A black toenail, medically known as melanonychia, is a common condition that can range from a minor issue to a sign of a serious underlying health problem. The discoloration represents a change in the nail plate or, more frequently, the nail bed beneath it. While often caused by simple mechanical damage, the appearance of a black color under the nail is a symptom, not a diagnosis, and should not be ignored.
Bruising Beneath the Nail
The most frequent cause of a black toenail is a subungual hematoma, the medical term for blood pooling between the nail bed and the nail plate. This results from trauma that causes blood vessels to rupture and bleed. The blood becomes trapped because the nail plate acts as a rigid barrier.
Trauma can be a single, acute event, such as stubbing a toe or dropping an object onto the nail. Alternatively, it results from repetitive micro-trauma, often seen in runners or hikers whose toes repeatedly jam against the front of their shoes. This constant friction causes repeated injury to the nail bed, eventually leading to bleeding.
A subungual hematoma typically appears dark red, purple, or black, often covering a significant portion of the nail. This discoloration is temporary and will gradually grow out with the nail. Since toenails take approximately 12 months for complete replacement, the discoloration may be visible for many months.
If the hematoma is small and pain is minimal, self-care measures like rest, elevation, and applying ice can help manage symptoms. If the blood accumulation is large, it can exert significant pressure, causing intense, throbbing pain. This requires medical attention for a procedure called trephination, where a small hole is made in the nail to drain the trapped blood and relieve the pressure.
Fungal Infections and Other Discolorations
Black toenails appearing without a history of injury are frequently caused by biological or chemical factors. Onychomycosis, a fungal infection often caused by the dermatophyte Tinea unguium, is a common culprit. The discoloration results from the buildup of fungal debris or pigment produced by the fungi themselves.
Fungal discoloration often begins as white, yellow, or brown patches before darkening to reddish-brown or black. Unlike a bruise, a fungal infection often results in a thickened, crumbly, or distorted nail texture. The nail may also separate from the nail bed, a condition known as onycholysis.
Beyond fungal growth, certain medications can cause the nail bed to darken. This drug-induced melanonychia is a side effect of some chemotherapy agents, antimalarials, and anti-seizure medications. This discoloration is caused by the stimulation of melanocytes, the pigment-producing cells, and can appear as a diffuse darkening or a defined longitudinal band. This type of discoloration is usually harmless and often resolves once the medication is stopped or the nail grows out.
Identifying Serious Causes and Seeking Care
While most black toenails are benign, certain signs suggest a more serious condition, such as subungual melanoma, a rare but dangerous form of skin cancer. A key indicator is discoloration that appears as a dark, irregular band or streak that does not correspond to an injury and does not grow out with the nail. The appearance may be a wide, dark, or variegated brown-to-black streak.
A particularly concerning symptom is Hutchinson’s sign, which occurs when the dark pigment extends onto the surrounding skin of the cuticle or nail fold. This spreading of pigment indicates the discoloration is caused by active melanocytes and warrants immediate professional evaluation. Any black discoloration that appears without a clear cause, changes size or shape, or is accompanied by bleeding or a painful nodule should prompt a visit to a healthcare provider.
Medical care is necessary if the pain from an injury is severe and throbbing, indicating significant pressure requiring drainage. A doctor should also be consulted if the injury was severe enough to damage the underlying nail matrix or if there are signs of infection, such as redness, swelling, or pus. Monitoring the growth of the nail is the simplest way to differentiate a bruise from a more serious, fixed discoloration.