A black toenail is usually harmless — but not always. Here's how to tell the difference and what to do about it.
A black toenail can be alarming — but in most cases, it's not a sign of anything serious. At ASG Foot & Ankle Specialists, we see black toenails regularly, and the vast majority are caused by trauma or fungal infection. That said, there are rare cases where a black toenail signals something that needs prompt attention.
Here's a complete guide to the causes of black toenails, how each is treated, and the warning signs that mean you should see a podiatrist right away.
The most common cause. Blood pools under the nail after trauma — dropping something on your toe, stubbing it, or repetitive pressure from running or tight shoes. The blood appears black or dark red/purple under the nail.
Common in runners, hikers, and athletes. Repeated impact of the toe against the front of the shoe causes micro-trauma and bleeding under the nail. Often affects the longest toe.
Advanced fungal infection can cause the nail to turn dark brown or black, in addition to thickening, crumbling, and separating from the nail bed. The discoloration is from debris and fungal pigments accumulating under the nail.
A rare but serious cause. Subungual melanoma is a type of skin cancer that develops under the nail. It appears as a dark streak or patch that doesn't move as the nail grows. It can be mistaken for a bruise — which is why any unexplained black toenail should be evaluated.
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The most common causes of a black toenail are: subungual hematoma (blood pooling under the nail from trauma), fungal nail infection (onychomycosis), repetitive pressure from running or tight shoes, and rarely, subungual melanoma (a type of skin cancer). Most black toenails are caused by trauma or fungal infection and are not dangerous, but any black toenail that appears without a clear cause, doesn't grow out, or has irregular borders should be evaluated by a podiatrist.
Most black toenails are caused by trauma (blood under the nail) and resolve on their own as the nail grows out. However, see a podiatrist if: the black area doesn't move as the nail grows, there was no injury to explain it, the discoloration has irregular borders or multiple colors, you have a streak of dark pigment running from the nail base to the tip, or you have diabetes or a weakened immune system.
A black toenail from trauma (subungual hematoma) often does fall off as the blood separates the nail from the nail bed. This is normal — a new nail will grow in its place over 6–12 months. Keep the area clean and protected. If the nail is painful, a podiatrist can drain the blood to relieve pressure.
Our board-certified podiatrists can quickly evaluate your toenail, rule out serious causes, and get you the right treatment. Three locations in the South Chicago suburbs — same-week appointments often available.