Black fingernails usually result from a bruise under the nail, but the discoloration can also signal a fungal infection, a medication side effect, or in rare cases, a type of skin cancer. The cause matters because some are harmless and resolve on their own, while others need prompt attention. Here’s how to tell the difference.
Bruising Under the Nail
The most common cause of a black fingernail is a subungual hematoma, which is blood trapped between the nail and the nail bed. Slamming your finger in a door, dropping something heavy on it, or repetitive pressure (like gripping tools or playing sports) can rupture tiny blood vessels underneath. The pooling blood creates pressure that often causes throbbing pain, and the nail turns dark purple, red, or black.
As the nail grows out, the dark spot moves toward the tip. Fingernails grow at roughly 3.5 mm per month, so depending on where the bruise sits, it can take three to six months to fully disappear. You’ll notice the discolored area slowly shifting forward week by week, which is actually a reassuring sign: it confirms the mark is a bruise, not something more concerning.
Most nail bruises don’t need treatment. But if the pain is severe or getting worse, that means the blood is building too much pressure. In that case, a provider can drain it by making a small hole in the nail. This works best within 24 to 48 hours of the injury. If the bruise covers more than half the nail or you suspect the finger might be fractured, get it evaluated sooner.
Fungal Infections
Not all dark nails involve trauma. Certain fungi and molds can colonize the nail plate and turn it black, dark grey, or greenish-black. The most common culprit is a common skin fungus called Trichophyton rubrum, which accounts for over half of cases. Other species, including Candida (a type of yeast) and various environmental molds, can also cause color changes ranging from yellow-green to deep black.
Fungal nail discoloration tends to look different from a bruise. It usually develops gradually without a memorable injury, and the nail may also become thickened, crumbly, or lifted from the bed. The color change often starts at one edge or the tip and spreads inward over weeks or months. If your nail is darkening and also changing texture, a fungal infection is a likely explanation. Prescription antifungal treatment is typically needed because over-the-counter options rarely penetrate the nail deeply enough.
External Staining
Sometimes the color is sitting on top of the nail rather than underneath it. Hair dyes (especially henna), tobacco, potassium permanganate, silver nitrate, and tar-based products can all stain nails brown or black. The pigment sticks to the surface of the nail plate and may appear as a diffuse wash of color or horizontal bands that correspond to when the exposure happened.
You can often identify staining because it affects multiple nails at once, the surrounding skin may also be discolored, and gently scraping or cleaning the nail surface lightens it. Once you stop the exposure, the stain grows out with the nail over a few months.
Medications That Darken Nails
Several medications can trigger dark streaks or diffuse darkening of the nails as a side effect. Chemotherapy drugs are the most common culprits, but antiretroviral medications (like zidovudine, used in HIV treatment), hydroxyurea (used for sickle cell disease and certain blood disorders), and some anti-seizure medications can also cause it. The darkening typically appears as lengthwise streaks running from the base of the nail to the tip, and it may affect several nails simultaneously.
If you notice nail changes after starting a new medication, mention it to your prescriber. The discoloration is usually cosmetic and reversible once the medication is stopped or changed, though it takes months for the nail to grow out completely.
Systemic Health Conditions
In some cases, black or brown nails reflect something happening elsewhere in the body. Chronic kidney disease can increase melanin production in the nail bed, turning the far end of the nail brown while the base stays white, creating a distinctive “half-and-half” pattern. Excessive fluoride intake can turn nails brown or black. Hormonal conditions that boost melanin production throughout the body can also darken the nails along with the skin.
These causes are less common, but they’re worth knowing about because the nail changes show up gradually, affect multiple nails, and don’t have an obvious external explanation like injury or staining.
Dark Streaks and Melanoma Risk
A single dark streak running lengthwise down one nail deserves careful attention, especially if it’s new or changing. Subungual melanoma is a rare but serious form of skin cancer that starts in the nail matrix (the tissue under the base of your nail where new nail cells form). It typically appears as a brown or black band that widens over time.
Dermatologists use a set of warning signs sometimes called the ABCDEF criteria to evaluate suspicious nail streaks:
- Age and ancestry: Peak incidence is between ages 50 and 70. People of African, Asian, Hispanic, and Native American descent are disproportionately affected. Among people of African descent, subungual melanoma accounts for up to 75% of all melanoma cases.
- Band characteristics: A brown-to-black streak wider than 3 mm, with uneven or blurry borders.
- Change: The band is getting wider, darker, or more irregular over weeks to months.
- Digit: The thumb and big toe are the most commonly affected.
- Extension: Pigment spreading from the nail onto the surrounding skin fold (called Hutchinson’s sign) is a red flag.
- Family history: A personal or family history of melanoma or atypical moles raises risk.
Nail melanoma occurs at similar rates across all racial groups, but it makes up a much larger share of total melanoma diagnoses in people with darker skin. This is partly because other types of melanoma (the sun-related kinds) are far less common in these populations, making the nail subtype proportionally more significant.
How to Tell Causes Apart
A few practical clues help narrow things down. A bruise almost always follows a specific injury, hurts initially, and then moves toward the tip of the nail over subsequent weeks. Watching the spot migrate forward is the simplest confirmation it’s a hematoma.
Fungal infections develop slowly without trauma, change the nail’s texture (thickening, crumbling), and may affect more than one nail. Staining affects the nail surface and often the skin around it, correlates with chemical exposure, and can sometimes be partially removed by cleaning.
Melanoma tends to be a single lengthwise streak on one nail, often on the thumb or index finger, that gradually widens. The surrounding skin may darken. Unlike a bruise, it doesn’t shift position with nail growth because the pigment is being continuously produced at the nail’s root.
If your nail turned black after an obvious injury, it’s very likely a bruise. If a dark band appeared without trauma, is only on one nail, and seems to be widening, have a dermatologist examine it. They can use a magnifying device called a dermatoscope to look at the pigment pattern in detail and determine whether a biopsy is warranted.
Normal Variants in Darker Skin
It’s worth noting that longitudinal dark streaks in the nails are a common, completely benign finding in people with more melanin in their skin. These streaks are caused by normal melanocyte activity in the nail matrix and may appear on multiple fingers. They tend to be stable in width and color over time. The key distinction is that benign streaks stay consistent, while melanoma-related streaks change.