A black toenail can have several causes, and while it is often harmless, some changes should be checked by a doctor. Common explanations include injury, pressure from shoes, fungal infections, and, less commonly, skin cancer.One of the most frequent causes is trauma. Hitting your toe or repeatedly putting pressure on it can cause blood to collect beneath the nail. This is called a subungual hematoma and may appear dark red, purple, brown, or almost black.

Tight or poorly fitting shoes can cause similar problems, especially for runners, hikers, or people who spend long periods on their feet. Repeated pressure may gradually darken the nail even when there is no memorable injury.A fungal infection can also change the appearance of a toenail. The nail may become yellow, brown, or dark, while also becoming thicker, brittle, crumbly, or separated from the nail bed.

Sometimes dark discoloration develops because of medications, certain medical conditions, or increased pigment production. In these cases, the appearance and pattern of the discoloration can provide important clues about the underlying cause.A less common but important possibility is melanoma of the nail. It can appear as a dark brown or black streak beneath the nail and may gradually become wider or change over time. It is particularly important to have unexplained discoloration evaluated.

Pay attention to changes such as a dark mark that does not grow out with the nail, pigment spreading onto the surrounding skin, bleeding, pain without an obvious injury, or a rapidly changing appearance.If you know the nail was injured and it is gradually growing out normally, it is often less concerning. However, if there is no clear explanation for a black toenail or the discoloration persists or changes, a healthcare professional should examine it.