Black lines running vertically from the base of your nail to the tip are usually caused by increased pigment production in the nail matrix, the tissue just beneath your cuticle where new nail grows. The medical term is longitudinal melanonychia, and in most cases the cause is completely harmless. The most common source of brown-to-black nail pigmentation is actually a bruise under the nail, followed by normal ethnic pigmentation, trauma, infections, and nutritional deficiencies. Rarely, a dark nail streak can signal melanoma, which is why understanding the differences matters.
Normal Pigmentation by Skin Tone
If you have darker skin, vertical dark lines on your nails may simply be part of your normal pigmentation. Among African Americans, the prevalence of these streaks reaches about 77%, and in one study of Black patients, the rate climbed from 2.5% in children under 3 to 96% in adults over 50. A Brazilian study of people with medium-to-dark skin tones found melanonychia in 73% of participants. By contrast, a study of over 2,400 Chinese adults found just 1.7% prevalence in those over 50 and none in patients under 20.
When pigmentation appears on multiple nails symmetrically, is consistent in width, and has been present for years, it’s almost certainly benign. These bands tend to darken gradually with age and require no treatment.
Bruises Under the Nail
A subungual hematoma, or bruise beneath the nail, is the single most common cause of brown-to-black nail discoloration. It can follow a single obvious injury (slamming your finger in a door) or develop from repeated, low-grade trauma you barely notice, like tight shoes pressing against a toenail during running.
The key feature of a nail bruise is that it moves. Because the discoloration sits within or beneath the nail plate, it gradually migrates toward the tip as the nail grows out, eventually disappearing when that section of nail is trimmed off. The color typically has sharp edges and ranges from red to purple-black. If you watch the spot over several weeks and it clearly shifts toward the free edge of your nail, a bruise is the most likely explanation.
Nutritional Deficiencies and Systemic Causes
Your nails can act as a window into what’s happening inside your body. Deficiencies in vitamin B12, vitamin D, and protein have all been linked to dark nail streaks. The mechanism is the same as ethnic pigmentation: the pigment-producing cells in your nail matrix become overactive, but in this case the trigger is a nutritional shortfall rather than genetics.
Certain systemic diseases can also flip this switch. Addison’s disease, which affects the adrenal glands, is a well-known cause. Thyroid disorders, Cushing’s syndrome, and conditions involving excess iron storage or bilirubin buildup can do the same. If dark lines appear on several nails at once and you also have fatigue, weight changes, or skin darkening elsewhere, a systemic cause is worth investigating with bloodwork.
Medications That Cause Dark Streaks
Chemotherapy drugs are the most common medications to cause nail darkening, but they’re not the only ones. Antiretroviral drugs used for HIV, certain anti-seizure medications, and drugs used alongside UV light therapy for skin conditions can all trigger melanonychia. In most medication-related cases, the lines appear on multiple nails and develop within weeks to months of starting the drug. The pigmentation typically fades after the medication is stopped, though it can take many months for a fully pigmented nail to grow out.
Infections and Skin Conditions
Fungal nail infections, bacterial infections (particularly those caused by Pseudomonas, which can turn nails greenish-black), and viral warts near the nail matrix can all stimulate pigment production. Inflammatory skin conditions like psoriasis, lichen planus, and chronic inflammation around the cuticle area are additional triggers.
These causes usually come with other visible signs: thickened or crumbly nails with fungal infections, redness and swelling with bacterial infections, or pitting and ridging with psoriasis. The dark line itself is a secondary effect of the underlying condition, and treating that condition typically resolves the pigmentation over time.
External Staining
Sometimes the explanation is on the surface rather than beneath it. Tobacco, henna, dirt, and certain chemicals can stain nails brown or black in a way that mimics melanonychia. The difference is that external staining tends to affect the outer nail surface unevenly, can often be partially scraped or filed away, and doesn’t form a clean vertical line running from cuticle to tip.
When a Dark Line Could Be Melanoma
Nail melanoma is rare, accounting for roughly 0.7% to 3.5% of all skin melanomas. It peaks between the ages of 50 and 70. But because it’s often mistaken for a bruise or normal pigmentation, it tends to be diagnosed later than melanomas elsewhere on the body, which worsens outcomes. When caught at an early stage, the five-year survival rate is around 97%. When diagnosis is delayed and the cancer advances, that number drops significantly, falling below 20% at stage IV.
Dermatologists use a set of warning signs sometimes called the nail ABCD criteria to flag suspicious streaks:
- A (Adult age): New streaks appearing after age 18, especially after age 50, deserve more scrutiny than those present since childhood.
- B (Brown bands on a brown background): A dark band with brownish discoloration spreading across the full width of the nail bed, rather than a crisp single line, is more concerning.
- C (Color in the skin around the nail): Pigment spreading from the nail into the cuticle or the skin alongside the nail is a red flag. This is sometimes called Hutchinson’s sign.
- D (One digit): A single affected nail is more suspicious than multiple nails showing similar streaks.
Other warning features include a band that is widening over time, has irregular or blurred borders, or shows multiple shades of brown and black within it. A streak that has been stable in width and color for years is far less concerning than one that appeared recently and is getting broader.
How Diagnosis Works
A dermatologist will first examine the streak visually and with a dermatoscope, a handheld magnifying tool with a light source that reveals patterns invisible to the naked eye. Bruises show a characteristic homogeneous color pattern and will have migrated toward the nail tip since they first appeared. Melanoma-suspicious lesions tend to show irregular lines and multiple colors.
If there’s any uncertainty, a biopsy of the nail matrix is the only definitive way to rule out melanoma. The procedure involves numbing the finger with a local anesthetic injected at the base, then gently lifting back the cuticle fold to expose the nail matrix underneath. A thin shave of tissue is taken from the point where the dark streak originates. The cuticle is sutured back into place, and the nail plate is repositioned over the exposed area. The specimen needs to be at least 1 millimeter thick to allow proper evaluation under a microscope.
Recovery involves some tenderness and a temporary nail irregularity as the biopsy site grows out. The nail may develop a ridge or slight indentation at the biopsy location, but in most cases it returns to normal appearance over several months as the nail fully regenerates.
What to Watch For Over Time
If you have a single dark line on one nail that appeared in adulthood, keep an eye on its width, color, and borders. Take a photo every few months so you can compare. Lines that stay the same width and color over a year or more are almost always benign. Lines that widen noticeably, develop uneven borders, darken in patches, or start spreading pigment into the surrounding skin should be evaluated by a dermatologist promptly. Multiple symmetrical lines across several nails, especially if you have darker skin, are far less likely to be problematic and more likely to reflect normal pigmentation, a systemic condition, or a medication effect.