A black nail is a symptom of an underlying issue, not a diagnosis itself. This dark discoloration occurs beneath the hard nail plate, making the nail appear black, blue, or deep purple. The two most common causes are acute trauma (subungual hematoma) and chronic fungal infection, though a rare but serious cancer can also present this way. Proper management and treatment rely entirely on accurately identifying the specific cause of the dark spot. Understanding the differences between these causes is the first step toward determining the appropriate course of action for your nail.
Identifying the Cause of Nail Discoloration
The appearance of the discoloration and the history of the injury provide clues for differentiation. A subungual hematoma results from blood pooling between the nail bed and the nail plate following an injury. This trauma can be a single, forceful event, like dropping a heavy object on a toe, or repetitive micro-trauma from activities such as running in tight shoes. The resulting discoloration is typically a deep red, purple, or black bruise that appears suddenly within hours of the injury, covering a localized area or the entire nail.
In contrast, a fungal infection develops slowly over weeks or months. Fungi feed on the nail’s keratin, leading to discoloration that may appear yellow, brown, or black. Unlike a hematoma, a fungal infection often causes the nail to become thickened, brittle, or crumbly. The discoloration typically starts at the edge and moves inward, usually lacking the acute pain associated with immediate trauma.
A rare but concerning cause is subungual melanoma, a form of skin cancer that begins in the nail matrix. This condition typically presents as a dark, vertical streak of pigment, often brown or black, running from the cuticle to the tip of the nail. The streak may be irregular in color or border and tends to widen over time, which is a warning sign. If the pigment extends beyond the nail plate and into the surrounding skin of the cuticle or nail fold (Hutchinson’s sign), it raises suspicion for this serious diagnosis.
Immediate Home Care for Traumatic Black Nails
If the black nail is the result of a recent, mild trauma and the pain is manageable, initial home care focuses on reducing swelling and pressure. Immediately applying the R.I.C.E. protocol—Rest, Ice, Compression, and Elevation—can help limit the bleeding under the nail. Applying a cold compress for up to 20 minutes every few hours during the first day will constrict blood vessels and reduce localized swelling.
To manage pain and inflammation, non-steroidal anti-inflammatory drugs (NSAIDs) can be taken as directed. The discoloration will not disappear quickly, but will grow out with the nail plate. Fingernails generally take four to six months to fully replace themselves, while toenails can require 12 to 18 months for the discoloration to be removed.
Avoid attempting to drain the trapped blood at home using non-sterile tools. This procedure, called trephination, introduces a high risk of bacterial infection to the nail bed and surrounding tissues. If the pain from the pressure becomes intense or throbbing, it signals the need for professional medical drainage in a sterile environment.
Professional Medical Interventions
When a subungual hematoma is large, covering more than 25% of the nail, and causing significant throbbing pain, a medical professional may perform a drainage procedure called trephination. This involves creating a tiny hole in the nail plate using a sterile needle or a heated cautery device to release the accumulated blood and relieve the painful pressure. This procedure is quick and typically results in immediate pain relief, and the small hole will remain until the nail grows out.
For a fungal infection, topical or oral antifungal medications are required, as the infection is deeply embedded in the nail plate. Prescription oral antifungals, such as Terbinafine, are often considered the most effective treatment due to their ability to reach the nail matrix through the bloodstream. This treatment requires consistent, long-term use, often lasting three to four months, as the medication must remain in the newly growing nail to clear the infection.
If subungual melanoma is suspected due to the appearance of a dark, changing streak, a doctor will perform a biopsy of the nail matrix. This involves removing a small tissue sample from the area where the nail grows to analyze the cells for malignancy. If the biopsy confirms cancer, treatment typically involves surgical excision, which may require removing the entire nail unit to ensure all cancerous tissue is eliminated.
Recognizing Signs That Require Immediate Medical Attention
A black nail requires immediate professional evaluation if the discoloration appeared without any history of trauma or injury. This unexplained pigment change may indicate a more systemic or serious underlying condition that needs prompt diagnosis. If the dark area under the nail is a vertical streak that is widening, changing color, or has blurred, irregular borders, it must be examined urgently.
Signs of a developing infection necessitate immediate medical care, including severe, throbbing pain that worsens despite home care measures. Other infection red flags include pus draining from beneath the nail, or spreading redness, warmth, and swelling in the skin surrounding the nail. Furthermore, any instance where the dark pigment extends onto the cuticle or the skin of the nail fold, known as Hutchinson’s sign, should be considered a medical emergency requiring immediate attention to rule out melanoma.