A black toenail is a common presentation that can cause alarm because the dark discoloration suggests bleeding or severe injury. This color change, medically known as onychodystrophy, is often a sign of an issue beneath the nail plate. The causes range from everyday occurrences like minor trauma to rare, but serious, medical conditions. Understanding the source of the black color—whether it is dried blood, a fungal byproduct, or pigment—helps determine the necessary course of action. While many instances are harmless and resolve naturally as the nail grows out, some require professional evaluation.
The Role of Physical Trauma
The most frequent reason a toenail turns black is a subungual hematoma, a bruise beneath the nail plate. This occurs when blood vessels in the nail bed rupture, causing blood to pool in the confined space beneath the nail. Acute, single-event trauma, such as dropping a heavy object on the toe or severely stubbing it, is a common cause of this immediate bleeding and discoloration.
Repetitive micro-trauma, often seen in athletes like runners, leads to a condition sometimes called “runner’s toe.” This results from the toe repeatedly sliding forward and hitting the inside of a shoe, especially if the footwear is too tight or too loose. The continuous friction damages small capillaries under the nail, causing minor, cumulative bleeding over time.
The resulting black or dark purple appearance is the color of clotted, dried blood trapped beneath the nail plate. This discoloration is temporary and will grow out with the nail, which typically takes several months for the toenail to fully replace itself. If the trauma is severe, the pressure from the pooled blood can cause intense pain or even lead to the nail separating entirely from the nail bed.
Infections and External Staining
Black discoloration can also arise from sources other than blood, including biological invaders and external factors. A fungal infection, known as onychomycosis, typically presents with yellow or white discoloration, but can sometimes appear dark brown or black. This darkening is generally due to debris buildup or advanced infection within the nail plate.
Certain types of fungi, such as those from the Scytalidium genus, can directly produce a black pigment within the nail structure itself, a condition known as fungal melanonychia. A secondary bacterial infection, most commonly caused by Pseudomonas bacteria, can colonize the damaged area, often leading to a greenish-black hue. The warm, moist environment of footwear provides an ideal breeding ground for these organisms.
Temporary black staining can also occur from external sources. Dyes from dark socks or poorly colored shoes, as well as prolonged use of certain dark nail polishes, can seep into the nail plate. Chemicals like potassium permanganate, sometimes used in foot soaks, can also cause superficial black staining. This discoloration is non-painful and will wear off or grow out with the nail.
The Significance of Pigmentation
When discoloration is not caused by blood or external sources, it is often a result of melanin production, a process called melanonychia. This appears as a dark, usually vertical, streak or band running from the cuticle to the tip of the nail. In individuals with darker skin tones, these pigmented streaks are a common, benign finding, representing normal melanocyte activity.
However, a pigmented streak that appears suddenly, changes rapidly, or involves only a single digit can be a sign of subungual melanoma, a rare form of skin cancer. Melanoma develops from the pigment-producing cells beneath the nail and is the most serious cause of a black toenail. Unlike a bruise, this discoloration is not linked to trauma and will not grow out with the nail.
A particularly concerning sign is Hutchinson’s sign, where black or brown pigment extends from the nail plate onto the surrounding skin of the cuticle or nail fold. While this sign can occasionally be seen in benign conditions, its presence in an adult’s single, new pigmented band is a strong indicator that melanoma should be ruled out. Additionally, certain medications, including some chemotherapy drugs and antimalarials, can cause diffuse, non-traumatic nail pigmentation.
Actionable Steps and When to See a Doctor
If a black toenail is clearly the result of recent, single trauma, home care involves monitoring for infection and managing pain. For a subungual hematoma, the discoloration should migrate distally as the nail grows, confirming it is dried blood. If the bruise causes severe throbbing pain, a medical professional can drain the blood to relieve pressure, but this must be done shortly after the injury occurs.
Seek medical attention if the cause of the black toenail is unknown or if the condition does not improve with time. Specific red flags necessitate a visit to a podiatrist or dermatologist. These warning signs include intense or increasing pain, noticeable swelling or discharge, or a black area covering more than half of the nail.
Consultation is also necessary if the discoloration appears as a dark streak that widens, changes color, or involves the skin surrounding the nail (Hutchinson’s sign). Individuals with underlying health conditions like diabetes, which can complicate foot injuries, should have any change in toenail color professionally evaluated immediately.