A black toenail can happen for many reasons. Sometimes it is harmless and temporary, but in some cases it can be a sign of an injury, infection, or a condition that needs medical attention.

1. Bruising Under the Nail (Most Common Cause)

A very common reason for a black toenail is a subungual hematoma — blood trapped underneath the toenail.

It can happen from:

Stubbing your toe.
Dropping something on your foot.
Wearing tight shoes.
Running or hiking (repeated pressure from the toe hitting the shoe).

Signs it is likely a bruise:

The dark area appeared after an injury.
The nail may feel sore or tender.
The discoloration may slowly move outward as the nail grows.

Toenails grow slowly, so it can take months for the dark mark to completely grow out.

2. Repeated Pressure From Shoes or Exercise

Athletes often develop black toenails, especially runners.

Causes include:

Shoes that are too small.
Long downhill walks or runs.
Toes repeatedly hitting the front of the shoe.
Long-distance activities.

This is sometimes called “runner’s toe.”

Prevention:

Wear properly fitted shoes.
Leave enough room for toes.
Keep toenails trimmed.
Use moisture-wicking socks.

3. Fungal Nail Infection

A fungal infection can sometimes cause dark discoloration, although it more commonly causes:

Yellow or white nails.
Thickened nails.
Brittle or crumbly texture.
Nail lifting.

A long-standing dark nail with changes in thickness or texture may need evaluation.

4. Melanoma Under the Nail (Rare but Important)

A dark streak or patch under a toenail can rarely be a type of skin cancer called subungual melanoma.

Warning signs include:

A dark brown or black streak that does not go away.
Color spreading or becoming wider.
Irregular borders.
Pigment appearing on the surrounding skin near the nail.
No history of injury.
A single affected nail, especially if it is new.

This does not mean every black toenail is cancer — most are not — but unusual or changing nail pigmentation should be checked.

5. Other Possible Causes

Black or dark toenails may also be related to:

Certain medications.
Repeated nail trauma.
Poor circulation.
Diabetes-related foot problems.
Some skin conditions.

When to See a Doctor

Seek medical advice if:

✅ The black spot appeared without an obvious injury
✅ The discoloration is growing or changing
✅ The pigment extends onto the skin around the nail
✅ The nail is painful, swollen, or draining
✅ You have diabetes or circulation problems
✅ The mark does not move outward as the nail grows

What You Can Do at Home (For a Likely Bruise)

Wear comfortable, roomy shoes.
Avoid pressure on the toe.
Keep the nail clean and dry.
Do not try to remove the nail yourself.
If there is severe pressure pain soon after an injury, a healthcare professional can sometimes drain trapped blood to relieve pressure.

Bottom Line

A black toenail is often caused by trapped blood from trauma or pressure, especially from tight shoes or exercise. However, a new dark mark without a clear cause, a changing streak, or pigment spreading around the nail should be evaluated to rule out more serious problems.

Black Toenail Here s What it means