A black line on your nail is usually a streak of pigment running from the base to the tip, and in most cases it’s harmless. The medical term is longitudinal melanonychia, and it’s especially common in people with darker skin tones, appearing as a normal finding in African American, Hispanic, Indian, and Japanese populations. That said, a black line can sometimes signal a fungal infection, a medication side effect, a systemic health condition, or, rarely, a type of skin cancer called subungual melanoma. The key is knowing what to watch for.
Pigment Lines vs. Tiny Bleeding Streaks
Not all black lines on nails are the same thing. The two most common types look similar but have very different origins.
A pigment line (melanonychia) is a brown-to-black band that runs vertically along the nail. It forms when pigment-producing cells in the nail root become more active or multiply. These lines tend to be consistent in color and width, and they grow out slowly with the nail.
A splinter hemorrhage, on the other hand, is a thin reddish-brown or black line caused by tiny burst blood vessels under the nail. These are usually 1 to 3 millimeters long and appear closer to the fingertip rather than running the full length of the nail. When they first form, they look reddish or purple, then darken to brown or black within a few days. Stubbing a toe, getting acrylic nails, or even minor repetitive trauma can cause them. They grow out with the nail and disappear on their own.
Common Harmless Causes
The most frequent reason for a black or brown line on the nail is simply your body’s normal pigment production. In people with darker skin, multiple pigment bands across several nails are so common they’re considered a normal variant, sometimes called ethnic melanonychia. Pregnancy can also trigger multiple bands across the nails due to hormonal shifts in pigment activity.
Chronic, low-grade trauma is another everyday culprit. Tight shoes pressing on toenails, habitual nail biting, or repetitive friction from work or sports can stimulate pigment cells in the nail root to produce melanin. These single-band streaks typically affect only the nail being irritated and fade once the source of trauma stops. Postinflammatory hyperpigmentation, where a nail darkens after an injury or inflammation has healed, works the same way.
Fungal Infections That Darken Nails
Certain fungi can cause black or dark brown discoloration that mimics a pigment line. This is called fungal melanonychia, and it’s caused by pigment-producing molds and yeasts rather than the typical fungi behind most nail infections. Researchers have identified at least 29 different fungal species capable of producing this effect. Unlike a simple pigment band, a fungal infection usually comes with nail thickening, crumbling, or a change in texture. If your doctor suspects a fungal cause, a short course of antifungal treatment can confirm the diagnosis: if the dark color clears up, fungi were the culprit.
Medications and Drug Reactions
Certain medications can trigger black or brown nail lines by activating the pigment cells in the nail root. Chemotherapy drugs are the most well-known offenders, particularly cyclophosphamide, hydroxyurea, and doxorubicin. When a single group of pigment cells gets activated, the result is one distinct vertical band. When the drug activates pigment cells more broadly, the entire nail can darken. These changes are cosmetic side effects, not dangerous on their own, and they typically reverse once the medication is stopped.
Systemic Health Conditions
Less commonly, black lines on multiple nails can be a clue to an underlying health condition. Addison’s disease, which affects the adrenal glands, is one of the classic associations. Autoimmune conditions like lupus, Sjögren syndrome, and rheumatoid arthritis have also been linked to nail pigmentation changes, likely because the underlying blood vessel inflammation affects the nail bed. HIV, chronic kidney disease, and chronic hepatitis round out the list of systemic conditions that can show up as darkened nail lines. In these cases, the nail changes are rarely the only symptom, so they tend to appear alongside other signs of the disease.
When a Black Line Could Be Melanoma
Subungual melanoma is the most serious possible cause of a dark nail line, and also the rarest. It accounts for a small fraction of all melanomas but deserves attention because early detection dramatically improves outcomes. Dermatologists use an ABCDEF framework to evaluate suspicious nail lines:
- Age: Most subungual melanomas appear in adults, with peak incidence between ages 50 and 70.
- Band characteristics: A band wider than 3 millimeters, with irregular or blurred borders, or one that’s very dark brown to black raises more concern than a thin, well-defined line.
- Change: A line that’s getting wider, darker, or more irregular over weeks to months is a red flag. Stable, unchanging lines are far more likely to be benign.
- Digit: The thumb, index finger, and big toe are the most commonly affected digits for melanoma.
- Extension: Pigment that spreads beyond the nail onto the surrounding skin of the cuticle or fingertip (called Hutchinson’s sign) is one of the strongest warning signs.
- Family history: A personal or family history of melanoma or atypical moles increases risk.
A single new dark band on one nail in an adult, especially one that’s changing, warrants evaluation. If a dermatologist can’t rule out melanoma through visual exam alone, a biopsy of the nail root may be recommended. Doctors try to avoid biopsying the part of the nail root responsible for forming the nail surface, since scarring there can cause a permanent ridge or groove. But when there are signs of progression or the diagnosis is uncertain, biopsy is the definitive next step.
What to Pay Attention To
If you’ve noticed a new black line on a nail, the most important thing is context. Multiple thin lines across several nails in someone with darker skin is almost always normal pigmentation. A single dark line that appeared after you started a new medication, banged your finger, or changed your footwear usually has an obvious explanation.
The lines worth watching are the ones that are new, on a single nail, and have no clear cause. Track whether the band is stable or changing. Take a photo in good lighting and compare it a month later. If the line is widening, darkening, developing irregular edges, or if you notice any pigment spilling onto the skin around the nail, that’s when to get it looked at promptly. A dermatologist can often distinguish benign from concerning causes with a dermoscopy exam, a painless close-up magnification of the nail, before deciding whether further steps are needed.