Onychomycosis is the medical term for a fungal infection of the nail, and it overwhelmingly affects toenails more than fingernails. It’s common: roughly 10% of the general population has it, and that number climbs to 20% in people over 60 and 50% in those over 70. The infection changes the nail’s color, thickness, and texture, and while it isn’t dangerous for most people, it doesn’t resolve on its own and can take over a year to fully treat.
What Causes It
The vast majority of toenail fungal infections are caused by a group of fungi called dermatophytes, organisms that feed on keratin, the protein your nails and skin are made of. One species in particular dominates: it’s detected in roughly 98% of infections, either alone (about 60% of the time) or alongside other fungi. That co-infection detail matters. Nearly 4 in 10 cases involve a mix of fungal species, which can complicate treatment and explain why some infections are stubborn.
Yeasts and non-dermatophyte molds account for a smaller share of cases and are more common in warmer climates. The fungus thrives in warm, moist environments, which is why toenails are hit far more often than fingernails. Feet spend hours inside shoes, creating ideal conditions for fungal growth.
Who’s Most at Risk
Age is the single biggest risk factor. Slower nail growth, reduced blood flow to the feet, and decades of cumulative exposure all contribute to rising rates with age. Beyond age, several conditions increase your odds significantly.
People with diabetes are nearly three times more likely to develop onychomycosis than people without it. Among those with diabetes, having peripheral vascular disease (poor circulation in the legs and feet) roughly doubles the risk again. Other risk factors include a weakened immune system, a history of athlete’s foot, psoriasis affecting the nails, excessive sweating, and regular use of shared showers or pools.
What It Looks Like
Onychomycosis doesn’t look the same in everyone, and the appearance depends on where the fungus enters the nail. The most common pattern starts at the tip or side of the toenail and works its way back toward the cuticle. You’ll notice the nail turning yellow or brownish-white, with debris building up underneath and the nail gradually thickening. Over time, the nail may become brittle, crumbly, or start separating from the nail bed.
A less common form shows up as white, chalky patches on the surface of the nail. This type affects only the top layer and tends to be easier to treat. In rare cases, the infection starts at the base of the nail near the cuticle and grows outward. This pattern is more often seen in people with weakened immune systems. In advanced infections, the entire nail can become thickened, discolored, and distorted.
How It’s Diagnosed
A thickened, discolored toenail isn’t always fungal. Psoriasis, trauma, aging, and other conditions can look nearly identical, so lab confirmation matters before starting treatment. Your doctor will typically clip or scrape a sample from the affected nail and send it for testing.
The most common initial test dissolves the nail material in a chemical solution and examines it under a microscope. This catches fungal infections about 80% of the time. A fungal culture, where the sample is grown in a lab, is more specific (99% accuracy when positive) but takes weeks to produce results and misses infections about 44% of the time. Newer DNA-based testing offers a good balance, with sensitivity and specificity both around 83 to 84%, and results come back faster. Your doctor may use a combination of methods to improve accuracy.
Oral Treatment Options
For moderate to severe infections, oral antifungal medication is the most effective approach. The standard course runs about 12 weeks. In a head-to-head trial, one oral antifungal cleared the fungus in 73% of patients by week 48, while the alternative cleared it in about 46%. Clinical improvement followed a similar pattern: 76% of the first group were cured or had only minimal remaining symptoms, compared to 58% of the second group.
Those numbers highlight an important reality. Even the best oral treatment doesn’t work for everyone, and the infection can persist or return. Your doctor will typically check liver function before and during treatment, since these medications are processed by the liver. The pills themselves are taken daily for the full 12-week course, but you won’t see the final result for months afterward, because the damaged nail needs to grow out completely.
Topical Treatment Options
Topical treatments are painted directly onto the nail and are generally reserved for mild to moderate infections, particularly when the fungus hasn’t reached the base of the nail or when oral medications aren’t an option. The challenge with topical therapy is penetration: the drug has to pass through the hard nail plate to reach the fungus underneath.
Newer prescription topical solutions have improved on older lacquer formulations, but cure rates remain lower than oral therapy. In large clinical trials, one widely studied topical achieved complete cure in 15 to 18% of adults, with the fungus itself cleared in 53 to 55%. Results varied by age and health status. In older adults over 65, the complete cure rate dropped to about 14%. In patients with diabetes, it was around 11 to 13%. These numbers reflect “complete cure,” meaning both the fungus is gone and the nail looks normal. Many more patients see meaningful improvement without hitting that strict benchmark.
Topical treatments require daily application for about 48 weeks, which demands patience and consistency. Missing applications reduces an already modest success rate.
Why Treatment Takes So Long
Even after the fungus is killed, the damaged portion of your toenail has to grow out and be replaced by healthy nail. Toenails grow slowly, roughly 1 to 2 millimeters per month, and a big toenail can take 12 to 18 months to fully replace itself. This means you may finish a 12-week course of oral medication and still have a discolored, thickened nail for many months afterward. That’s normal. The new nail growing in from the base should look healthy and clear. If it doesn’t, the infection may not have been fully eliminated.
Preventing Reinfection
Recurrence is one of the most frustrating aspects of onychomycosis. The same conditions that caused the initial infection, warm shoes, damp feet, exposure to contaminated surfaces, persist after treatment ends. Reinfection rates are high enough that prevention deserves real attention.
Keep your feet dry. Change socks when they get damp, choose moisture-wicking materials, and alternate shoes so each pair has time to dry out fully between wears. Treat athlete’s foot promptly, since the same fungi cause both conditions, and untreated skin infection can reinfect a healing nail. Use antifungal powder or spray inside your shoes. Wear sandals in communal showers, locker rooms, and pool areas. Trim nails straight across and keep them short so there’s less surface area for fungus to colonize.
If you’ve had onychomycosis once, you’re at higher risk for getting it again. Some doctors recommend periodic use of a topical antifungal on the nails even after successful treatment to reduce the chance of recurrence.