Dark toenails, appearing in shades of purple, brown, or black, can be a noticeable and sometimes concerning change. While often cosmetic, a darkened toenail can also signal underlying conditions requiring attention. Understanding these factors is important for foot health.
Injury and Impact
Physical trauma is a frequent cause of dark toenails, typically resulting in a subungual hematoma (blood under the nail). This can occur from a single injury, such as stubbing a toe or dropping an object. Repetitive microtrauma also contributes, particularly from activities like running or wearing ill-fitting shoes. The pooled blood initially appears red or purple, gradually changing to darker shades like brown or black as it clots.
Pain and tenderness often accompany a subungual hematoma due to the pressure from trapped blood. In many cases, the discoloration will grow out naturally as the nail grows, taking several months for resolution. Severe trauma can lead to the nail lifting from its bed or detaching entirely, which warrants professional evaluation.
Infections
Infections are another common reason for toenail discoloration. Fungal infections, known as onychomycosis, can cause dark discoloration, ranging from yellow-brown to black. This darkening often results from debris accumulating under the nail or from fungal pigment production. Other signs include nail thickening, brittleness, and sometimes a foul odor.
Bacterial infections can also affect toenails, with Pseudomonas aeruginosa being a notable example. This bacterium often leads to a greenish-black discoloration due to pigment production. These infections commonly develop in moist environments or in nails compromised by trauma or other conditions.
Systemic Health Issues
Dark toenails can also be a manifestation of broader systemic health conditions. Certain medical issues can impact nail color, including kidney disease, which may cause the lower half of the nails to turn white and the upper half to appear pigmented. Heart conditions and diabetes, particularly when affecting circulation, can also contribute to nail discoloration. Nutritional deficiencies, such as a lack of vitamin B12 or iron, have been associated with nail color changes.
Medications can also cause nail discoloration as a side effect. Chemotherapy drugs are known to cause nail changes, including darkening. Certain antimalarial drugs and tetracycline antibiotics can also lead to nail pigmentation. These changes are often temporary and resolve after discontinuing the drug, though a healthcare provider should be consulted.
Pigmentation Concerns
Pigment production can also lead to dark toenails. Benign melanonychia presents as a longitudinal dark streak, resulting from increased melanin deposition in the nail matrix. This condition is often harmless and more prevalent in darker skin tones, where it can be a normal finding.
Conversely, subungual melanoma is a serious skin cancer originating under the nail. Key warning signs include pigment spreading to the nail fold or cuticle (Hutchinson’s sign). Other concerning features are irregular borders, rapid growth, or single-digit involvement without clear trauma. Any suspicious changes warrant immediate medical evaluation.
External factors can also temporarily darken toenails. Dark nail polish, dyes from clothing, or cleaning products can stain the nail plate. These external stains typically affect the nail surface and are distinguished from internal causes by their temporary nature or by growing out with the nail.