Although a toenail falling off from running is not uncommon and is sometimes even considered a badge of honor among certain runners, it can be painful and, if not managed well, can even increase the risk of infection. The primary reason why runners’ toenails fall off is that a subungual hematoma develops under the nail.

Repetitive trauma caused by running could lead to discolored, painful nails. Sweaty socks and dirty shoes along with tight laces are some other potential factors that lead to runners losing toenails. Toenail loss is best treated with professional medical help rather than attempting self-treatment at home.

Runner’s toenail is primarily caused by repeated stress on your toenails as you run. Rather than resulting from one acute misstep while you run, runner’s toenail or toenails falling off after running is typically because of the accumulated microtraumas your toe experiences during the gait cycle.

Runners’ toenails can fall off due to major trauma, repetitive minor trauma, or repeated friction on the toe. If you develop a black toenail, consult a doctor for proper treatment, which may include draining blood to relieve pressure.

When enough blood collects beneath the nail, it starts to separate the nail from the toe itself. “In other words, the blood is pushing the toenail off of the toe,” says Paull. “The toenails are attached to the toe around the edges, and when enough blood collects, it creates enough force to separate the nail.”