A black line on your fingernail is usually a harmless streak of pigment called melanonychia. It’s extremely common in people with darker skin tones and often requires no treatment at all. In rare cases, though, a dark nail streak can signal melanoma, which is why understanding the difference matters.
Why Dark Lines Form in Nails
The nail plate itself is translucent. When pigment-producing cells at the base of your nail become activated, they deposit color into the nail as it grows, creating a vertical stripe that runs from the cuticle toward the tip. This can appear brown, dark brown, or black depending on how much pigment is involved.
The most common causes are all noncancerous:
- Normal pigmentation: People with medium to dark skin naturally develop these bands. Longitudinal melanonychia appears in 10 to 20% of Japanese individuals and roughly 1.4% of lighter-skinned populations. In a study of 68 Hispanic patients with dark nail lines, nearly 69% of cases were attributed to normal skin pigmentation alone. Multiple bands across several nails are a strong indicator that this is simply your body’s baseline.
- Nail trauma: Repeated injury from nail biting, picking, or friction from tight shoes can stimulate pigment cells. Even habitual tapping on a keyboard can do it over time.
- Moles: A small mole (nevus) can form in the nail matrix, producing a single dark band on one nail.
- Bruising: A bruised nail sometimes appears as a dark streak or spot. These typically grow out with the nail over several months.
Other Medical Causes
Beyond normal pigmentation and trauma, a range of health conditions can trigger dark nail lines. Fungal infections are one overlooked cause. At least 21 species of pigment-producing fungi have been linked to melanonychia. These organisms produce melanin that gets deposited in the nail, sometimes mimicking a simple pigmented band. If a dark line appears alongside nail thickening, crumbling, or a change in texture, a fungal infection is worth investigating.
Certain medications also cause nail darkening. Chemotherapy drugs, antiretroviral therapy for HIV, and antimalarial medications like hydroxychloroquine can all produce brown or black bands across one or more nails. These typically fade after stopping the medication, though it can take months for the nail to grow out completely.
Hormonal conditions, including Addison’s disease, Cushing’s syndrome, and overactive thyroid, can trigger melanonychia. So can vitamin B12 deficiency, pregnancy, and inflammatory skin conditions like psoriasis and lichen planus. When dark lines appear on multiple nails simultaneously and you haven’t had them before, a systemic cause is more likely than a local one.
Splinter Hemorrhages Look Different
If what you’re seeing looks less like a smooth vertical line and more like a tiny sliver of wood trapped under your nail, it’s likely a splinter hemorrhage. These are small bleeds under the nail that appear as thin, reddish-brown lines typically 1 to 3 millimeters long. They usually show up closer to the tip of the nail rather than running its full length. When they first form, splinter hemorrhages look reddish or purple, then darken to brown or black within a few days.
The key difference: splinter hemorrhages move with the nail as it grows and eventually disappear at the free edge. A true pigmented band stays anchored at the cuticle because the pigment-producing cells are in the nail matrix. Splinter hemorrhages are usually caused by minor trauma, though they can occasionally point to a heart valve infection or clotting issue if they appear on multiple nails without explanation.
When a Dark Line Could Be Melanoma
Subungual melanoma, a type of skin cancer that forms under the nail, is rare but serious. It produces a brown or black streak that typically starts at the cuticle and grows toward the tip. Dermatologists use a set of criteria to evaluate suspicious nail bands, and knowing them can help you decide whether to get yours checked.
The features that raise concern:
- Width of 3 millimeters or more with blurry or irregular borders
- Color variation within the band, such as areas of light brown mixed with very dark brown or black
- Change over time, including widening, darkening, or new irregularity in a band that was previously stable
- A single affected nail, especially the thumb or big toe
- Hutchinson’s sign: pigment that spreads beyond the nail onto the surrounding skin of the cuticle or nail fold. This is one of the most important warning signs.
- Age between 40 and 70, which is when subungual melanoma most commonly appears
- Personal or family history of melanoma or atypical moles
Subungual melanoma accounts for a larger proportion of melanoma cases in African American, Asian, and Native American populations, where it can represent up to one third of all melanomas diagnosed. This is partly because melanoma on sun-exposed skin is less common in these groups, making the nail a relatively more frequent site.
What To Look For Over Time
If you have multiple dark bands across several nails and darker skin, you’re almost certainly looking at normal ethnic melanonychia. No testing or treatment is needed, though it’s reasonable to mention it at your next dermatology visit for confirmation.
A single new dark line on one nail deserves closer attention. Take a photo of it with good lighting and a ruler or coin for scale. Check it again in a month or two. A band that stays the same width, color, and pattern is far less worrying than one that’s changing. If it widens, if the borders become uneven, if the color becomes streaky or varied, or if pigment starts leaking onto the skin around the nail, get it evaluated promptly. A dermatologist can examine the band with a dermatoscope and, if needed, biopsy the nail matrix to rule out melanoma.
One reassuring detail: the nail grows slowly, roughly 3 to 4 millimeters per month for fingernails. A dark line caused by a one-time injury or a temporary medication will gradually grow out and disappear over four to six months. A line that persists because the source is in the nail matrix will remain indefinitely, which is normal for pigmented bands but warrants evaluation if the appearance is changing.