A black toenail is almost always caused by blood pooling under the nail after some kind of injury. This is called a subungual hematoma, and it’s the single most common reason a toenail turns dark. Less often, the discoloration comes from a fungal infection, a medication side effect, or, rarely, a type of skin cancer that develops under the nail. The cause matters, so it’s worth understanding what each looks like.
Trauma: The Most Common Cause
When something hits, crushes, or repeatedly presses on your toenail, the tiny blood vessels in your nail bed break open and leak. Because your nail plate is tightly bonded to the tissue beneath it, that blood has nowhere to go. It pools in the small space and creates pressure, which is why a freshly injured toenail can throb intensely. The trapped blood turns the nail dark red, purple, or black depending on how much has collected.
Obvious injuries cause this: dropping something heavy on your foot, stubbing your toe hard, or getting your foot stepped on. But the damage doesn’t have to be sudden. Runners, hikers, and people who spend long hours on their feet frequently develop black toenails from repetitive, low-grade pressure. Shoes that are too tight in the toe box, or that allow your foot to slide forward on downhill terrain, push the nail into the front of the shoe with every step. Over miles or hours, that’s enough to cause bleeding underneath.
If you can trace the discoloration back to a specific event or activity, trauma is almost certainly the explanation. The nail may eventually loosen and fall off on its own. A full toenail takes up to 18 months to regrow, and after an injury, regrowth can take anywhere from 6 months to 2 years.
When the Pain Is Severe
A large hematoma creates significant pressure under the nail, and the pain can be disproportionate to the size of the injury. If the pressure is bad enough, a doctor can relieve it with a simple procedure called trephination, where a small hole is made in the nail plate to let the blood drain. This works best within 24 to 48 hours of the injury and carries very low risk of complications. Trying to drain it yourself, however, can introduce bacteria and lead to infection.
Fungal and Bacterial Infections
Nail fungus is common and can cause a range of discoloration, including dark brown or black. Bacterial infections tend to produce green or black pigment. The key difference from trauma is the additional signs: an infected nail typically becomes thickened, brittle, crumbly, or misshapen. It may separate from the nail bed and develop a noticeable smell. The discoloration also tends to develop gradually rather than appearing after one event.
Dermatophytes (a group of fungi) cause most nail infections, but yeast, bacteria, and molds can all be responsible. Fungal nail infections don’t resolve on their own, so if your toenail is darkening along with these other changes, it’s worth getting evaluated. Treatment usually takes months because any medication has to work as the new, healthy nail slowly grows in.
Medication Side Effects
Certain drugs can darken your nails as a side effect. Chemotherapy drugs and some HIV medications are the most well-known culprits, often producing brown or black bands that run lengthwise across the nail. Antimalarial medications like hydroxychloroquine can cause a brownish discoloration of the nail bed. If you started a new medication and noticed your nails changing color afterward, the drug is a likely explanation.
Underlying Health Conditions
Dark streaks or patches on the nails sometimes reflect something happening elsewhere in the body. Conditions that can trigger nail darkening include vitamin B12 deficiency, Addison’s disease (an adrenal disorder), thyroid problems, lupus, scleroderma, and HIV. Pregnancy can also cause temporary nail pigment changes. In most of these cases, the nail discoloration is one of several symptoms, not the only one.
Subungual Melanoma: Rare but Serious
The possibility that worries most people searching this question is cancer, specifically subungual melanoma. This is genuinely rare, but it does exist and is worth knowing about because early detection matters.
Subungual melanoma typically appears as a dark vertical streak running from the base of the nail toward the tip, almost as if someone drew a line on the nail with a brown or black marker. Over time, the streak may widen, become uneven in color, or develop irregular borders. The nail might crack, split, or lift away from the nail bed. In more advanced cases, the skin around the nail starts to darken too, a feature called Hutchinson sign, which is a strong indicator that the pigment is coming from melanoma rather than a benign cause.
A few features help distinguish melanoma from a simple bruise. A hematoma from trauma will grow out with the nail over months and eventually disappear. Melanoma doesn’t move with nail growth. It stays in place, or it widens. And while a bruise typically involves a broad area of discoloration, melanoma more often presents as a defined streak. If you have a dark band on your nail that wasn’t caused by an injury, that hasn’t changed position as the nail grows, or that has been getting wider, those are reasons to have it looked at promptly. A biopsy is the only way to confirm or rule out melanoma.
Preventing Black Toenails From Activity
If you’re a runner or hiker dealing with recurring black toenails, the fix is almost always in your footwear. Your shoes should have enough room in the toe box that you can wiggle your toes comfortably. Many people size their running shoes a half size up from their casual shoes to account for foot swelling during exercise. A wider toe box design can also reduce pressure on the big toe and second toe, which take the most impact.
Lacing technique matters too. The “heel lock” method, where you create a loop with the top eyelets to anchor your heel in place, prevents your foot from sliding forward on downhill stretches. Keeping your toenails trimmed short and straight across also reduces the surface area that catches on the shoe. These adjustments won’t help a nail that’s already damaged, but they can stop the cycle of losing the same toenail season after season.