A darkened toenail, ranging from dark brown and purple to black, is a common concern. While discoloration can sometimes indicate a serious underlying medical condition, most cases result from relatively harmless events. Since the nail plate is translucent, any color change is usually due to an issue in the nail bed or matrix underneath, where the nail is formed. Understanding the origin of the discoloration—whether it is blood, an organism, or pigment—is the first step in addressing the change.
Physical Injury and Immediate Causes
The most frequent cause of a sudden dark toenail is trauma, resulting in a subungual hematoma, which is a bruise beneath the nail plate. This happens when an injury ruptures small blood vessels in the nail bed, causing blood to pool in the confined space between the nail and the tissue below. This pooling leads to a visible dark-red, blue-purple, or black patch.
Trauma can be acute, such as dropping a heavy object, or chronic, resulting from repeated minor impacts. Repetitive microtrauma is common in athletes or from wearing ill-fitting shoes that cause the toes to press against the front of the shoe box. The pressure from the trapped blood can cause throbbing pain, which a medical professional can relieve by draining the hematoma through a small hole in the nail (trephination). If the pain is mild, the dark patch will simply grow out over several months as the blood is reabsorbed and the new, clear nail plate replaces the damaged area.
Fungal Infections and Other Organisms
Another common reason for toenail darkening is onychomycosis, a fungal infection that thrives in the warm, moist environment created by socks and shoes. While these infections typically cause the nail to turn white or yellow, they can also result in dark discoloration. This darkening is often secondary, caused by a buildup of dark debris underneath the nail plate or by certain fungi that produce melanin pigment.
A fungal infection is distinct from trauma because it is accompanied by other structural changes. The nail plate often becomes thickened, brittle, and may crumble at the edges. The infection can also cause onycholysis, the separation or lifting of the nail from the nail bed, sometimes accompanied by a foul odor. Treatment involves anti-fungal medications, which may be topical for mild cases or oral for more severe infections, as the condition will not resolve on its own.
Pigmentation Changes and Systemic Conditions
Darkening can also result from increased pigment production, a condition known as melanonychia, which presents as a dark brown or black vertical stripe running the length of the nail. This occurs when melanocytes, the pigment-producing cells in the nail matrix, are activated to deposit melanin into the growing nail plate. Longitudinal melanonychia is a benign finding in most cases, particularly in people with naturally darker skin tones, where it is often considered a normal process.
The discoloration can also be a side effect of internal issues, including certain diseases or medications. Systemic conditions like Addison’s disease, chronic renal failure, or HIV infection can lead to generalized nail darkening or multiple pigmented streaks. Additionally, some pharmaceutical agents, such as certain chemotherapy drugs, can cause transverse bands of discoloration that move outward as the nail grows.
This category also includes subungual melanoma, a rare but serious form of skin cancer arising from melanocytes in the nail matrix. The signs are often summarized using the “ABCDEF” mnemonic. A particularly concerning sign is the extension of the black or brown pigment from the nail plate onto the surrounding skin, known as Hutchinson’s sign, which strongly suggests malignancy.
Triage: When to Consult a Medical Professional
While many instances of a dark toenail are benign, seeking professional medical advice is important for ruling out serious conditions. Consult a physician if the dark discoloration appears suddenly without any known injury. This is especially true if the dark patch or stripe does not grow out with the nail over time, which would be expected of a bruise or hematoma.
Other red flags include a pigmented stripe that is rapidly changing, widening, or has irregular, blurred borders. Any dark discoloration that spreads from the nail plate onto the cuticle or the surrounding skin needs immediate evaluation. Also, consult a doctor if the darkening is accompanied by signs of infection, such as swelling, persistent pain, redness, or the presence of pus.