A black or dark brown line running from the base of your nail to the tip is called longitudinal melanonychia. In most cases, it’s a harmless streak caused by pigment-producing cells in the nail matrix, but it can occasionally signal something more serious, including nail melanoma. The difference between a benign line and a dangerous one comes down to specific features you can check yourself.
Why Dark Lines Form in Nails
The nail matrix, the tissue just beneath your cuticle where new nail grows, contains cells that produce melanin. When these cells become activated or increase in number, they deposit pigment into the nail plate as it grows outward, creating a visible vertical stripe. This activation can happen for several reasons: normal ethnic variation, repeated physical trauma, certain medications, hormonal changes (especially during pregnancy), or, rarely, melanoma forming beneath the nail.
The line itself is pigment locked into the nail plate, so it moves forward as the nail grows. Fingernails grow at roughly 3.5 mm per month, while toenails grow at about 1.6 mm per month. That growth rate matters because a line caused by a one-time event, like an injury, will eventually grow out and disappear. A line that persists or widens is being continuously fed by active cells at the base.
The Most Common Cause: Normal Pigmentation
Ethnicity is the single biggest factor. Nearly all people of African descent develop dark brown or black nail pigmentation by age 50. It also affects up to 20% of Japanese people and is common in other groups with darker skin tones, including Hispanic and Native American populations. In these cases, you may see lines on multiple nails, and they tend to be uniform in color and width. This is completely benign and requires no treatment.
Children and teenagers occasionally develop these bands too, often triggered by minor, repeated trauma to the nail (like biting or picking at it). These childhood streaks frequently fade on their own over time.
Other Harmless Causes
Several medications can trigger dark nail lines. Chemotherapy drugs are the most well-known culprits, but antiretroviral medications used for HIV, certain antibiotics in the tetracycline family, and antimalarial drugs can all cause it. When a medication is responsible, the lines typically appear on more than one nail at roughly the same time and fade after the drug is stopped, though it takes months for the nail to fully grow out.
Hormonal shifts during pregnancy can also activate melanin production in the nail matrix, producing temporary streaks that resolve after delivery. Fungal nail infections occasionally cause dark discoloration too, though they more commonly turn nails yellow or white and make them thick and crumbly rather than producing a clean, defined line.
Splinter Hemorrhages: A Different Kind of Line
Not every dark line under a nail is melanin. Splinter hemorrhages are thin, reddish-brown streaks caused by tiny blood clots in the small capillaries beneath the nail plate. They run vertically, just like pigment bands, but they’re usually shorter (not spanning the full length of the nail) and have a distinctly reddish tone rather than true brown or black.
The most common cause is simple injury to the nail. If you recently jammed your finger or subjected your nails to repetitive impact, a splinter hemorrhage is the likely explanation and will grow out on its own. However, splinter hemorrhages that appear without any history of trauma can sometimes point to other conditions, including infections of the heart valves or autoimmune disorders, so unexplained ones are worth mentioning to your doctor.
When a Black Line Could Be Melanoma
Subungual melanoma, a cancer that starts in the nail matrix, is rare but serious. It accounts for a larger proportion of melanoma cases in people with darker skin. The peak incidence is between the ages of 50 and 70, and it most commonly affects the thumb or big toe.
Dermatologists use an ABCDEF screening framework to evaluate suspicious nail lines:
- Age and ancestry: Peak risk in the 50 to 70 age range, with higher rates in African American, Asian, and Native American populations.
- Band characteristics: A brown-to-black band wider than 3 mm with irregular or blurred borders is more concerning than a thin, uniform line.
- Change: A band that is getting wider, darker, or more irregular over weeks or months. A nail condition that doesn’t improve despite treatment also raises concern.
- Digit: The thumb and big toe are the most commonly affected.
- Extension of pigment: Pigment that spreads beyond the nail onto the surrounding skin of the cuticle or nail fold. This is called Hutchinson’s sign and is one of the strongest warning signals.
- Family history: A personal or family history of melanoma or atypical moles increases risk.
No single feature on this list confirms melanoma. A wide band on one nail in a 60-year-old with no other explanation is more suspicious than narrow, stable lines across several nails in a 25-year-old. The combination of features matters.
What Hutchinson’s Sign Looks Like
This is the single most important visual clue. If the dark pigment from your nail band bleeds into the skin around the nail, particularly the cuticle area or the skin along the sides of the nail, that discoloration of surrounding skin is Hutchinson’s sign. Benign nail pigmentation stays within the nail plate. Melanoma can spread pigment into the tissue around it. If you notice darkening of the skin next to a pigmented nail, get it evaluated promptly.
How Doctors Evaluate a Suspicious Line
A dermatologist will first examine the line using a dermatoscope, a handheld magnifying device with a light source that reveals color patterns not visible to the naked eye. Many benign lines can be confidently identified at this stage without any procedure.
If the line looks suspicious, the next step is a biopsy of the nail matrix. This involves lifting or partially removing the nail plate to access the pigmented cells at the base. Several techniques exist, including punch biopsy (taking a small circular sample) and a method sometimes called the “trap door” approach, where the nail fold is reflected back to give a direct view of the matrix so the dermatologist can precisely target the pigmented area. The procedure is done under local anesthesia. Recovery depends on how much tissue was sampled, but you can generally expect the nail to look abnormal for several months as it regrows.
What You Should Watch For
If you have a single dark line on one nail that appeared recently and you’re over 40, keep a close eye on it. Take a photo with your phone next to a ruler so you can track any changes in width or color over the coming weeks. A line that stays the same width and color for months is far less worrying than one that is actively changing.
Seek evaluation from a dermatologist if the band is wider than 3 mm, if it has uneven coloring (streaks of different shades within the band), if the borders are blurred or irregular, if the pigment is spreading onto the skin around the nail, or if the nail itself is cracking, splitting, or distorting near the line. Multiple uniform bands across several nails, on the other hand, almost always reflect normal pigmentation or a systemic cause like medication.