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 three miles) per run, Dr. Lepow says.
In most cases, black toenails from running 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—during which 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.
Afterward, though, could be another story, due to the pressure of the blood in that blister under the nail. “The more blood under the nail, the more it's going to hurt.” says Dr. Lepow.
In most cases, you don’t need to do anything for a black toenail.
Treatment for black toenails caused by running simply depends on how much pain they’re causing you.
“In mild cases, no treatment is needed at all, and the black nail will simply grow out,” Dr. Lepow explains. It will take about six to nine months to grow out completely, according to the American Osteopathic College of Dermatology.
For black toenails causing lots of pain, though, see your podiatrist. Your doctor may poke a few tiny holes in the nail to drain some of the blood and reduce the subsequent pressure and pain, says Dr. Parthasarathy. (Definitely do not try this at home.) It’s best to see your doc within a few days of noticing the black spot, since you want to get in before the blood dries. (Bonus: Along with less time dealing with pain, that can also reduce the chances that your nail will loosen and fall off, she says.)
If your black toenail is not hurting, you can continue to run as usual—just keep it trimmed short, especially any loose portions, to guard against further trauma, says Dr. Parthasarathy. Resist the urge to pull it off yourself (😱), which can cause more trauma if it’s not yet ready to come off, says Dr. Lepow.
If—and when—the toenail comes off, you should clean the area with soap and water twice a day, and then cover it with a bandage to protect the area while it heals, says Dr. Lucas. (If it’s bleeding when it falls off, apply pressure first to stop it.) You may also want to apply petroleum jelly to the area to help keep the skin moist as it heals.
Here’s how to reduce your chances of getting the dreaded black toenail.
While you may not be able to fully prevent a black toenail from occurring if you’re running long or often, there are some things you can do to make it less likely to happen.
First, keep your toenails short—the nail should be flush with the end of your toe.
“The less the nail extends beyond, it's going to be less likely to hit on the end of the shoe while you're running,” says Dr. Lucas. (Just make sure to cut straight across. Leaving curves or sharp edges can leave you vulnerable to an ingrown toenail, says Dr. Lepow).
Then take a close look at your running shoes. Shoes that are too tight—especially those that pinch in the toe box—can make that repetitive trauma more likely. You should make sure your running shoes give you about a half inch of room from where your longest toe ends. (Determine this while you are standing, not sitting, so your toes are fully forward.)
“This should give enough space for the toes of the foot to slide forward and back within the shoe,” Dr. Lepow says. You might need your shoes to go a little bigger, though, if your feet have a tendency to swell more when you exercise, or in hot temperatures.
Your socks play a role too. Thicker socks can be cushioning, but just make sure the thickness is not making the shoe too tight, Dr. Lepow explains. If you can, try on your socks with your running shoes so you can gauge how they will fit when you run. (The experts at a running store can help you make the right choice.)
Here’s when black toenails may indicate something more serious.
While black toenails caused by running are generally harmless, it’s possible that something else is going on. Having a doctor check out any pigmentation change on your skin is always a smart idea. If you’re a runner, though, and your black toenail doesn’t come with any more worrisome signs (see below), your doctor will take photos of your toe every month or so and will be looking for the discoloration to start to clear out as the new nail begins to grow.
If a toenail turns black and it’s not the running-related subungual hematoma caused by that repetitive microtrauma, your doctor might worry about something more serious, like a rare form of skin cancer called subungual melanoma. According to a study published in Dermatology, melanoma under the nail makes up only 3% of all melanomas.
In the case of melanoma, it’d likely appear as a brown or black vertical line, and you likely wouldn’t see a clearing at the base of your nail as time goes on. In fact, you may even see pigment spreading to the tissue surrounding the nail, says Dr. Lucas.
While subungual melanoma is the most concerning fear with a black toenail, there are other conditions that can cause it too. In some cases, a fungal infection—say, you were walking barefoot one too many times in your gym’s locker room—can blacken your toenails, says Dr. Lepow. It’s also possible that blackening under your toenails can simply be a natural pigment change as you get older.
While some causes of black toenails can be serious, the vast majority of time runners experience one, they can simply point to their form of exercise as the most likely cause. So if you run often—especially if you’ve recently upped your mileage—and notice a black toenail, don’t freak out. Chances are the repetitive trauma of pounding the pavement is to blame. But if you don’t want to sport a black toenail, you can play the prevention game: Definitely check your socks and your shoe fit to prevent another one from popping up. And it’s still a good idea to loop in a doctor so they can monitor and help treat it.
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