Black lines running vertically on your nails are usually a condition called longitudinal melanonychia, caused by pigment-producing cells in the nail matrix depositing melanin into the nail plate as it grows. In most cases, these lines are completely harmless. But because a small percentage of cases can signal melanoma or other health conditions, it’s worth understanding what’s behind them.
What These Lines Actually Are
The dark streak you see is melanin, the same pigment that colors your skin and hair, embedded in the nail plate. Cells called melanocytes sit in the nail matrix (the tissue under your cuticle where the nail forms), and when they’re activated, they deposit pigment into the nail as it grows outward. The result is a vertical brown or black band that runs from the base of your nail toward the tip.
These lines can appear on fingernails or toenails. They can be narrow or wide, light brown or jet black, and they can show up on one nail or several at once. The pattern and appearance matter, because different causes tend to look different.
The Most Common Causes
For most people, black nail lines come down to one of a few benign explanations.
Skin tone and genetics are the single biggest factor. Melanonychia is extremely common in people with darker skin. Up to 77% of Black adults over age 20 have some degree of nail pigmentation, and nearly 20% of Japanese adults do as well. In white populations, the prevalence drops to about 1.4%. Among people of Caribbean descent, nail discoloration by age 50 is considered normal. If you have medium to dark skin and notice pigmented bands across multiple nails, this is very likely a normal variation.
Nail trauma is another frequent trigger. Repeated pressure or injury to the nail, whether from biting, picking, tight shoes, or even regular manicures, can activate melanocytes and produce a dark streak. A bruise under the nail (subungual hematoma) can also appear as a dark spot or line, though bruises tend to be more blotchy and grow out with the nail over several months.
Moles can form in the nail matrix just like they form elsewhere on your skin. A nail mole (nevus) produces a stable, uniform brown or black band that stays roughly the same width and color over time.
Age spots (lentigos) in the nail matrix work the same way they do on sun-exposed skin, creating areas of increased pigment that show up as dark streaks.
Splinter Hemorrhages Look Similar but Aren’t
Not every dark line on your nail is pigment. Splinter hemorrhages are tiny streaks of bleeding under the nail that can look like thin reddish-brown or black lines running vertically. They’re usually only 1 to 3 millimeters long and tend to appear closer to the tip of the nail. When fresh, they look reddish or purple, then darken to brown or black within a few days.
The most common cause is simple nail trauma: stubbing a toe, getting acrylic nails applied, or even minor repetitive impacts. Splinter hemorrhages also show up frequently with nail psoriasis, lichen planus, and in people taking blood thinners. In rare cases, they can signal a heart valve infection called endocarditis, which produces splinter hemorrhages in 15% to 33% of affected people. The key difference is that splinter hemorrhages move outward as your nail grows and eventually disappear, while pigment-based lines stay in place because the melanocytes keep producing melanin.
Health Conditions That Cause Nail Pigmentation
A number of systemic conditions can activate melanocytes in the nail matrix. Hormonal disorders like Addison’s disease, Cushing’s syndrome, and overactive thyroid are known triggers. Connective tissue diseases such as lupus and scleroderma can cause nail darkening too. HIV infection is another recognized cause, as are inflammatory skin conditions like psoriasis and lichen planus.
Nutritional deficiencies play a role as well. Vitamin B12 deficiency is one of the more common nutritional causes of nail hyperpigmentation. Pregnancy can also trigger temporary melanonychia due to hormonal shifts.
Certain medications are well-documented culprits. Chemotherapy drugs, antiretroviral therapy for HIV, and antimalarial medications can all stimulate melanocytes in the nail matrix. The drug essentially causes melanocytes to produce melanin in a way they wouldn’t normally, creating either a single dark band or diffuse darkening across the entire nail plate. This type of discoloration often improves after the medication is stopped, though it can take months for the affected nail to grow out completely.
Fungal Infections Can Turn Nails Dark
Most people associate fungal nail infections with yellow or white discoloration, but certain fungi actually produce melanin and can create brown-to-black nail pigmentation. This is called fungal melanonychia, and it’s relatively rare compared to typical nail fungus. Over 20 species of pigment-producing fungi have been identified as causes, along with at least 8 species of non-pigmented fungi that can still darken the nail through other mechanisms.
The most commonly responsible organisms produce either a dark band resembling a pigment streak or diffuse brown-to-black staining of the nail plate. This matters because fungal melanonychia can mimic the appearance of melanoma, making accurate diagnosis important. Unlike pigment from melanocytes, fungal pigmentation responds to antifungal treatment.
When a Black Line Could Be Melanoma
Subungual melanoma, a type of skin cancer that starts under the nail, is the concern that brings most people to search for this topic. It’s rare in the general population, but it accounts for up to one-third of all melanoma cases in Black, Asian, and Native American individuals. It most commonly appears between the ages of 40 and 70.
Dermatologists use a set of warning signs sometimes called the ABCDEF criteria to evaluate suspicious nail bands:
- A (Age and ancestry): Peak incidence is in the 50s to 70s age range. Higher risk in African American, Asian, and Native American populations.
- B (Band characteristics): A brown or black band wider than 3 millimeters with uneven color or irregular borders.
- C (Change): A band that’s getting wider, darker, or more irregular over time, or a nail that isn’t improving despite treatment for another suspected cause.
- D (Digit): The thumb, big toe, and index finger are most commonly affected.
- E (Extension): Pigment spreading beyond the nail onto the surrounding skin of the cuticle or nail fold. This is called Hutchinson’s sign and is one of the most important red flags.
- F (Family history): A personal or family history of melanoma or atypical moles.
A single new dark band on one nail in an adult, especially one that’s widening or has blurry edges, warrants evaluation. Multiple uniform bands across several nails in a person with darker skin is a very different picture and almost always benign.
How Nail Lines Are Diagnosed
A dermatologist will start with a detailed look at the nail using a dermatoscope, a magnifying tool with polarized light that reveals details invisible to the naked eye. Dermatoscopy can help distinguish regular pigment patterns from irregular ones, identify whether the pigment originates from the upper or lower part of the nail matrix, and detect subtle pigment spread onto the surrounding skin that might not be obvious otherwise.
If the appearance raises concern, the next step is a biopsy of the nail matrix. Several techniques exist depending on the width and location of the band. For narrow bands under 3 millimeters, a small punch biopsy is standard. Wider or more suspicious bands may require a shave biopsy or a longer incision along the side of the nail. The tissue sample is examined under a microscope to determine whether the melanocytes are normal, overactive, or cancerous.
A biopsy sounds intimidating, but it’s done under local anesthesia and is the only way to definitively distinguish between a harmless mole in the nail matrix and early melanoma. For most people with dark lines on their nails, a visual exam and dermatoscopy are enough to confirm a benign cause without any need for biopsy.