A black toenail, medically known as melanonychia, can be a startling discovery. While this discoloration may appear concerning, the cause is often benign and related to common physical activity or minor injury. Understanding the source of the dark color is important because, while most cases resolve on their own, some instances signal an underlying condition requiring professional medical attention. The black appearance is generally due to blood trapped beneath the nail plate or, less frequently, pigment created by an infection or abnormal cell growth.
The Most Common Reason Trauma
The most frequent cause of a black toenail is a subungual hematoma, which is essentially a bruise under the nail plate. This condition results from trauma that causes small blood vessels in the nail bed to rupture and bleed. The blood pools between the nail and the tissue underneath, causing pressure, pain, and the characteristic dark discoloration.
Acute, high-impact injury, such as dropping a heavy object on the foot or severely stubbing a toe, is a common culprit. The color initially appears reddish-purple and then darkens to brown or black as the blood coagulates. Chronic, repetitive microtrauma is also a primary mechanism, often seen in runners or athletes who wear ill-fitting shoes that cause the toes to repeatedly strike the front of the shoe. This repeated pressure, sometimes called “runner’s toe,” causes cumulative damage to the blood vessels beneath the nail.
Unlike a stain, a subungual hematoma will move outward as the nail grows, eventually disappearing when the affected section is trimmed away. This process can take several months, depending on the rate of nail growth. If the blood collection covers more than 25% of the nail or causes throbbing pain, it may need to be drained by a medical professional to relieve the pressure.
Discoloration Due to Fungal Infections
Dark discoloration can also be caused by onychomycosis, the medical term for a fungal nail infection. While most fungal infections result in yellow, white, or green discoloration, certain types of fungi can lead to black or brown nails. This specific phenomenon is sometimes referred to as fungal melanonychia.
Specific molds, known as dematiaceous fungi, produce melanin, a brown-to-black pigment, within their cell walls. This pigment, rather than blood, creates the dark appearance on or under the nail plate. Fungal infections often present with other signs, such as nail thickening, a brittle texture, or separation of the nail from the nail bed.
The discoloration from a fungal infection often appears irregularly or affects the entire nail, unlike the defined spot of a recent bruise. These infections require specific topical or oral antifungal treatments, as they will not resolve on their own like a hematoma. A healthcare provider can confirm a fungal infection by examining a sample of the nail debris under a microscope.
When Black Nails Indicate Serious Health Issues
While less common than trauma or fungus, a black toenail can occasionally signal a serious underlying health concern, most notably subungual melanoma. This is a rare form of skin cancer that develops in the pigment-producing cells of the nail matrix. The black or brown coloring is caused by an overproduction of melanin by the cancerous cells.
Melanoma often presents as a dark, vertical band or streak running the length of the nail that does not grow out or change position over time. Unlike a bruise, a melanoma streak may have irregular borders and vary in pigmentation. A defining feature is Hutchinson’s sign, where the pigment spreads beyond the nail plate and onto the surrounding skin of the cuticle or nail fold.
Other causes of dark streaks, known as longitudinal melanonychia, include certain systemic conditions or medications, such as chemotherapy drugs. Any new, unexplained, or rapidly expanding dark band warrants immediate medical evaluation.
Knowing When to See a Doctor
Most black toenails resulting from a definite injury can be managed at home by resting and waiting for the discoloration to grow out with the nail. However, certain signs indicate the need for prompt professional attention to prevent complications or rule out serious disease.
You should consult a doctor if you notice any of the following:
- The discoloration covers a large portion of the nail and is accompanied by severe or throbbing pain, requiring surgical drainage.
- Signs of a potential infection are present, such as increasing redness, swelling, warmth, or pus around the nail.
- The black discoloration appears without any known injury, persists without growing out, or changes in size or shape.
- A dark streak appears that expands to the skin around the nail (Hutchinson’s sign), which requires screening for subungual melanoma.
- You have underlying health issues like diabetes or poor circulation and notice any change in nail appearance.