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Why Do Runners Get Black Toenails?

My girlfriend is a marathoner and it seems like she always has black toenails. It’s gross. Why does this keep happening, and is there anything I can do to help her prevent them?


Suck it up and find a new fetish? We kid, we kid. (Sort of…)

First, take comfort in the fact that you’re not the only guy whose girlfriend’s toenails have crossed over to the dark side. Studies suggest anywhere from 2.5 to 14 percent of marathoners experience the phenomenon researchers like to call “jogger’s toe.”

Most black nails are the result of a hematoma (a bruise) under the nail. The bruise can cause swelling, and separate the nail from the nail bed. The thing is, once the nail is black, there’s not much you can do except wait for it to fall off and get replaced, naturally. She could paint the offending nail—“teams of collegiate runners have painted their nails dark colors for cosmetic purposes,” this paper says—but that’s up to her.

Of course, scarier things could cause a nail to turn black. It could be the result of a fungal infection, or even worse, a malignant melanoma that’s developed under the nail. A visit to the doctor will quickly rule out either of these diagnoses.

Let’s say she has jogger’s nail, which she likely does. It’s the result of repeated trauma to the nail. If she were to maintain a cadence of 180 steps per minute while running a four-hour marathon in ill-fitting shoes, she could potentially slam her toe into the front of her sneakers 21,600 times. That’s quite a beating.

What you can do, oh helpful boyfriend, is take her to buy new shoes. As the paper cited above advises, “lacing should be tight enough to keep the foot from sliding forward without restricting circulation, and the anterior toe box should be high and long enough to allow unrestricted dorsal flexion of the toes and minor forward slippage.” Also: learn to love the toe as a symbol of your girlfriend’s commitment to fitness. Black toenails happen to the best of runners, even when shod in a proper pair of shoes.

THE BOTTOM LINE: A minor bruise under the nail is probably causing the blackness. Try new shoes! Or a slick of trendy gunmetal gray polish should do the trick.