A black toenail indicates changes beneath the nail plate, often due to bleeding, bruising, or pigment alterations. While many causes are benign, it can stem from various factors, ranging from minor injuries to more serious underlying conditions.
Physical Injury and Trauma
One of the most frequent reasons for a black toenail is physical injury, resulting in a subungual hematoma. This occurs when blood becomes trapped and pools beneath the nail plate, often appearing as dark red, purple, or black discoloration. This type of trauma can be from a single, blunt force, such as dropping a heavy object on the toe or stubbing it forcefully.
Repetitive microtrauma is another common cause, particularly in athletes. Activities like running, hiking, or sports involving sudden stops and turns can lead to the nail repeatedly hitting the inside of a shoe. This repeated impact can cause small blood vessels under the nail bed to rupture, leading to discoloration. The discolored area will gradually grow out with the nail over several months as the nail grows.
Toenail Infections
Toenail infections, both fungal and bacterial, can also lead to discoloration. Fungal infections (onychomycosis) can cause the nail to thicken, crumble, and change color, appearing brownish, yellowish, or even black. These infections are common and thrive in warm, moist environments like sweaty shoes or public showers.
Bacterial infections can also affect the toenail, such as Pseudomonas aeruginosa. This bacterium can produce a greenish-black discoloration of the nail, sometimes accompanied by a distinct odor. Such infections are often acquired in damp conditions or through shared nail tools.
Systemic Conditions and Other Factors
Beyond injuries and infections, several other factors can contribute to toenail discoloration. Subungual melanoma, a rare but serious form of skin cancer, can manifest as a dark, growing streak under the nail. This streak often runs vertically from the cuticle to the tip and may show irregular borders or extend to the surrounding skin, a sign known as Hutchinson’s sign. Unlike bruises, this discoloration does not grow out with the nail or fade.
Certain medications, including some chemotherapy drugs and antimalarials, can cause nail darkening as a side effect. Benign hyperpigmentation, a natural increase in pigment cells, can also result in harmless dark streaks or generalized darkening of the nail bed. This is more commonly observed in individuals with darker skin tones and involves multiple nails.
Seeking Professional Medical Advice
Consult a healthcare professional if a black toenail appears without a known injury, or if the discoloration is accompanied by severe pain, swelling, pus, or a foul odor. Also seek medical attention if the dark spot is spreading rapidly, if there is a persistent dark streak extending to the cuticle, or if there is any suspicion of melanoma.
A healthcare provider can accurately diagnose the cause of the black toenail through a physical examination and, if necessary, further tests like a nail biopsy or fungal culture. Early diagnosis allows for appropriate treatment, which can range from conservative care for minor trauma to specific medications for infections, or more involved interventions for serious conditions like melanoma.