A dark line running vertically along your nail is called longitudinal melanonychia, and in most cases it’s completely benign. It happens when pigment-producing cells in the nail root deposit melanin into the nail plate as it grows, leaving a visible streak that can range from light brown to black. The line typically runs from the base of the nail toward the tip. While the vast majority of these lines are harmless, a small percentage are caused by melanoma under the nail, which is why any new or changing dark line deserves attention.
Why Dark Lines Are Common in Darker Skin
The single most common cause of dark nail lines is simply having more active pigment cells in the nail matrix. This is so prevalent in people with darker skin tones that nearly all Afro-Caribbean individuals develop brown-black nail pigmentation by age 50. It affects up to 20% of Japanese people as well, and is frequently seen in Hispanic and Indian populations. In these cases, the lines often appear on multiple nails at once, which is actually a reassuring sign. White-skinned individuals develop these lines far less often, which means a new dark streak in this group warrants closer evaluation.
Pregnancy can also trigger multiple pigmented bands across several nails due to hormonal changes in melanin production. These typically fade after delivery, though they can persist.
Trauma and Bleeding Under the Nail
Not every dark mark on a nail comes from pigment cells. Injury is one of the most common culprits. A subungual hematoma, essentially a bruise trapped under the nail plate, can look like a dark spot or streak after you jam a finger in a door or stub a toe. These marks grow out with the nail over weeks to months and eventually disappear.
Splinter hemorrhages are a related but distinct pattern. These are tiny streaks of blood under the nail that look like thin reddish-brown lines running in the direction of nail growth, usually 1 to 3 millimeters long. They tend to appear closer to the tip of the nail. When they first form, they look red or purple, then darken to brown or black within a few days. Stubbing your toe, getting acrylic nails applied, or even minor repetitive trauma can cause them. They move with the nail as it grows and resolve on their own.
Splinter hemorrhages that appear without any obvious injury can sometimes signal an underlying condition. Nail psoriasis, lupus, and blood vessel inflammation are all associated with them. People taking blood thinners like aspirin or warfarin see them more frequently, and they show up in 60% to 70% of people on certain cancer-targeted therapies.
Medications That Cause Nail Pigmentation
Certain drugs activate the pigment-producing cells in the nail root, causing brown-to-black bands to appear on one or more nails. Cancer chemotherapy agents are the most common offenders. Hydroxyurea, a medication used for blood disorders and some cancers, is particularly well known for this side effect. Antiretroviral drugs used for HIV, certain antimalarials, and systemic retinoids can also trigger it. The pigmentation usually develops gradually after starting the medication and may fade once the drug is stopped, though it can take months for the affected nail to grow out completely.
Fungal Infections That Darken Nails
Fungal nail infections don’t always look like the thick, yellow, crumbling nails most people picture. Some fungal species produce dark pigments that stain the nail plate brown or black, closely mimicking a pigmented streak. More than 20 species of pigment-producing fungi and at least 8 non-pigmented species have been linked to dark nail discoloration.
A few clues help distinguish fungal melanonychia from other causes. The nail is more likely to be thickened, crumbly, or lifted from the nail bed. Inflammation around the nail is common. The discoloration may be more diffuse rather than a crisp, well-defined line. A lab test where a small nail sample is examined under a microscope can confirm the presence of fungal organisms and distinguish infection from a pigment-cell problem.
Systemic Diseases Linked to Dark Nail Lines
Less commonly, dark nail bands can be a visible sign of an internal condition. Addison’s disease, where the adrenal glands don’t produce enough hormones, causes widespread skin and nail darkening. Autoimmune connective tissue diseases, including lupus, Sjögren syndrome, and systemic sclerosis, have been documented in patients with multiple pigmented nail streaks. Chronic kidney disease and chronic hepatitis are other associations. Peutz-Jeghers syndrome, a genetic condition that causes intestinal polyps, also presents with characteristic nail and lip pigmentation.
In these situations, the nail lines are rarely the only symptom. Fatigue, joint pain, skin changes, or digestive issues typically accompany the nail findings and point toward the underlying diagnosis.
When a Dark Line Could Be Melanoma
Subungual melanoma, a cancer that starts in the pigment cells of the nail matrix, is the most serious cause of a dark nail line. It’s rare overall but disproportionately affects people with darker skin, where it accounts for a larger share of all melanoma diagnoses. It most commonly appears on the thumb or big toe.
Certain features raise concern. A single band that is wider than 3 millimeters, has irregular or blurred borders, or contains multiple shades of brown and black deserves prompt evaluation. A band that is rapidly widening or darkening over weeks to months is more worrisome than one that has stayed the same for years. One of the most important warning signs is called Hutchinson’s sign: pigment that spills beyond the nail onto the surrounding skin of the cuticle or fingertip. This discoloration of the skin around the nail strongly suggests melanoma.
Other red flags include a dark line appearing for the first time in an adult over 50, a band on only one nail (especially in a lighter-skinned person), and any bleeding, nail cracking, or separation of the nail from the bed without clear injury.
How a Dark Nail Line Is Evaluated
A dermatologist will typically start by examining the nail with a dermatoscope, a handheld magnifying device that illuminates the nail plate and reveals the pattern of pigment underneath. Certain patterns, like regular, evenly spaced lines, suggest a benign cause. Irregular spacing, variable color, and disrupted patterns point toward something that needs further investigation.
If the appearance is suspicious, a nail matrix biopsy is the next step. This involves removing a small sample of tissue from the root of the nail where the pigment originates. Several techniques exist, including a small circular punch through the nail plate or a thin shave of the matrix tissue. The sample is then examined under a microscope to determine whether the pigment cells are normal, forming a benign mole, or showing signs of melanoma. The procedure is done under local anesthesia, and while it can cause temporary nail deformity, it provides a definitive answer.
For lines that appear clearly benign, particularly multiple symmetrical bands in someone with darker skin, no biopsy may be needed. Instead, your dermatologist may photograph the nail and monitor it over time, looking for any changes in width, color, or border.