Black lines on fingernails are pigmented streaks running from the base of the nail to the tip, caused by melanin deposited into the nail plate as it grows. They range from completely harmless to, in rare cases, a sign of nail melanoma. The most common cause by far is normal melanin activity, especially in people with darker skin tones. But certain features of these lines, like their width, color variation, and whether they affect one nail or many, help distinguish routine pigmentation from something that needs medical attention.
Why Black Lines Form in Nails
The nail grows from a tissue called the matrix, tucked under the base of your nail. Melanocytes (pigment-producing cells) live in this matrix, and when they become more active or multiply, they deposit melanin into the nail plate as it grows outward. This creates a vertical brown-to-black streak that travels the length of the nail. The medical term is longitudinal melanonychia.
The streak can be a single thin line or a broader band. It can appear on one nail or several. The pattern matters because it points toward different causes, from genetics to medications to, occasionally, cancer.
Normal Pigmentation Is the Most Common Cause
The single most frequent reason for black lines on nails is physiologic melanonychia, meaning the melanocytes in your nail matrix are simply more active than average. This is extremely common in people with darker skin. It typically shows up as multiple bands across several nails, often on both hands. The lines tend to be uniform in color and width, and they don’t change much over time.
Pregnancy can also trigger these streaks, as hormonal shifts increase melanin production throughout the body. Chronic minor trauma to a nail, like repeated tapping or pressure from tight shoes (more common on toenails), can activate melanocytes in the same way. In all of these scenarios, the lines are benign and don’t require treatment.
Medications That Cause Nail Pigmentation
Certain drugs activate melanocytes in the nail matrix, producing brown or black bands that can appear on one or more nails. Chemotherapy drugs are the most well-known culprits, with hydroxyurea being particularly associated with nail darkening. Antimalarial medications and some antibiotics, including minocycline, can do the same.
Drug-induced lines generally affect several nails at once, often with multiple bands per nail. They typically fade after the medication is stopped, though it can take months for the discolored nail to fully grow out.
Splinter Hemorrhages Look Similar but Aren’t
Tiny bleeds under the nail, called splinter hemorrhages, can mimic the appearance of melanin streaks. These are thin, reddish-brown to black lines that run vertically, usually 1 to 3 millimeters long, and they tend to appear closer to the tip of the nail rather than running its full length. When fresh, they look reddish or purple, then darken to brown or black within a few days.
The most common cause is simple trauma: jamming a finger, getting acrylic nails applied, or even vigorous hand use. However, splinter hemorrhages can occasionally signal a more serious problem. Between 15% and 33% of people with endocarditis, an infection of the heart valves, develop splinter hemorrhages. If you notice them on multiple nails without any obvious injury, that’s worth mentioning to a doctor.
Fungal Infections Can Darken Nails Too
Most people associate nail fungus with thick, yellowish nails, but certain fungi actually produce melanin and can create dark brown or black streaks that look remarkably similar to melanin-based lines. At least 21 different fungal species have been linked to this type of pigmented nail infection. Some are common nail fungi that happen to produce pigment; others are rarer species that are naturally dark-colored when they grow.
Fungal melanonychia is more likely if the nail is also thickened, crumbly, or has an unusual texture alongside the dark coloring. A doctor can confirm or rule out a fungal cause with a nail clipping or culture.
Underlying Health Conditions
Several systemic conditions can cause dark streaks on the nails. Addison’s disease, where the adrenal glands don’t produce enough hormones, triggers widespread darkening of the skin and nails. HIV infection has been linked to nail pigmentation as well. These are less common causes, and they almost always come with other noticeable symptoms, like fatigue, weight changes, or skin darkening in other areas of the body.
When a Black Line Could Be Melanoma
Subungual melanoma, a type of skin cancer that starts under the nail, is rare but serious. It accounts for a small fraction of all melanomas, but it’s often diagnosed late because people assume the dark streak is harmless. Dermatologists use a set of clinical criteria, sometimes called the ABCDEF rule, to evaluate suspicious nail lines.
Features that raise concern include:
- Width of 3 millimeters or more with uneven or blurry borders
- Color variation within the band, such as areas of light brown mixed with very dark brown or black
- Change over time, including a band that’s getting wider, darker, or more irregular
- A single nail affected, especially in someone without darker skin
- Pigment spreading beyond the nail onto the surrounding skin fold (known as Hutchinson’s sign), which is one of the most specific warning signs for melanoma
Subungual melanoma typically affects a single digit. It’s more common on the thumb or big toe, and it tends to appear in adults over 50. A new, solitary dark band in someone with lighter skin deserves prompt evaluation.
How Doctors Evaluate a Suspicious Line
A dermatologist will examine the line’s width, color pattern, borders, and whether pigment extends beyond the nail. They’ll also consider your age, skin tone, number of nails involved, and any recent medication changes. A specialized magnifying tool called a dermatoscope lets them see the pigment pattern in more detail, which helps distinguish benign melanocyte activation from abnormal growth.
If the line looks suspicious or the diagnosis isn’t clear from examination alone, the next step is a biopsy of the nail matrix. This involves numbing the finger and taking a small tissue sample from the base of the nail where the pigment originates. It’s a minor outpatient procedure, but it provides a definitive answer. Not every dark line needs a biopsy. The goal of the clinical evaluation is to identify which lines can be safely monitored and which need tissue sampling to rule out melanoma.
What to Watch For
If you have dark skin and notice uniform bands across multiple nails, this is almost certainly normal pigmentation. If you’re taking a medication known to cause nail darkening, the connection is likely straightforward. The lines to pay attention to are new, solitary, dark bands on a single nail, particularly ones that are wider than a few millimeters, have uneven color, or are changing over weeks to months. Pigment that bleeds onto the skin around your nail is the clearest red flag. Photographing the line periodically can help you track whether it’s stable or evolving.