A darkened toenail, medically termed melanonychia, is a common concern. This discoloration is a visual sign that pigment, typically blood or melanin, has become trapped beneath the nail plate. While a black toenail is most frequently the result of a minor injury, it should never be dismissed without careful consideration. Understanding the underlying cause is the first step, as the spectrum ranges from a simple bruise to a rare but serious health condition.
The Most Common Cause: Subungual Hematoma
The majority of black toenails are caused by a subungual hematoma, which is essentially a bruise under the nail plate. This occurs when blood vessels in the underlying nail bed rupture, leading to a pooling of blood that discolors the nail. The black or dark purple appearance is the color of this trapped blood.
This injury often results from a single, acute event, such as stubbing a toe or dropping an object onto the foot. Equally common, especially among athletes, is chronic microtrauma. This occurs when the repetitive impact of the toe sliding forward in a shoe causes repeated small injuries to the nail bed, often during activities like long-distance running or hiking with ill-fitting footwear.
Acute injury often causes throbbing pain and pressure, as the accumulating blood has nowhere to escape beneath the rigid nail. If the hematoma is minor and painless, the discoloration will simply grow out as the toenail naturally advances. Since toenails grow slowly, taking approximately six to nine months for full replacement, the dark spot will gradually move toward the tip.
Simple management for a minor, non-painful hematoma involves the RICE method: rest, ice, gentle compression, and elevation. If the blood accumulation is large and causes significant throbbing pain, a medical professional may perform a procedure called trephination. This involves creating a small hole in the nail plate to drain the trapped blood, instantly relieving the pressure.
Non-Traumatic Causes of Toenail Discoloration
Not all black toenails result from physical injury; other conditions can also cause the nail to darken. One common non-traumatic source is a fungal infection, specifically Tinea unguium (onychomycosis). Although fungal infections typically present as yellow or white discoloration, the buildup of debris and pigment can sometimes make the nail appear dark brown or black.
Certain systemic health issues can also manifest as nail discoloration due to their effect on circulation or pigmentation pathways. Conditions like diabetes, kidney disease, heart disease, or anemia can lead to changes in nail health and color. Furthermore, some medications, including certain chemotherapy agents or antimalarial drugs, can induce temporary pigmentation changes in the nail unit that resolve after the drug is discontinued.
Recognizing Nail Melanoma
While rare, a black mark under the toenail can signify subungual melanoma, a cancer that originates from the pigment-producing cells of the nail matrix. Unlike a hematoma (a pool of blood), melanoma is an uncontrolled growth of pigmented cells, often appearing as a dark, linear stripe known as longitudinal melanonychia. Early identification of this condition is crucial.
To distinguish potential melanoma from benign causes, clinicians often rely on the ABCDEF criteria. The “B” highlights a brown-to-black band that is wide (typically 3 millimeters or more) and has irregular or blurred borders. “C” refers to a change in the band’s size or color over time, or a failure to change despite expected nail growth.
The most concerning sign is “E,” which stands for the extension of pigment onto the skin surrounding the nail, known as the Hutchinson sign. When this dark pigment bleeds into the cuticle or the lateral nail folds, it suggests that the melanocytes are spreading beyond the nail matrix. Unlike a hematoma, melanoma pigmentation often does not migrate uniformly with the growth of the nail plate.
When to Seek Professional Medical Attention
Knowing when to move from observation to professional consultation is important. If the discoloration appeared without known trauma, or if a dark mark persists and does not grow out with the nail over several months, a medical evaluation is necessary. You should also seek immediate care if you experience severe, unrelenting pain or pressure in the toe that suggests a large, acute hematoma.
Signs of potential infection, such as increasing redness, warmth, swelling, or the presence of pus or foul odor around the nail, also necessitate a doctor’s visit. Any black or dark spot that begins to spread to the surrounding skin—the Hutchinson sign—requires prompt assessment to rule out melanoma. A doctor may perform trephination for a painful hematoma or, in more complex cases, take a biopsy of the nail bed to confirm a diagnosis.