Black toenail: fungus, bruising, or something that needs a doctor

Bruising under the nail

A subungual haematoma is blood trapped beneath the plate, usually from a dropped object or from repeated impact in running shoes that are half a size too small. It often appears suddenly, can be tender for a few days, and moves toward the tip as a defined patch as the nail grows outward over the following weeks.

It needs no specific treatment beyond better-fitting shoes and time, though a large, very painful haematoma covering most of the nail bed should be seen the same day, since pressure sometimes needs to be released to relieve pain.

Advanced fungal infection

Long-standing fungal nails can darken to brown or black as debris and pigment-producing organisms build up under the plate. The giveaway is context: the nail is also thickened and crumbly, the border of the discolouration is irregular rather than a clean defined patch, and it arrived gradually over months rather than overnight.

This pattern is common in nails that have been infected for a year or more without treatment, particularly in people who did not notice the earlier yellow or brown stage.

Other causes worth knowing

Certain medications, some skin conditions, and rarely a benign or malignant growth in the nail matrix can also cause dark discolouration. These are far less common than bruising or fungus but are the reason black nails deserve more caution than yellow ones.

How it is diagnosed

A clinician can usually distinguish a bruise from fungus by history and appearance alone, sometimes confirmed with dermoscopy. If a longitudinal dark band is present with no clear injury, a biopsy of the nail matrix may be recommended to rule out a pigmented lesion, which is a precaution rather than a sign that something serious is likely.

Comparing the possibilities

Most black toenails resolve into one of two categories, but the details differentiate them clearly if you know what to look for.

Treating the fungal kind

If a clinician confirms fungus, treatment is the same as for any advanced case: reduce the thickness by filing, treat the surrounding skin with an antifungal cream, and commit to a daily nail treatment for months while the damaged plate grows out. Dark discolouration that has been present for years usually takes a full growth cycle, nine to twelve months, to fully disappear.

What goes wrong

The main mistake is assuming any black nail is 'just a bruise' and ignoring it indefinitely. Bruises grow out and move toward the tip within a few months; if a dark area is not moving, is widening, or has been present for over three months with no change in position, it needs a professional look rather than more waiting.

When to get it checked promptly

Book an appointment rather than self-treating if any of the following apply, since these patterns fall outside the routine bruise-or-fungus picture.

• A dark band running lengthways from the cuticle to the tip, especially if it widens or the pigment spreads onto the surrounding skin
• Black discolouration with no memory of injury and no thickening
• Bleeding, an open sore, or pain that keeps increasing rather than settling
• You have diabetes, neuropathy or poor circulation

Frequently asked questions

Keep reading

• Toe fungus signsEarly warning signs, look-alike conditions, and what to do in the first month.
• Thick yellow toenailsFungus, trauma, age or polish? Identify the cause and pick the right fix.
• Onychomycosis explainedThe clinical picture: types, stages, diagnosis and every treatment route.
• Fungus and diabetesWhy diabetes changes the plan, and the daily foot-check routine that protects you.

Browse all nail fungus guides

Sources

• Mayo Clinic — Nail fungus
• MedlinePlus — Nail injuries and disorders
• NHS — Fungal nail infection