Black discoloration under a toenail indicates a change in the nail unit or underlying tissue. While often harmless, this color change can signal a more serious underlying condition. Understanding the causes is important for proper assessment and timely intervention.
Blood Under the Nail
One of the most frequent reasons for a black toenail is a subungual hematoma, which is a bruise beneath the nail plate. This occurs when small blood vessels under the nail rupture, leading to a collection of blood. Direct trauma, such as dropping a heavy object on the toe, stubbing it, or even repeated minor impacts from ill-fitting footwear during activities like running, can cause this.
The blood trapped beneath the nail initially appears red or purple, but as it dries and oxidizes, it darkens, often turning deep brown or black. The discoloration typically spreads as the blood pools, and it will gradually grow out with the nail over several months. In most cases, a subungual hematoma resolves on its own, requiring no specific treatment unless there is pain or pressure.
Fungal Infections
Fungal infections, medically known as onychomycosis, are another common cause of toenail discoloration. These infections occur when fungi invade the nail plate or the nail bed. While fungal infections most commonly cause yellow, white, or brown discoloration, they can also lead to a black appearance. This darker hue can result from the accumulation of debris under the nail, a secondary bacterial infection, or the fungus itself producing pigments.
Beyond discoloration, fungal infections can manifest with other symptoms. The affected nail may become thickened, brittle, or crumbly, and it might even separate from the nail bed. These infections are contagious and can spread to other nails or to the skin. Effective treatment often requires antifungal medications, as the infection is unlikely to clear on its own.
Other Reasons for Discoloration
Less common, but important, factors can contribute to a black toenail. Subungual melanoma, a rare but serious form of skin cancer, can appear as a dark streak or patch under the nail. Unlike a bruise, this discoloration often has irregular borders, varies in color, and may extend into the skin surrounding the nail, a specific sign known as Hutchinson’s sign. This type of melanoma does not grow out with the nail like a hematoma, and its presence requires immediate medical evaluation.
Certain medications can also induce nail discoloration as a side effect. Some chemotherapy drugs, antimalarial medications, and certain antibiotics can cause nail pigmentation changes. Additionally, chronic irritation or infection from an ingrown toenail can lead to localized discoloration. The persistent inflammation and potential for bacterial growth can cause darkening of the nail or the skin directly beneath it.
When to See a Doctor
Consulting a healthcare professional is advisable if the cause of a black toenail is unclear or if concerning symptoms are present. Seek medical attention if the discoloration appeared without any known trauma or injury. Also seek evaluation if the dark area does not appear to grow out with the nail or if its shape or size changes over time.
Any dark streak that extends from under the nail onto the surrounding skin, known as Hutchinson’s sign, warrants prompt medical assessment. Similarly, pain, swelling, redness, or pus around the nail suggests an infection that requires treatment. If the nail begins to detach from the nail bed, or if there is any suspicion of a serious condition like melanoma, seek medical advice promptly for accurate diagnosis and appropriate care.