Toenail fungus is an infection that gets under and into the nail, causing discoloration, thickening, and brittleness. It’s extremely common, and a type of mold called a dermatophyte is responsible for about 90% of cases. The medical name is onychomycosis, and while it’s not dangerous for most people, it rarely resolves on its own and can worsen over months or years without treatment.
What Causes It
Fungi thrive in warm, moist environments. Your shoes create exactly that kind of environment, which is why toenails are far more commonly affected than fingernails. The dermatophytes that cause most infections are the same family of fungi behind athlete’s foot. In fact, athlete’s foot is one of the most common entry points: the fungus spreads from the skin around your toes into the nail bed, usually through a tiny crack or separation between the nail and skin.
Less commonly, yeasts and nondermatophyte molds cause toenail infections. These matter because they sometimes resist standard antifungal treatments, which is one reason doctors may want to confirm the specific type of fungus before starting treatment.
What It Looks Like
The first sign is usually a white or yellow streak near the tip of the nail. As the infection progresses deeper, the nail becomes thicker and more discolored, often turning yellow, brown, or greenish. The texture changes too: infected nails get brittle, crumbly, and ragged at the edges. In more advanced cases, the nail may separate from the nail bed entirely, lifting up and creating a gap underneath.
The skin beneath and around the nail can also thicken. This abnormal growth makes the nail harder to trim and can make wearing shoes uncomfortable. Without treatment, the infection can spread to other toenails, though it often starts on a single toe, most commonly the big toe.
Who Gets It
Risk increases with age, largely because nails grow more slowly and thicken as you get older, giving fungi more time to establish themselves. People with diabetes, poor circulation, or weakened immune systems face higher risk and more serious consequences from infection. Psoriasis affecting the nails also makes fungal infection more likely.
Lifestyle factors play a big role. Walking barefoot in damp public areas like pools, gym showers, and locker rooms exposes you directly to the fungi. Wearing shoes that trap sweat, having a history of athlete’s foot, or even a minor nail injury can all open the door to infection. If someone you live with has toenail fungus or athlete’s foot, that increases your exposure too.
Is It Fungus or Something Else?
Several conditions mimic toenail fungus, and roughly half of nail problems that look fungal turn out to be something else. Nail psoriasis is one of the most common lookalikes, but there are key differences. Psoriasis tends to affect fingernails more than toenails, and almost always shows up alongside psoriasis symptoms elsewhere on the body, like scaly patches on the elbows or scalp. Psoriasis also causes distinctive small pits (tiny depressions) in the nail surface, and sometimes produces reddish-brown splotches called “oil spots” underneath the nail. Neither of those features occurs with fungal infections.
Fungal infections, by contrast, typically show yellow or white streaks running along the nail, thickening of the nail and the skin underneath, and brittleness. If you also have cracked, peeling skin between your toes (athlete’s foot), that’s a strong clue pointing toward fungus rather than psoriasis or trauma.
Trauma to the nail from stubbing your toe or wearing tight shoes can also cause discoloration and lifting. A doctor can usually tell these apart, but lab confirmation is often needed to be sure.
How It’s Diagnosed
Because so many nail conditions look alike, doctors generally confirm toenail fungus with a lab test before recommending treatment. The simplest method involves clipping off a small piece of the affected nail and examining it under a microscope after treating it with a chemical solution that makes fungal elements easier to see. A fungal culture can identify the exact species, though cultures come back negative in up to 30% of cases even when fungus is present.
A nail biopsy with special staining is considered the most accurate single test. DNA-based testing is significantly more sensitive than culture and can detect both common and uncommon fungal species, though it’s not yet available everywhere.
Treatment Options
Treatment depends on how much of the nail is affected. For mild to moderate infections, topical antifungal solutions applied directly to the nail are the first option. These require daily application for a full 48 weeks. Complete cure rates are modest: the most effective topical option clears the infection entirely in about 15% to 18% of people. Other topical treatments have complete cure rates in the single digits. That may sound low, but “complete cure” is a strict standard requiring both a clear nail and negative lab tests. Many more people see meaningful improvement even if they don’t hit that benchmark.
For more extensive infections, oral antifungal medications are more effective but carry a greater risk of side effects. Treatment typically lasts several months, and the nail takes even longer to grow out and look normal again since toenails grow slowly, roughly replacing themselves over 12 to 18 months.
Laser treatment has generated interest but hasn’t proven to be a reliable cure. Results are mixed, and even when the treatment initially works, the fungus often returns. Laser is not currently considered a first-line treatment.
Recurrence is a frustrating reality with every approach. The same environment that caused the infection in the first place (warm, dark shoes) is still there after treatment ends, so reinfection is common without preventive habits.
How to Prevent It
Prevention comes down to keeping your feet dry and limiting your exposure to the fungi. Wear moisture-wicking socks and change them if they get sweaty. Give your shoes at least 24 hours to dry between wearings, and choose breathable materials like canvas or mesh when possible. An antifungal powder or spray in your socks and shoes adds another layer of protection.
In public spaces, always wear flip-flops or shower sandals in locker rooms, shared showers, pool decks, and gym floors. Never share nail clippers, shoes, or unwashed towels.
Keep your toenails trimmed short and cut straight across. Short nails give fungi less surface to colonize and reduce the chance of debris collecting underneath. Disinfect your nail clippers after each use: soak them for five minutes in a solution of one tablespoon bleach per cup of water if you have an active infection, or wipe them with 70% rubbing alcohol if you don’t.
If you develop athlete’s foot, treat it immediately before it has a chance to spread to the nails. When you’re undergoing treatment for toenail fungus, disinfect or throw away shoes you wore before starting treatment. UV shoe sanitizers work well for this. Wash all socks in hot water and detergent. And if someone in your household has a fungal infection on their feet or nails, they should be treating it at the same time to prevent everyone from passing it back and forth.