Many runners suffer from black toenails, which is caused by bleeding under the toenails. Poor-fitting shoes are usually the culprit.
The most common culprit for black nails is repetitive trauma, which can result from running or from wearing any type of ill-fitting footwear. If a black nail crops up shortly after a workout or a day spent in too-tight or too-loose shoes, this is likely the cause.
Repetitive trauma ranges from mild (think: a small, painless, black-and-blue discoloration beneath the nail), to severe (large, bloody blisters between your nail and nail plate), explains Sutera. Dropping a heavy object (say, a dumbbell) onto your foot can burst the blood vessels under your nail bed and cause blood to pool underneath, too.
But other issues besides trauma can cause black toenails as well.
Fungal infections—like athlete's foot—can spread to your toenails and turn them shades of yellow, blue, green, brown, purple, and black, Sutera explains. This range in color is unique to fungus, as is the presence of subungual debris—a chalky white substance that lines the nail bed and often carries a funky odor.
If you think you may have a fungal infection, head to your doctor—he or she can clip and biopsy a portion of your nail to confirm a diagnosis. Treatment options vary depending on the severity of the infection. Mild cases are often addressed with topical medications, while more aggressive fungi require oral medication or even laser treatment.