A black toenail is almost always caused by one of three things: a bruise from trauma, a fungal infection, or an underlying health condition. The most common cause by far is blood trapped beneath the nail after an injury, a condition called a subungual hematoma. While most cases are harmless and resolve on their own, certain patterns of discoloration can signal something more serious, including a rare form of skin cancer.
Trauma and Bruising Under the Nail
The most frequent reason for a black toenail is simple injury. When something heavy lands on your toe, or your toe repeatedly jams against the front of a shoe, tiny blood vessels beneath the nail break and bleed. That blood gets trapped between the nail plate and the nail bed, creating a dark bruise that can look black, deep purple, or reddish-brown. The hallmark symptom is sudden, throbbing pain caused by the pressure of pooled blood pressing against the nail from below.
This kind of bruise can appear within hours of an injury. In mild cases, the discoloration stays put, and the blood gradually grows out with the nail over several months. More severe hematomas can cause intense pressure that worsens over the first day or two. If the pain is getting worse or the dark area is expanding, a healthcare provider can relieve the pressure by making a small hole in the nail, a procedure called trephination. This works best within 24 to 48 hours of the injury. Never attempt to drain a nail yourself with a pin or paper clip, as the risk of infection and further damage is high.
Toenails grow slowly, roughly 1.6 millimeters per month. That means a fully damaged toenail can take up to 18 months to grow out completely. If the nail matrix (the tissue that produces the nail) was damaged in the injury, regrowth may be even slower, and the new nail may come back thicker or slightly misshapen.
Runner’s Toe and Repetitive Stress
You don’t need a single dramatic injury to get a black toenail. Repetitive microtrauma from athletic activity is one of the most common causes, especially among runners, soccer players, basketball players, and dancers. Each stride or sudden stop pushes the toenail into the front or side of the shoe, and over miles or hours, that friction causes bleeding under the nail. The result looks identical to a bruise from dropping something on your foot, but it builds up gradually rather than appearing all at once.
Ill-fitting shoes are the usual culprit. A good rule of thumb when buying running shoes is to leave enough space in the toe box so your toes aren’t pressed against the front, but not so much space that your foot slides forward with each step. Getting your feet professionally sized at a running store helps, especially since foot size can change over time. Adjusting your lacing technique to keep your foot more securely in place can also reduce the friction that leads to blackened nails. Construction workers and others who spend long hours on their feet in tight footwear face similar risks.
Fungal Infections
Fungal nail infections, known as onychomycosis, are another common cause of dark toenails. While most fungal infections turn nails yellowish-brown rather than truly black, advanced or untreated infections can darken significantly. The fungi involved break down keratin, the protein that makes up your nail, by producing enzymes that burrow into the nail structure.
The most common pattern starts at the tip of the nail and works its way back toward the base. As the infection progresses, the nail thickens, becomes brittle, and may separate from the nail bed. That gap between the nail and the bed can become a reservoir for bacteria and additional fungi, which deepens the discoloration. In other cases, white patches appear on the nail surface and eventually spread, making the nail rough, soft, and crumbly. Yeast infections from Candida can also affect nails, typically starting with redness and swelling in the skin around the nail before invading the nail itself.
People with diabetes, vascular disease, or weakened immune systems are more vulnerable to toenail fungus. If your black toenail is also thick, crumbly, or has an unusual texture, a fungal infection is a likely explanation worth getting evaluated.
Health Conditions That Affect Nail Color
Several systemic health issues can show up as toenail discoloration. Conditions that impair circulation, including diabetes, kidney disease, and vascular disease, can alter the color and health of your nails over time. In diabetes, nails more commonly turn yellowish, but blackening does occur in some cases. Heart infections and psoriasis can also produce darkened nails.
Nutritional deficiencies play a role too. A vitamin B12 deficiency, for instance, can cause nails to darken. If you have a black toenail that you can’t link to an injury or infection, and especially if multiple nails are affected, an underlying health condition could be the reason.
When Black Nails Could Be Melanoma
The most serious possible cause of a black toenail is subungual melanoma, a rare form of skin cancer that develops in the nail matrix. It accounts for roughly 0.7% to 3.5% of all melanoma cases worldwide, but the risk varies significantly by ethnicity. In people with lightly pigmented skin, it makes up about 3% of melanoma diagnoses. In people of African descent, it accounts for up to 75% of all melanoma cases, making awareness especially important in that population.
The key visual difference between melanoma and a bruise is the pattern. A hematoma looks like a dark smudge or blotch that appeared after a specific injury and gradually grows out with the nail. Melanoma typically appears as a dark longitudinal streak, a line running from the base of the nail toward the tip. That streak may widen over time, develop irregular borders, or extend into the skin surrounding the nail. Melanoma grows slowly over weeks to months, while a bruise shows up within hours.
If you notice a dark streak under your nail that you can’t explain with an injury, or if a dark spot isn’t growing out with the nail over time, get it evaluated promptly. The width and borders of the pigmented band are among the key features a provider will assess. Early detection makes a significant difference in outcomes for this type of cancer.
How to Tell What’s Causing Yours
A few simple observations can help you narrow down the cause. Think about whether you can connect the discoloration to a specific event, like stubbing your toe, dropping something on it, or breaking in new shoes. If the dark area appeared quickly after trauma and looks like a bruise, it’s almost certainly a hematoma. You may also notice the blood moving forward as the nail grows, or see it when you trim the nail.
If the nail is thickened, crumbly, or has an unusual texture along with the dark color, a fungal infection is more likely. If a dark line appeared without any injury and isn’t growing out or is getting wider, that warrants a closer look from a dermatologist. Multiple discolored nails without a clear cause point toward a systemic issue worth investigating.
For simple bruising, the main treatment is patience. Keep the nail clean, wear shoes that give your toes room, and let the nail grow out. If pain is severe or worsening in the first couple of days, a provider can drain the blood relatively quickly. For fungal infections, over-the-counter topical treatments exist, but prescription options tend to be more effective for infections that have penetrated deep into the nail.