A thick, discolored toenail sends many people straight to antifungal products — but not every damaged nail is fungal. Distinguishing fungus from trauma matters, because treating the wrong thing wastes time and money. Here’s how to think about it. Our toenail page covers fungal treatment.

What toenail fungus tends to look like

Fungal nails often start at the tip or side and spread inward, becoming thickened, yellow-brown or white, and brittle or crumbly. It commonly affects more than one nail over time and may accompany athlete’s foot. It tends to persist and progress rather than grow out.

What nail trauma tends to look like

Trauma-related nail changes are usually linked to an event or a pattern — stubbing a toe, dropping something, or repetitive pressure from running or tight shoes. You might see bruising (a dark red-to-black discoloration under the nail), thickening of a single nail, or a nail that lifts. Trauma changes often grow out over time as the nail replaces itself.

Clues that help tell them apart

• Number of nails: Fungus often spreads to several; trauma is usually one.
• History: A clear injury or repetitive pressure points to trauma.
• Color: Bruise-like red/black suggests blood from trauma; yellow-brown crumbly debris suggests fungus.
• Course: Trauma tends to grow out; fungus tends to persist or worsen.

Why the two can overlap

Confusingly, a traumatized nail can also become infected, because injury creates entry points for fungus. That’s one reason self-diagnosis is unreliable.

Getting a definitive answer

A podiatrist can examine the nail and, when needed, test a sample to confirm whether fungus is present. That diagnosis determines the right treatment — and prevents months of treating the wrong problem.

Frequently Asked Questions

This article is for general education and is not a substitute for a diagnosis. Medically reviewed by Dr. Shawn S. Rabbani, DPM.