Why toes or toenails can turn black
Blackening of a toe or toenail can have several causes, ranging from a minor injury to a serious problem involving blood flow or infection. The appearance alone is not enough to determine the cause, so persistent or unexplained discoloration should be assessed by a health professional.
One common cause of a black toenail is trauma. Stubbing a toe, having something strike the nail or repeatedly wearing tight shoes can cause bleeding beneath the nail, known as a subungual haemorrhage. The trapped blood can make the nail appear red, purple, brown or black. Repeated pressure from poorly fitting footwear can cause similar injury even when a person does not remember a specific accident.
The discoloration from an injury should gradually move toward the end of the nail as the nail grows. A dark mark that does not move with nail growth, continues to change or develops without an obvious injury may require further examination.
Nail separation and fungal infections
Another condition affecting toenails is onycholysis, in which the nail plate separates from the nail bed. It can result from repeated trauma, prolonged exposure to water, chemical exposure, skin conditions, fungal infections and some systemic diseases. The separated part of the nail often appears white or opaque rather than simply black. Secondary infections can cause further discoloration.
Fungal nail infections, known medically as onychomycosis or tinea unguium, can also change the appearance of toenails. They commonly cause thickening, brittleness and discoloration and are more likely in people with conditions such as diabetes, poor circulation or a weakened immune system.
These conditions should not automatically be treated with antifungal medicines simply because a nail has changed color. Different nail disorders can look similar, and medical assessment may be needed to establish the cause.
When blackening may signal a circulation problem
A dark or black toe can sometimes indicate a serious loss of blood supply. Peripheral artery disease, or PAD, occurs when arteries supplying the legs become narrowed or blocked. Severe loss of blood flow can lead to tissue death, known as gangrene.
Other signs of significant circulation problems can include a foot or leg that is unusually cold, wounds that heal slowly, changes in skin color, reduced hair growth and slow-growing toenails.
Diabetes can increase the risk of serious foot problems because it can damage nerves and blood vessels. Nerve damage may reduce sensation, making it harder for someone to notice an injury, while reduced blood flow can make wounds more difficult to heal.
Gangrene is a more serious possibility. It occurs when body tissue dies because of inadequate blood supply or a serious infection. Affected tissue can become dark red, purple or black and may also become cold, numb, swollen or painful. Gangrene requires urgent medical attention.
Melanoma is rare but should not be overlooked
A dark streak or patch beneath a toenail can rarely be caused by subungual melanoma, a form of skin cancer that develops under the nail. It often appears as a dark brown or black band and can gradually become wider or more irregular.
Because melanoma is uncommon and several non-cancerous conditions can also cause dark nails, a dark nail should not automatically be assumed to be cancer. But a new or changing dark mark without an obvious explanation should be examined by a healthcare professional.
When to seek medical care
Anyone with unexplained blackening of a toe or toenail should consider getting it assessed, particularly if the discoloration persists or changes.
Medical attention is more urgent when the toe or foot becomes black along with severe or worsening pain, numbness, coldness, swelling, an open wound, signs of infection or rapidly changing skin color. These can be signs of gangrene or severe circulation problems.
People with diabetes should be particularly careful about changes in the color, temperature, skin or nails of their feet. The CDC recommends seeking medical care for changes in foot color or temperature, sores, infections and other foot problems rather than waiting for them to become more serious.
The key point is that black toes or toenails are a symptom, not a diagnosis. The appropriate treatment depends on whether the underlying cause is an injury, fungal infection, nail disorder, circulation problem or another condition.
