause, does not grow out or keeps widening, have a dermatologist look at it.
Related
- What your nails say about your health
- When to worry about your nails
- Nail fungus: early signs
- Nail color chart
- CheckMole: black line under a fingernail, bruise or melanoma
Sources
- Ricardo JW, Bellet JS, Jellinek N, et al. Evaluation and diagnosis of longitudinal melanonychia: a clinical review by a nail expert group. J Am Acad Dermatol. 2025. PubMed 40023404
- Alessandrini A, Dika E, Starace M, et al. Diagnosis of melanonychia. Dermatol Clin. 2021. PubMed 33745638
- Jefferson J, Rich P. Melanonychia. Dermatol Res Pract. 2012. PubMed 22792094
- Levit EK, Kagen MH, Scher RK, et al. The ABC rule for clinical detection of subungual melanoma. J Am Acad Dermatol. 2000. PubMed 10642684
- Baran R, Kechijian P. Hutchinson's sign: a reappraisal. J Am Acad Dermatol. 1996. PubMed 8543700
- Rodriguez-Cerdeira C, Martinez-Herrera E, Cortes-Lopez PN, et al. Fungal melanonychia: a systematic review. Microorganisms. 2024. PubMed 38930478
This article is for general education and is not medical advice. It does not replace a consultation with a qualified clinician. If a symptom worries you, see a doctor.