When it comes to black toenail causes, the most common one is repeated trauma to the nail — stubbing your toe, dropping something on it, or your toes banging against the front of your shoes during running or hiking. Less often, a black toenail can come from a fungal infection or, rarely, a pigmented growth under the nail that needs medical attention. If the dark colour appeared without any injury, or it doesn’t grow out with the nail, get it checked promptly.
See a doctor promptly if you notice any of these:
- Dark discolouration with no injury you can remember
- A dark streak or spot that is spreading, changing shape, or not growing out with the nail
- Severe pain, swelling, redness, or pus around the nail
- A black toenail plus a wound that won’t heal, especially if you have diabetes
- Bleeding under the nail that doesn’t stop, or the nail lifting badly
What a black toenail is and why it happens
A black toenail is a toenail that has turned dark brown, purple, or black, either all over or in a patch. The most common cause is a bruise under the nail — doctors call it a subungual hematoma. When you stub your toe hard, drop something heavy on it, or your toes repeatedly jam into the toe box of your shoes, small blood vessels under the nail break and blood pools underneath. That trapped blood looks dark through the nail, and the colour can be dramatic even when the injury felt minor at the time.
Runners and hikers know this one well. On long downhills, your foot slides forward with every step and your longest toe taps the front of the shoe thousands of times. You might not notice anything during the run, then wake up the next day to a dark, tender toenail. Tight shoes, shoes a half-size too small, and worn-out socks that let your foot slide around all make it more likely.
Not every black toenail is a bruise, though. Fungal infections can darken a nail over weeks or months, usually along with thickening, crumbling, or a yellowish tinge at the edges. And rarely, dark pigment under a nail can be a melanocytic growth — the kind of thing that needs a dermatologist to look at, especially if it’s a single dark streak that starts at the cuticle and runs to the tip. That’s uncommon, but it’s the reason an unexplained black toenail shouldn’t just be ignored.
What helps a black toenail
For a simple bruise under the nail, the first 48 hours are about calming it down. Rest the foot, ice it for 15 minutes at a time with a towel as a barrier, and keep it elevated when you can. An over-the-counter pain reliever can take the edge off the throbbing. If the pressure under the nail is severe — that intense, pulsing ache that makes a shoe unbearable — a doctor can drain it safely in the office. Don’t try to drill or poke the nail yourself; according to the Cleveland Clinic, draining a subungual hematoma is a quick clinic procedure, and doing it at home risks infection.
After the first couple of days, switch to practical protection. Wear roomy shoes that don’t press on the toe, keep the nail trimmed straight across, and cushion the toe with a bit of moleskin if shoes rub it. The bruised nail grows out slowly — toenails grow roughly a millimetre a month.
If the nail loosens or falls off, keep the nail bed clean and covered until a new nail grows in, and watch for signs of infection like increasing redness or warmth. If fungus is the cause instead, over-the-counter antifungal treatments help mild cases, but stubborn or widespread fungal nails usually need prescription treatment from a clinician — and treating it early keeps it from spreading to other nails.
A bruise follows an injury and grows out with the nail. A fungal nail thickens and spreads slowly. A pigmented streak that doesn’t move as the nail grows is the one to get checked quickly.
A daily routine while your toenail heals
Start the day by checking the toe — a quick look for new redness, swelling, or drainage takes ten seconds and catches infection early. Wash your feet with warm water and mild soap, dry them thoroughly (especially between the toes), and put on clean, moisture-wicking socks. If the nail is tender, a silicone toe cap or a bit of padding before you put your shoes on makes the whole day more comfortable.
Choose shoes with a wide, deep toe box while the nail heals, and lace them so your foot can’t slide forward. If you’re a runner, this is the time to check your shoe size honestly — many runners need a half-size up from their everyday shoes, and that extra room is what stops the next black toenail.
In the evening, let your feet air out for a while after you take your shoes off. If the toe is still sore, a short soak in warm water can soothe it, but skip the soak if there’s an open wound or the nail has come off — in that case, keep it clean, dry, and covered instead.
Frequently asked questions
Will a black toenail go away on its own?
A bruised nail usually does, but slowly. The dark colour grows out with the nail, and sometimes the damaged nail falls off and a new one grows in underneath.
Is a black toenail always caused by injury?
No. Injury is the most common cause, but fungal infections can darken nails too, and certain medications or health conditions can change nail colour. The key question is whether you can tie it to a specific stub, run, or tight shoe — if you can’t, get it examined.
Should I drain a black toenail myself?
No. Draining a bruise under the nail with a needle or a heated pin is a common home remedy you’ll see online, but it carries a real infection risk and you can damage the nail bed. If pressure pain is severe, a clinician can do it sterilely in minutes.
Can running shoes really cause black toenails?
Absolutely — it’s one of the classic runner’s injuries. Shoes that are too small, laced too loosely so the foot slides, or worn out in the toe box all let the toes bang forward on downhills. A proper fit, with about a thumb’s width between your longest toe and the shoe’s end, prevents most cases.
When is a black toenail an emergency?
It’s an emergency if there’s uncontrolled bleeding, signs of infection like spreading redness and fever, or if you have diabetes and any foot wound or nail problem — diabetes makes foot injuries riskier and slower to heal, so don’t wait those out at home.
Sources
- Cleveland Clinic — Subungual Hematoma (bruise under the nail)
- American Academy of Dermatology — Nail changes and when to see a dermatologist
- Mayo Clinic — Ingrown toenails and toenail injuries
This article is for information only and is not medical advice. If your black toenail appeared without injury, is changing, or is painful, talk to a healthcare professional.