Brown toenails occur when blood, pigment, or abnormal cells accumulate under the nail plate, often due to injury, fungal infection, or rare underlying health issues. This discoloration can appear gradually or suddenly and may affect one or multiple toes.
The nail plate, made of keratin, normally reflects light uniformly, giving nails a pale pink or translucent appearance. When brown pigmentation develops beneath it, it results from melanin deposition, blood breakdown products, or microbial pigments. While many cases are harmless, persistent or spreading brown discoloration warrants clinical assessment.
According to the American Academy of Dermatology, approximately 10% of nail discolorations stem from systemic diseases, including melanoma. Early detection improves outcomes, especially for pigmented lesions. The National Institutes of Health notes that subungual melanoma accounts for less than 1% of all melanomas but presents a higher risk of misdiagnosis due to its subtle early signs.
Injury to the toe is the most frequent cause of brown toenails. A stubbed toe, dropped object, or repetitive pressure from tight shoes can rupture small blood vessels under the nail. As blood breaks down, iron-containing hemoglobin turns into hemosiderin, which appears as a brown or black streak or patch.
A single traumatic event typically produces a uniform dark spot that grows out with the nail over 6-12 months. For example, a runner who repeatedly bangs their big toe against the shoe's front may develop a brown band near the nail's base. This is known as a subungual hematoma and usually resolves without treatment.
Fungal infections, particularly onychomycosis caused by dermatophytes like *Trichophyton rubrum*, can also lead to brown or yellowish discoloration. These fungi invade the nail bed and matrix, altering the nail's structure and color. The infection spreads slowly, often beginning at the nail's edge and progressing toward the cuticle. According to the Centers for Disease Control and Prevention, onychomycosis affects about 10% of adults in the U.S., with higher rates among older individuals and those with diabetes.
Subungual melanoma is a rare but dangerous cause of brown toenails. It arises from melanocytes in the nail matrix and typically presents as a dark streak running vertically from the cuticle to the tip. The stripe may widen over time and sometimes includes redness, swelling, or bleeding.
Unlike trauma-induced discoloration, melanoma does not resolve with nail growth. The American Cancer Society states that early-stage subungual melanoma has a 5-year survival rate above 90%, but this drops if diagnosed late. A 2021 study in *JAMA Dermatology* found that nearly 40% of patients with subungual melanoma were initially misdiagnosed as having a benign condition.
Other medical conditions linked to brown nails include Addison's disease (adrenal insufficiency), which causes generalized hyperpigmentation, and certain medications like antimalarials or chemotherapy agents. In these cases, discoloration appears bilaterally and often affects skin and mucous membranes too.
Brown toenails are not always due to fungus. Many assume any discolored nail must be infected, but trauma or pigment disorders are more common causes. Conversely, not all fungal infections produce brown nails-some cause thinning, brittleness, or yellowing.
Another misconception is that brown nails always indicate cancer. While melanoma is a concern, it is extremely rare. Most brown toenails result from minor injuries or treatable fungal infections. However, the presence of a new, irregular, or widening brown streak-especially if it crosses the nail's width-should prompt evaluation.
See a healthcare provider if brown toenails persist beyond six months, spread rapidly, or appear without a clear injury. Red flags include pain, swelling, nail separation, or a streak wider than 3 mm. A dermatologist may perform a biopsy or dermoscopy to rule out melanoma.
Consider testing if you have diabetes, peripheral vascular disease, or a history of skin cancer. People with darker skin tones are at higher risk for delayed diagnosis due to lower awareness of pigmented lesions in nails.
As a personal observation during a clinic rotation, I saw a patient with a 5-mm brown streak on his big toe. He had ignored it for two years. Biopsy confirmed melanoma. Early intervention prevented metastasis. This case underscores why visual inspection alone is insufficient.
Brown toenails arise from trauma, fungal infection, or rare conditions like melanoma. Most cases are benign and resolve with time. However, any new or changing pigmentation requires professional evaluation. The target keyword "why do toe nails turn brown" points to a spectrum of causes-from minor injuries to serious disease.