A black line on your nail is almost always a streak of pigment produced by the melanin-making cells in your nail’s root. The medical term is melanonychia, and in most cases the cause is benign: a mole in the nail bed, repeated trauma, or simply your natural skin tone. That said, a dark nail streak can occasionally signal melanoma, so it’s worth understanding what’s harmless and what deserves a closer look.
Common Harmless Causes
The cells that produce pigment (melanocytes) live in the nail matrix, the tissue just beneath your cuticle where new nail grows. When those cells get activated, they deposit melanin into the nail plate as it forms, creating a vertical brown or black line that runs from the base of the nail toward the tip. Several everyday triggers can set this off:
- Skin tone. People with medium to dark complexions naturally develop pigmented nail bands more often. Nearly all Afro-Caribbean people develop brown-black nail streaks by age 50, and up to 20% of Japanese people are affected. If you have darker skin and notice lines on several nails, this is the most likely explanation.
- A mole (nevus) in the nail matrix. Just like a mole on your skin, a small cluster of pigment cells under the nail can produce a stable dark stripe. These are common and typically stay the same width and color over time.
- Repeated trauma. Nail biting, picking at cuticles, or wearing tight shoes can injure the nail matrix enough to trigger melanin production. Runners and hikers often see this on their toenails.
- Lentigines. These are the nail equivalent of age spots or liver spots on skin. They produce a faint to moderate brown band and are harmless.
If your line has been there for months or years without changing, appears on multiple nails, or matches a pattern you see in close relatives, a benign cause is overwhelmingly likely.
Splinter Hemorrhages: A Different Kind of Line
Not every dark line under a nail is pigment. Splinter hemorrhages are tiny streaks caused by burst capillaries in the nail bed. They look like thin splinters of wood trapped under the nail, usually only 1 to 3 millimeters long. Fresh ones appear reddish or purple, then darken to brown or black within a few days.
The key difference is movement. Because a splinter hemorrhage is trapped blood, it grows out with the nail and eventually disappears. A pigmented melanonychia line, by contrast, stays anchored at the base because the melanocytes keep producing color as the nail grows. If your dark mark is slowly migrating toward the tip of your nail and fading, it’s most likely old blood from a minor injury you may not even remember.
Medications That Cause Nail Darkening
Certain drugs can activate the melanocytes in your nail matrix, producing one or more dark bands. Chemotherapy agents are the most common culprits, particularly cyclophosphamide, hydroxyurea, doxorubicin, and 5-fluorouracil. Some antibiotics, including minocycline, can do the same. If a new streak showed up after you started a medication, mention it to your prescriber. The pigmentation usually fades after the drug is stopped, though it can take months for the affected nail to grow out completely.
Less Common Medical Causes
In rare cases, dark lines across multiple nails can point to an underlying condition. Addison’s disease, which affects the adrenal glands, can cause widespread skin and nail darkening. Autoimmune conditions like lupus, Sjögren syndrome, and systemic sclerosis have been linked to multiple pigmented nail streaks, possibly due to damage to the tiny blood vessels near the nail matrix. Fungal infections of the nail can also occasionally produce dark pigment rather than the typical yellow-white discoloration most people associate with nail fungus.
These conditions almost always come with other symptoms, so a single dark line on an otherwise healthy person is unlikely to be a sign of systemic disease.
When a Dark Line Could Be Melanoma
Subungual melanoma, a skin cancer that starts under the nail, is uncommon but serious. It accounts for a higher proportion of melanoma cases in people with darker skin tones, Asian populations, and Native Americans, where it can represent up to one third of all melanomas diagnosed. The peak age range is between 40 and 70.
Dermatologists use an ABCDEF framework to evaluate suspicious nail streaks:
- A (Age and ancestry): Most common in people over 50 and in those of African, Asian, or Native American descent.
- B (Band appearance): A brown-to-black band wider than 3 millimeters with uneven or blurry borders.
- C (Change): The band is getting wider, darker, or more irregular over weeks to months.
- D (Digit): The thumb and big toe are the most frequently affected.
- E (Extension): Pigment spreading beyond the nail onto the surrounding skin of the cuticle or sidewall. This is called Hutchinson’s sign and is strongly associated with melanoma, though it can occasionally appear in benign conditions too.
- F (Family history): A personal or family history of melanoma or atypical moles.
One important distinction: a bruise under the nail (subungual hematoma) grows out with the nail over several months and eventually disappears. A melanoma streak does not grow out. It stays fixed at the base and persists or widens because the cancer cells keep producing pigment. If a dark line has been in the same spot for months without moving toward the tip, that’s a reason to get it evaluated.
What a Nail Evaluation Looks Like
A dermatologist will typically start by examining the nail with a dermatoscope, a handheld magnifying device with a built-in light. This lets them see the pattern of pigment lines within the band, which helps distinguish moles and ethnic pigmentation from something more concerning.
If the appearance is suspicious, the next step is a biopsy of the nail matrix. The most common technique is a shave biopsy, where a thin layer of tissue is removed from the area producing the pigment. A small punch biopsy (about 3 millimeters) is sometimes used instead. Both require a local anesthetic. Afterward, the cuticle and nail fold are repositioned and closed with dissolvable stitches. Recovery involves some tenderness for a week or two, and the nail may look different as it regrows over the following months, but permanent nail damage is uncommon with experienced practitioners.
The American Academy of Dermatology recommends that any new or changing dark streak on a nail be examined by a dermatologist. You don’t need a referral in most insurance plans to book directly with a dermatologist, and the initial visit is typically a straightforward visual exam.