A dark line running lengthwise on your nail is usually caused by increased melanin production in the nail matrix, the tissue at the base of your nail where new nail cells form. The most common explanation is completely benign, especially if you have darker skin. But because a small percentage of these lines turn out to be nail melanoma, understanding what yours looks like and how it behaves over time matters.
What the Line Actually Is
The pigment-producing cells in your nail matrix normally sit dormant. When something activates them, they start depositing melanin into the nail as it grows, creating a vertical streak that runs from the base of the nail to the tip. This is called longitudinal melanonychia. The line can range from light brown to jet black, and it can be narrow as a thread or wide enough to cover a large portion of the nail.
There are two distinct processes behind it. In one, the number of pigment cells stays normal but they simply produce more melanin. This is what happens with ethnic variation, pregnancy, trauma, and medication side effects. In the other, the number of pigment cells actually increases, which is what happens with moles (nevi) in the nail and, in rare cases, melanoma. In children, moles account for about half of all dark nail lines. In adults, they account for roughly 12%.
Skin Tone and Ethnic Variation
If you’re Black, Hispanic, South Asian, Japanese, or have darker skin in general, a dark line on one or more nails may simply be part of your normal pigmentation. This is the single most common cause. In a study of 68 Hispanic patients with dark nail lines, nearly 69% had lines caused by their natural skin pigmentation alone. These lines typically appear on multiple nails rather than just one, and they often develop gradually during adulthood without any other changes to the nail.
Trauma and Physical Injury
Stubbing your toe, wearing tight shoes, habitual nail biting, or picking at your nails can all trigger a dark line. Physical trauma damages tiny blood vessels under the nail or activates pigment cells in the matrix. When blood vessels break, you may see what’s called a splinter hemorrhage: a thin, reddish-brown line that runs in the direction of nail growth, usually 1 to 3 millimeters long and closer to the tip of the nail. These start out red or purple and darken to brown or black within a few days. They’re harmless and grow out as the nail grows.
Chronic, repeated trauma (like tight shoes pressing on a toenail over months) can produce a more persistent pigmented band that lasts as long as the irritation continues.
Fungal Infections
Certain fungal species can produce dark pigment that mimics a melanin streak. A common nail fungus called Trichophyton rubrum occasionally produces a diffusible black pigment, creating a dark toenail that looks nearly identical to a pigmented line from melanocytes. In the Hispanic patient study mentioned above, about 7% of dark nail lines were caused by fungal infection. A clue is that fungal lines tend to come with other nail changes: thickening, crumbling, or separation of the nail from the nail bed.
Medications and Nutritional Deficiencies
Several medications can activate dormant pigment cells in the nail matrix. When a drug is the cause, the pattern is distinctive: multiple nails are affected, often with several bands on each nail. Chemotherapy drugs are well-known triggers, but antimalarials and certain other medications can do it too. The lines typically fade after the medication is stopped, though it takes months for the affected nail to fully grow out.
Vitamin B12 deficiency is another recognized cause. When B12 levels drop, levels of a protective molecule called glutathione also fall. Glutathione normally keeps an enzyme involved in melanin production in check. Without enough of it, melanin production ramps up, and the excess pigment can show up as dark bands across multiple nails. This is reversible with B12 supplementation, though it takes time for the new, unpigmented nail to replace the old growth.
Pregnancy and Hormonal Changes
Pregnancy can cause dark bands on multiple nails, driven by the same hormonal shifts that darken the linea nigra on the abdomen or deepen the color of the areolae. These lines are harmless and typically fade after delivery, though they may take several months to grow out completely.
Splinter Hemorrhages From Systemic Conditions
While trauma is the most common cause of splinter hemorrhages, certain medical conditions can produce them too. Nail psoriasis and lichen planus frequently cause these tiny bleeds. Inflammation of the blood vessels, seen in conditions like lupus, is another trigger. Endocarditis, an infection of the heart valves, causes splinter hemorrhages in 15% to 33% of affected people. If you notice splinter hemorrhages on multiple nails without any history of injury, it’s worth getting checked for an underlying condition.
When a Dark Line Could Be Melanoma
Nail melanoma (subungual melanoma) is rare, but it’s serious and often caught late because people assume the line is harmless. It accounts for a disproportionately high share of melanoma in people with darker skin. In Black, Asian, and Native American populations, nail melanoma makes up as much as one third of all melanoma cases.
Dermatologists use an ABC framework to evaluate suspicious nail lines:
- Age: Peak incidence is between the 50s and 70s. Higher risk in Black, Asian, and Native American individuals.
- Band characteristics: A brown-to-black band that’s 3 millimeters or wider, with uneven or blurred borders, is more concerning than a thin, uniform line.
- Change: A band that’s getting wider, darker, or more irregular over weeks or months raises concern, especially if the nail itself isn’t responding to treatment for other conditions.
One particularly important warning sign is pigmentation that spreads beyond the nail itself onto the surrounding skin of the cuticle or nail fold. This is called Hutchinson’s sign, first described in 1886, and it strongly suggests melanoma. There is a benign look-alike (pseudo-Hutchinson sign), but it can only be confirmed after melanoma has been ruled out.
How a Suspicious Line Is Evaluated
A dermatologist will first examine the nail with a dermatoscope, a magnifying tool with a light that reveals pigment patterns invisible to the naked eye. If the pattern looks concerning, the next step is a biopsy of the nail matrix, the tissue where the pigment originates. This involves numbing the finger or toe and removing a small sample of tissue from under the base of the nail. Several techniques exist depending on the size and location of the line, ranging from a small punch sample to a narrow strip of tissue taken lengthwise along the nail.
Biopsies of the nail matrix can temporarily affect how the nail grows back, sometimes leaving a slight ridge or groove. But when melanoma is a possibility, the diagnostic information is worth it. If caught early, nail melanoma has a much better prognosis than when it’s discovered after it has grown deeper or spread.
What to Look For on Your Own Nail
A single dark band on one nail that appeared recently and is changing deserves evaluation. A band that’s wider than 3 millimeters, has blurry or irregular edges, or varies in color from one side to the other is more concerning than a thin, uniform, light-brown line. Pigment spreading onto the skin around the nail is the most urgent sign.
On the other hand, if you have darker skin and notice faint lines on several nails that have been stable for years, that pattern is consistent with normal ethnic pigmentation. Similarly, if the line appeared after an injury or during pregnancy, the cause is likely straightforward. Context matters: your skin tone, the number of nails affected, whether the line is changing, and whether you’re on any medications that could explain it all help distinguish the common from the concerning.