Black Toenail: What It Means and What to Do About It
A black toenail looks alarming. Usually it isn’t. But sometimes it is, and the two situations need completely different responses.
Here’s how to know which one you’re dealing with.
The most common cause: blood under the nail
Subungual hematoma. Blood pooling between the nail and the nail bed after trauma. You stubbed your toe, dropped something on your foot, or wore tight shoes during a long run. The blood has nowhere to go, so it pools and darkens, turning the nail black or deep purple.
Runners get this constantly. It’s so common in marathons it has a nickname: “runner’s toe.”
If you remember an impact or your shoes were tight recently, this is almost certainly what you’re looking at.
What to do if it’s trauma-related
Small black spots covering less than a quarter of the nail? Leave it alone. The blood reabsorbs, the nail grows out, done. Takes 6 to 9 months for the full nail to cycle through.
Larger pooling, especially with throbbing pain in the first 24 to 48 hours, is a different story. The pressure buildup is what hurts. A doctor can drain it by making a small hole through the nail (trephination), which immediately relieves the pain. This sounds worse than it is. The nail itself has no nerve endings.
Don’t try to drain it yourself with a needle at home. The infection risk isn’t worth it.
When it’s not trauma
This is the part that matters more.
If you can’t explain the blackness with an injury, and especially if it’s been there for weeks without growing out, get it checked. A few possibilities:
Fungal infection. Nail fungus can turn a nail black, not just yellow or white. Usually comes with thickening and brittleness too.
Melanoma. Subungual melanoma is rare but real. It starts as a dark streak running lengthwise down the nail, not a spreading blot. The streak originates from the nail matrix (the base). If you see a dark vertical line that appeared without trauma, see a dermatologist. Not next month. This week.
The Hutchinson sign is one thing doctors look for: pigmentation spreading from the nail onto the surrounding skin. That’s a red flag.
Medications and systemic conditions. Certain chemotherapy drugs, antimalarials, and silver-containing supplements can darken nails. So can Addison’s disease and HIV in some cases.
A quick way to think about it
Trauma you remember + pain that’s fading + nail growing out normally = wait it out.
No trauma + no growth + dark streak running top to bottom = see a doctor now.
Protecting the nail while it heals
If the nail is loose after trauma, don’t rip it off. Leave it in place as long as possible. It protects the sensitive nail bed underneath while the new nail grows in.
Keep the area clean and dry. If the nail is cracked and catching on socks, trim the loose edge carefully with clean clippers and cover with a bandage.
Give it time. Toenails are slow. The black will grow out toward the tip over months, and eventually clip off with the nail. Watching for it to disappear week by week will drive you crazy. Check monthly instead.