A black discoloration under a nail can be a concerning symptom, often prompting individuals to seek information about its origin. While many instances of a black nail are benign, resulting from common injuries, others can signal more serious underlying health conditions that warrant prompt medical evaluation. Understanding the various potential causes is important for determining the appropriate course of action.
Trauma-Induced Discoloration
The most frequent reason for a nail to appear black is a subungual hematoma, a bruise beneath the nail plate. This occurs when an injury, such as stubbing a toe, dropping a heavy object on the nail, or repetitive microtrauma from ill-fitting shoes, causes small blood vessels under the nail to rupture. Blood pools in the space between the nail and nail bed, leading to a dark red, purple, or black appearance.
The pressure from the trapped blood can cause significant pain and throbbing. In most cases, the discolored area will gradually grow out with the nail as the nail plate replaces itself, a process that can take several months for fingernails and up to a year or more for toenails. For minor cases, self-care involves resting and elevating the affected digit, applying ice to reduce swelling and pain, and using over-the-counter pain relievers. If the hematoma is large and causes severe pain, a medical professional can perform trephination to drain the blood and relieve discomfort.
Other Explanations for Discoloration
Beyond direct trauma, several other factors can lead to a nail turning black, each with distinct characteristics.
Fungal infections (onychomycosis) are a common cause of nail discoloration, sometimes including black or dark hues. Fungi can accumulate debris or produce pigments under the nail, causing darkening. Affected nails may also become thickened, brittle, crumbly, or misshapen, and they might separate from the nail bed. A foul odor can also be present with some fungal infections.
Subungual melanoma, a rare but serious skin cancer, develops under the nail. It often appears as a dark brown or black streak running vertically along the nail. Unlike a bruise, this streak typically does not grow out, may widen, or involve the surrounding skin (Hutchinson’s sign). The nail itself might also show signs of splitting, cracking, or general deformation.
Certain medical conditions and medications can also cause nail discoloration, though less commonly. Systemic diseases like diabetes, kidney disease, heart disease, anemia, and some autoimmune conditions (e.g., psoriasis) can cause nail darkening. Various medications, including certain chemotherapy drugs, antimalarials, and some antibiotics, can induce black or brownish nail discoloration as a side effect. These drug-induced changes often appear as longitudinal or transverse bands and typically resolve once the medication is stopped and the nail grows out.
Knowing When to Act
While many black nail instances are not alarming, certain signs warrant medical attention. Consult a healthcare professional if a black nail appears without injury or if discoloration does not improve or grow out. Seeking medical advice is also important if there is severe or increasing pain, or if signs of infection develop, such as redness, swelling, pus, or warmth around the nail.
Other concerning indicators include discoloration spreading beyond the nail plate to surrounding skin or cuticle, changes in the dark spot’s size or shape, or if the nail lifts without clear cause. Individuals with underlying health conditions like diabetes should also have nail changes evaluated promptly. A healthcare provider can accurately diagnose the cause of the black nail and recommend appropriate management.