A black discoloration under a nail can be concerning. While often harmless, it can occasionally signal a more serious underlying health issue. Understanding the various reasons a nail might turn black helps determine when medical attention is needed.
Injury-Related Causes
The most frequent reason for a nail to appear black is a subungual hematoma, essentially a bruise under the nail. This occurs when direct trauma causes blood vessels beneath the nail to rupture and bleed. Common incidents include dropping a heavy object, stubbing a toe, wearing ill-fitting shoes, or impacts during sports.
Blood collecting under the nail plate can cause significant pain and throbbing due to pressure buildup. The discoloration typically starts as red or purple, then gradually darkens to brown or black as blood clots. As the nail grows, this discolored area slowly moves outward with the nail plate, eventually clearing. Fingernails usually take 4 to 6 months to clear, while toenails can take 6 to 9 months, or even up to a year.
Infections and Benign Discolorations
Beyond injuries, non-traumatic factors can lead to a nail turning black. Fungal infections (onychomycosis) are a common culprit. While often causing white or yellowish discoloration, severe fungal infections can lead to dark debris under the nail, making it appear black or very dark brown. These infections frequently cause the nail to thicken, become brittle, or separate from the nail bed.
Longitudinal melanonychia is another benign condition, presenting as a dark streak running the length of the nail. This occurs when pigment-producing cells (melanocytes) in the nail matrix create excess melanin incorporated into the growing nail. It is common and usually harmless in individuals with darker skin tones, sometimes appearing in multiple nails from childhood.
External chemical stains can discolor the nail. Dyes, ink, certain topical medications like silver nitrate, or prolonged dirt exposure can stain the nail surface. This discoloration is typically superficial and will grow out with the nail over time.
Serious Medical Conditions
A black nail can sometimes indicate a serious medical condition. Subungual melanoma, a rare form of skin cancer, can manifest as a black or dark brown streak under the nail. Warning signs include a rapidly changing appearance, irregular borders, or discoloration spreading to the surrounding skin or cuticle (Hutchinson’s sign). Unlike a bruise, a melanoma streak typically does not grow out with the nail or fade.
Certain systemic medications can cause nail discoloration as a side effect. Some chemotherapy drugs, antimalarial medications, and minocycline are known to cause such changes. These drug-induced discolorations often affect multiple nails and may appear as diffuse pigmentation or transverse bands.
Underlying systemic health conditions can affect nail health. Conditions like certain autoimmune diseases, kidney disease, or adrenal insufficiency can cause nail changes. Though black discoloration is less typical than other colors, these changes often indicate a broader health issue.
When to Consult a Doctor
Consult a doctor for a black nail if there is persistent or worsening pain after an injury, or if discoloration occurred without clear trauma. Any dark streak that does not grow out with the nail, changes in size or color, or spreads to the cuticle or surrounding skin (Hutchinson’s sign) should be promptly evaluated. Unexplained or recurring black nails, or any concern about melanoma, require medical assessment. A doctor can diagnose the cause, which may involve examination, imaging, or a biopsy. Early detection allows for appropriate treatment.