Toe fungus is a fungal infection that grows in, under, or on the toenail. It typically starts as a white or yellow-brown spot beneath the tip of the nail and, over time, causes the nail to thicken, discolor, and crumble. The medical name is onychomycosis, and it affects roughly 4% of the general population, with older adults nearly five times more likely to develop it than younger people.
What It Looks Like
The infection usually begins at the outer edge of the nail and works its way inward. In its early stages, you might notice only a small discolored patch that’s easy to ignore. As the fungus spreads deeper into the nail, the changes become harder to miss:
- Thickened nail that’s difficult to trim
- Yellow, brown, or white discoloration that doesn’t grow out
- Brittle, crumbly, or ragged edges
- Misshapen nail that may curve or warp
- Separation from the nail bed, where the nail lifts away from the skin underneath
- A foul smell coming from the affected toe
Not every thick or discolored toenail is fungal. Nail psoriasis, repeated trauma from tight shoes or running, and simple aging can produce similar-looking changes. One way doctors distinguish fungal infections is by looking at the border where the nail separates from the bed: in fungal cases, that edge tends to be jagged with spiked projections, while trauma produces a clean, linear border.
What Causes It
A group of fungi called dermatophytes are responsible for about 90% of toenail infections. These organisms feed on keratin, the protein that makes up your nail. They thrive in warm, moist environments, which is why toenails are hit far more often than fingernails and why the infection is so common among people who spend time in communal showers, pools, or locker rooms.
Yeasts and environmental molds account for the remaining cases. Molds are cultured almost exclusively from toenails and make up around 8% of nail infections. In most cases, the fungus enters through a tiny crack or gap between the nail and the nail bed, often after a minor injury you may not even notice.
Who Gets Toe Fungus
Age is one of the strongest risk factors. Older adults have slower nail growth, reduced blood flow to the feet, and decades of cumulative exposure to fungal organisms. Beyond age, several health conditions significantly raise your odds. A large case-control study using data from the All of Us Research Program found that people with toe fungus were far more likely than controls to also have:
- Athlete’s foot (present in about 21.5% of cases, with 11 times the odds compared to controls)
- Obesity (present in 46.4% of cases, roughly 2.5 times the odds)
- Peripheral vascular disease (3 times the odds)
- Venous insufficiency (3.4 times the odds)
- Diabetes (3.3 times the odds)
Athlete’s foot and toe fungus are caused by the same family of fungi. Having one makes the other much more likely, since the infection can spread from the skin between your toes directly into the nail. Tobacco use and a weakened immune system (from HIV, cancer treatment, or immunosuppressive medications) also increase risk.
How It’s Diagnosed
A doctor can often suspect toe fungus based on appearance, but visual inspection alone isn’t reliable enough to start treatment. Confirmation requires a lab test on a nail clipping or scraping. The most common approach uses a chemical solution (potassium hydroxide) that dissolves the nail material so fungal structures become visible under a microscope. This test is quick and inexpensive, with about 61% sensitivity and 95% specificity, meaning it rarely gives a false positive but can miss some infections.
If the initial test is unclear, a fungal culture can identify the exact organism involved. Cultures are highly specific (99%) but take several weeks to grow and miss infections roughly half the time. A nail biopsy, where a clipping is examined under the microscope after special staining, offers the best balance of sensitivity (84%) and specificity (89%). Your doctor may use one or a combination of these methods depending on how straightforward your case is.
Treatment Options
Toe fungus is notoriously stubborn. The nail grows slowly (a big toenail takes 12 to 18 months to fully replace itself), so even successful treatment requires patience before you see a clear, healthy nail.
Topical Antifungals
Medicated nail lacquers and solutions are applied directly to the affected nail, usually once daily. They work best for mild infections that haven’t reached the base of the nail or spread to multiple toes. Because the nail plate is a dense barrier, topical treatments have lower cure rates than oral medications. They’re often chosen when the infection is limited or when oral drugs aren’t safe due to other health conditions.
Oral Antifungals
Prescription pills taken for one to three months are the most effective treatment for moderate to severe infections. They work systemically, reaching the nail through the bloodstream from underneath. Most patients take the medication for about 6 to 12 weeks, though the nail itself won’t look fully normal until it has grown out completely months later.
These medications require liver function monitoring. Your doctor will typically check blood work before starting treatment and periodically during the course, since the drugs can occasionally cause liver enzyme elevations. Most people tolerate the treatment well, but the need for monitoring is one reason doctors prefer to confirm the diagnosis with a lab test before prescribing.
Other Approaches
In severe cases where the nail is extremely thickened or painful, temporary removal of part or all of the nail can allow topical medication to reach the infection directly. Laser treatment is marketed at many podiatry offices, but evidence for its effectiveness remains limited compared to standard antifungal therapy.
What Happens If You Ignore It
Toe fungus won’t resolve on its own. Left untreated, it tends to spread to additional nails and can serve as a persistent source of athlete’s foot. The nail may become so thick that it presses against the inside of your shoe, causing pain when you walk. In people with diabetes or poor circulation, a damaged, fungal nail creates an entry point for bacteria, raising the risk of secondary skin infections in the surrounding tissue. For otherwise healthy people, the consequences are mostly cosmetic and comfort-related, but the longer the infection is established, the harder it becomes to clear.
Preventing Infection and Reinfection
Fungal spores are remarkably resilient and can survive on socks, shoes, shower floors, and nail tools for extended periods. Preventing toe fungus, or keeping it from coming back after treatment, requires attention to a few key areas.
Laundry temperature matters more than most people realize. Washing socks and towels in hot water at 60°C (140°F) or higher for at least 45 minutes kills dermatophytes effectively. A warm 30°C cycle, which many machines default to for energy savings, does not inactivate fungal spores. If your washer can’t reach that temperature, soaking socks in diluted bleach (roughly one part bleach to ten parts water for 10 minutes) achieves 100% disinfection.
Shoes are a common source of reinfection. Spraying the inside of shoes with a disinfectant and allowing them to dry fully before wearing them again helps break the cycle. Rotating between two or more pairs gives each one time to air out. Wearing moisture-wicking socks and choosing breathable footwear keeps your feet drier and less hospitable to fungi.
In shared spaces like gym showers or pool decks, wearing sandals or shower shoes creates a barrier between your feet and contaminated surfaces. Nail clippers and files should never be shared, and your own tools should be cleaned between uses. Standard steam sterilization (121°C for 30 minutes) is the most reliable method for metal instruments, but even wiping them with a hydrogen peroxide or bleach solution significantly reduces fungal contamination.