The sudden appearance of a dark or black spot beneath a big toenail can be alarming, often prompting an immediate search for answers. This discoloration, known medically as subungual pigmentation, occurs beneath the hard nail plate, attached to the nail bed tissue. While many causes are benign and reflect a minor injury, the symptom should never be ignored due to the possibility of more serious underlying conditions. Investigating the cause requires understanding whether the black color comes from dried blood (indicating trauma) or from pigment-producing cells (pointing toward biological changes).
The Primary Cause of Black Toenails: Trauma and Pressure
The most frequent reason a big toenail turns black is a subungual hematoma, essentially a bruise under the nail plate. This occurs when damaged blood vessels in the nail bed leak blood into the space between the nail and the underlying tissue. The iron in the blood turns dark as it dries and clots, creating the characteristic black or deep purple color.
Acute trauma, such as dropping a heavy object on the toe or stubbing it forcefully, is a common culprit leading to immediate pain and discoloration. The big toe is also susceptible to chronic microtrauma from repetitive activities, often termed “runner’s toe.” This happens when the toe repeatedly hits the front of a shoe during activities like running or prolonged walking, especially with ill-fitting footwear. The constant pressure slowly damages the capillaries, causing gradual bleeding that may not be noticed immediately.
If the blood collection is small and not causing significant discomfort, the body will reabsorb it over time. The discoloration will slowly grow out with the nail, which can take several months for a toenail to fully replace. When a hematoma covers 25% to 50% or more of the nail bed, or causes intense, throbbing pain, professional intervention may be required. A medical procedure called trephination involves creating a small hole in the nail plate to allow the trapped blood to drain, immediately relieving the pressure and pain. This procedure should never be attempted at home due to the risk of infection and permanent damage.
Pigmentation and Biological Causes
When trauma is not the cause, the dark color is usually due to the deposition of pigment, specifically melanin, a process known as melanonychia. The most concerning biological cause is subungual melanoma, a rare form of skin cancer originating from the pigment-producing cells of the nail matrix. Melanoma typically presents as a dark, vertical band or streak running the length of the nail, often with irregular borders. A sign of melanoma is the spread of the pigment onto the skin surrounding the nail, known as Hutchinson’s sign.
Another biological cause is an advanced fungal infection (onychomycosis), which usually begins as a yellow or white discoloration. Certain types of fungi, such as nondermatophytic molds like Scytalidium and Alternaria, produce dark pigments that lead to a brown or black appearance. This discoloration often comes with other symptoms, including nail thickening, brittleness, and the nail lifting away from the nail bed. Unlike a hematoma, which grows out, fungal discoloration usually affects the nail structure itself and may spread.
Systemic medications can also stimulate melanocytes to produce pigment, resulting in dark discoloration (drug-induced melanonychia). This is commonly associated with certain chemotherapy agents, such as doxorubicin and cyclophosphamide, or antibiotics like minocycline. The pigment may appear as diffuse darkening across the entire nail, or as transverse or longitudinal bands. The discoloration usually begins several weeks after starting the medication and typically resolves as a new, healthy nail grows out once the causative drug is stopped.
When to Seek Professional Diagnosis
While most black toenails are minor injuries that heal with time, knowing the “red flag” symptoms is important to rule out serious conditions. Seek an evaluation from a podiatrist or dermatologist if the black spot or streak appears without any known injury. Any dark discoloration that does not grow out with the nail or that changes rapidly in width, shape, or color warrants professional attention.
It is especially important to see a healthcare provider if the dark pigment spreads from the nail plate onto the surrounding skin of the toe, a classic sign associated with subungual melanoma. Severe pain, swelling, or signs of infection, such as pus or warmth, require prompt medical care. Preventative measures include ensuring shoes provide adequate space for the toes and keeping toenails trimmed straight across to minimize friction.