If you’ve been ramping up your running routine, you’re probably bracing yourself for a few of the unpleasant-but-common side effects of increasing your mileage—blisters, muscle tightness, and next-day soreness, to name a few.
One thing you probably weren’t thinking of? Toenails that turn black, and, in some cases, even fall off.
Um, what? Sorry to be the bearer of bad news, but if you run on the regular, you can pretty much expect a blackened toenail or two at some point in time, Ronald Lepow, D.P.M., an assistant professor of orthopedic surgery at Baylor College of Medicine in Houston and a lifelong runner, tells SELF.
“When you get your first one, it’s kind of a sign that you’ve moved your training to a higher level,” he says.
That’s because black toenails in runners tend to become more common the more you run, and the longer you go. People who run a mile or two at a time, and a couple days a week, are less likely to experience them than those who train multiple days of the week, hitting at least a 5K (roughly 3 miles) per run, Dr. Lepow says.
In most cases, black toenails in runners are not something to worry about, and for some people, it just comes with the miles (some runners even consider it a badge of honor to get their first one), but they can be annoying and pretty darn unsightly when flip flop season rolls around. Here’s everything you need to know about black toenails in runners.
How can running turn your toenails black?
The repetitive trauma due to the mechanics of running is the most common cause of black toenails in people who run, Jennifer Lucas, M.D., a dermatologist at the Cleveland Clinic, tells SELF.
“Typically, their toes are hitting somewhere in their shoes, often in the front of their shoes, and that pressure is pushing on the toenail plate itself,” she explains. “It basically causes bruising or bleeding underneath the toenail in the bed of the toenail.”
The official name for this is a subungual hematoma, which pretty much just means a blood blister under the nail, says Dr. Lepow. “That black color you see is really the dried blood,” he explains.
Pressure is to blame for this microtrauma, whether it’s from shoes that are too small or socks that are too tight, both of which can cause your toes to smack up against the sides of your running shoes, he says.
That’s why prolonged bouts of downhill running—where your foot is being pushed forward more with each stride—can increase the chances of getting a black toenail. So can running in hot weather; hot temperatures cause your feet to swell, which can increase the pressure in your shoe, says Dr. Lepow.
You might notice a subungual hematoma after your run first as a small black spot, but over the next few days, the discoloration can grow in size, Priya Parthasarathy D.P.M., a member and spokesperson of the American Podiatric Medical Association, tells SELF. The drying of the blood can cause your nail plate to separate and loosen, meaning your toenail can actually fall off weeks or even months afterward.
Your big toe and second toe are most often affected, since they tend to be the longest—and thus more likely to rub up against the sides of your shoes, says Dr. Lucas. All this banging, smacking, and rubbing sounds pretty painful, but it’s actually pretty likely you wouldn’t even feel it while it’s happening.
“Oftentimes, it doesn’t hurt,” says Dr. Lucas. It’s not like the obvious, stop-in-your-tracks pain you feel when you drop something on your foot. “It’s just that your toe keeps touching that part on your shoe,” she says—a discomfort that’s not enough to get your attention while you’re running.