A black or dark brown vertical line on your nail is called longitudinal melanonychia, and it’s usually harmless. The most common causes are nail trauma, moles within the nail, genetics, and normal aging. In rare cases, though, a dark line can signal a type of skin cancer called subungual melanoma, which is why any new or changing line deserves attention.
The Most Common Causes
The vast majority of dark nail lines fall into a handful of noncancerous categories. A bruised nail or repeated minor trauma (from nail biting, picking, or tight shoes) can activate the pigment-producing cells in your nail matrix, leaving a visible streak as the nail grows out. A mole beneath the nail does the same thing. Age spots that form in the nail bed are another frequent culprit, as are certain skin conditions like psoriasis and lichen planus.
Genetics plays a major role. People with medium to dark skin tones naturally have more active pigment cells, and vertical nail lines are extremely common in this group. Multiple lines across several nails are typical and almost always benign.
Splinter Hemorrhages Look Similar
Sometimes what looks like a dark line is actually a tiny bleed under the nail, called a splinter hemorrhage. These appear as thin streaks, usually 1 to 3 millimeters long, running in the direction of nail growth. They start out reddish or purple and darken to brown or black within a few days. The key difference is that splinter hemorrhages tend to appear near the tip of the nail and move outward as the nail grows, eventually disappearing on their own. A true pigment line, by contrast, stays anchored in the same spot at the base of the nail because the color is being produced continuously by cells in the nail matrix.
Medical and Nutritional Triggers
A number of medications can switch on pigment production in the nail. Certain chemotherapy drugs, antiretrovirals used for HIV, antimalarials, and the blood disorder medication hydroxyurea are well-documented causes. These drugs activate clusters of melanocytes in the nail matrix, producing single or multiple bands that range from light brown to black. The lines often fade once the medication is stopped, though it can take months for a nail to fully grow out.
Vitamin B12 deficiency, thyroid disorders, Addison’s disease, Cushing’s syndrome, and pregnancy can also trigger nail pigmentation. Fungal infections are another possibility, particularly on toenails. Fungal lines tend to look different under magnification: they often appear as a diffuse discoloration toward the tip of the nail rather than a crisp, well-defined band, and they may come with flaky scaling on the nail surface or yellowish-white streaks.
When a Line Could Be Melanoma
Subungual melanoma is rare, but it’s the reason dermatologists take dark nail lines seriously. It accounts for a disproportionately high share of melanoma cases in African American, Asian, and Native American populations, where it can represent up to one third of all melanomas diagnosed. The median age at diagnosis is around 66, though it can occur earlier.
Dermatologists use a set of warning signs sometimes called the ABCDEF criteria to screen for it:
- Age and ancestry: Peak incidence is between the 50s and 70s, with higher rates in people of African, Asian, or Native American descent.
- Band characteristics: A brown-to-black band wider than 3 millimeters, with uneven or blurry borders.
- Change: A band that grows wider, darker, or more irregular over time, or a nail problem that doesn’t improve with treatment.
- Digit: The thumb and big toe are the most commonly affected.
- Extension of pigment: Color that bleeds beyond the nail onto the surrounding skin fold (known as Hutchinson’s sign) is a red flag.
- Family history: A personal or family history of melanoma or atypical moles raises risk.
Nail plate destruction, dark dots or globules within the band, and ulceration are additional features that point toward melanoma rather than a benign cause or a fungal infection.
What Happens if You Get It Checked
If a dermatologist sees a line that looks suspicious, the next step is typically a biopsy of the nail matrix, the tissue at the base of the nail where the line originates. For lines narrower than 3 millimeters, a small punch biopsy is a standard approach. Wider or more concerning bands may require a shave biopsy of the matrix to get a larger tissue sample.
The biopsy itself is done under local anesthesia. You can expect some soreness and a temporary nail deformity as the nail regrows, but the nail usually recovers fully over several months.
Why Early Detection Matters
Subungual melanoma caught at an early stage has a survival rate of about 95%. When diagnosis is delayed and the cancer has spread, that number drops to as low as 15%. The challenge is that nail melanoma is easy to dismiss or miss entirely. A dark streak on a toenail can go unnoticed for months or years, especially if it’s hidden under nail polish.
A single new dark line on one nail, particularly if it’s wider than 3 millimeters, has irregular borders, or is changing in any way, warrants a dermatology visit. Multiple uniform lines across several nails, especially in someone with darker skin, are far less concerning and are usually a normal variant. The distinction between “watch and wait” and “get it checked now” comes down to whether the line is solitary, new, and evolving.