Black or dark brown lines running vertically down your fingernail are called melanonychia, and they’re usually harmless. The most common causes are normal pigment deposits, minor nail trauma, and genetics, particularly in people with darker skin tones. That said, a new or changing dark line can occasionally signal something more serious, including nail melanoma, so it’s worth understanding what to look for.
Why Dark Lines Appear in Nails
The cells that produce pigment (melanin) live in the nail matrix, the tissue just beneath the base of your nail. When those cells become activated, they deposit melanin into the growing nail plate, creating a vertical streak that travels outward as the nail grows. The streak can be light brown, dark brown, or black depending on how much melanin is produced and your natural skin tone.
Several things can trigger this pigment production:
- Genetics and skin tone: People with medium to dark skin develop these lines so commonly that they’re sometimes called “ethnic melanonychia.” Among people of Caribbean descent, nail discoloration is common by age 50. Nearly 20% of people of Japanese descent have benign melanonychia. Prevalence increases with age across all darker skin types.
- Repeated trauma: Nail biting, picking at cuticles, or friction from tight shoes can irritate the nail matrix enough to activate pigment cells.
- Moles: A small mole (nevus) can form in the nail matrix, producing a consistent pigmented band.
- Bruising: A direct injury to the nail bed can cause a dark streak or spot as blood is trapped beneath the nail.
- Age spots: Lentigos, the same type of spots that appear on sun-exposed skin, can develop in the nail.
These benign causes share a few reassuring features: the bands tend to be narrow (under 3 mm), uniform in color, and stable over time. Having lines on multiple nails also points toward a harmless explanation, since melanoma almost always affects a single nail.
Medications That Cause Nail Lines
Certain drugs can activate pigment cells in the nail matrix, producing one or more dark vertical bands or even darkening the entire nail plate. Chemotherapy drugs are the most well-known culprits, particularly doxorubicin, cyclophosphamide, hydroxyurea, and 5-fluorouracil. Antimalarial medications and antiretroviral therapy for HIV can also cause it. The discoloration typically appears weeks to months after starting the medication and often fades after the drug is stopped, though it can take many months for the affected nail to grow out completely.
Splinter Hemorrhages Look Different
Not every dark line in a nail comes from pigment. Splinter hemorrhages are thin, reddish-brown streaks caused by tiny burst blood vessels under the nail. They look like small splinters embedded beneath the nail surface and run vertically along the direction of nail growth.
A single splinter hemorrhage after bumping your hand is nothing to worry about. But when they appear on multiple nails without an obvious injury, they can signal an underlying condition. Between 15% and 33% of people with endocarditis (a heart valve infection) develop splinter hemorrhages. They also show up in autoimmune conditions like lupus and antiphospholipid syndrome, which cause blood vessel inflammation. Up to 35% of people with lichen planus report them, and they’re common in people on certain cancer medications called kinase inhibitors (appearing in 60% to 70% of patients taking those drugs). Chronic kidney disease is another association.
When a Line Could Be Melanoma
Subungual melanoma, cancer arising from pigment cells in the nail matrix, is rare but serious. It accounts for a small fraction of all melanomas, but it’s disproportionately common in Black, Asian, Hispanic, and Native American populations, where it represents up to one-third of all melanoma cases. Peak incidence falls between ages 50 and 70.
Dermatologists use a set of criteria called the ABCDEF rule to evaluate suspicious nail lines:
- A (Age and ancestry): Most common between ages 50 and 70, and in African American, Asian, and Native American individuals.
- B (Band characteristics): A band that is 3 mm or wider, brown to black in color, with blurred or irregular borders.
- C (Change): The band is getting wider, darker, or more uneven over time, or it isn’t improving with treatment for a presumed benign cause.
- D (Digit): The thumb and index finger are most commonly affected. A single digit is more suspicious than multiple digits.
- E (Extension): Pigment spreading beyond the nail onto the surrounding skin fold, known as Hutchinson’s sign, is a red flag.
- F (Family history): A personal or family history of melanoma or atypical moles increases risk.
By contrast, features that suggest a benign cause include onset at a young age, lines on several nails, narrow bands with sharp and even borders, and no change over months or years.
Other Medical Causes
Dark nail lines can occasionally reflect systemic health issues beyond cancer. Hormonal conditions like Addison’s disease, Cushing’s syndrome, and overactive thyroid can trigger excess melanin production in the nails. Nutritional deficiencies, especially vitamin B12, are another recognized cause. Fungal and bacterial nail infections sometimes produce dark pigment as well, though they’re more likely to cause general discoloration, thickening, or crumbling rather than a clean vertical line. Inflammatory skin conditions like psoriasis and lichen planus can also affect the nails. Pregnancy-related hormonal shifts occasionally darken the nails temporarily.
How Nail Lines Are Evaluated
A dermatologist will typically start by examining the nail with a dermatoscope, a handheld magnifying device with a light. This allows them to see the pattern, color, and borders of the pigmented band in detail. In many cases, especially when the band is narrow, uniform, and stable, no further testing is needed.
If the line looks suspicious, a nail matrix biopsy is the next step. This involves removing a small sample of tissue from the base of the nail where the pigment originates. Several techniques exist, including punch biopsy, shave biopsy, and a method called the “trap door” approach, which lifts part of the nail to give the dermatologist a direct view of the nail matrix for a precise, targeted sample. The procedure is done under local anesthesia. Recovery involves some temporary nail deformity as the nail regrows, which can take several months.
A biopsy is the only definitive way to distinguish between a benign mole in the nail matrix and early melanoma. Because darker-skinned patients tend to have wider and darker bands even when the cause is benign, clinical evaluation alone can be particularly tricky in this group. Research has found that darker-skinned patients receive more biopsies, partly because their bands are more likely to have features that overlap with melanoma warning signs.