Nail mycosis, formally called onychomycosis, is a fungal infection of the fingernails or toenails that causes discoloration, thickening, and brittleness. It affects roughly 4% of the general population and is by far the most common nail disorder, though many cases go undiagnosed. Toenails are affected far more often than fingernails.
What Causes It
A group of fungi called dermatophytes causes the vast majority of cases. These organisms have a specialized ability: they can break down keratin, the tough structural protein that makes up your nails. The dominant species is Trichophyton rubrum, responsible for most toenail infections worldwide. Dermatophytes account for about 90% of toenail infections and 50% of fingernail infections.
The remaining cases come from yeasts (mainly Candida albicans, responsible for about 2% of infections and more common in fingernails) and a miscellaneous group of environmental molds like Fusarium and Aspergillus, which together account for roughly 8% of nail infections.
How Fungi Break Down Your Nail
Keratin is one of the toughest proteins in the body. It’s reinforced by chemical bonds called disulfide bonds that lock the protein chains together. Dermatophytes produce specialized enzymes that first break these bonds, loosening the keratin structure. Once the nail’s defenses are weakened, a second wave of enzymes cuts the protein chains into smaller and smaller fragments until individual amino acids are released. The fungus feeds on these amino acids to fuel its growth, gradually hollowing out the nail plate from within.
This process explains why infected nails become progressively thicker and more crumbly over time. The fungus is literally digesting the nail’s structural material and replacing it with debris.
What It Looks Like
Nail mycosis takes several distinct forms depending on where the fungus enters the nail. The most common pattern starts at the tip or side of the nail and works backward toward the cuticle. You’ll notice a yellowish or brownish discoloration at the free edge that slowly expands, along with a buildup of chalky debris under the nail that pushes it upward.
A second pattern, called white superficial onychomycosis, appears as chalky white patches on the surface of the nail plate. This type is easier to treat because the fungus hasn’t penetrated deeply. A rarer and more stubborn form begins near the cuticle and moves outward, often signaling a weakened immune system. In advanced cases, the entire nail becomes thickened, discolored, and distorted to the point where it’s painful or difficult to trim.
Who Gets It
Age is one of the strongest risk factors. Nails grow more slowly as you get older, giving fungi more time to establish themselves, and circulation to the extremities declines. Older adults are nearly five times more likely to develop onychomycosis than the general population.
Diabetes is another major risk factor. People with diabetes develop nail fungus at roughly 2.8 times the rate of non-diabetic individuals, and some studies put the prevalence in diabetic groups as high as 31.5%. Elevated blood sugar leads to thicker nails and accelerated buildup of material beneath the nail plate, both of which create a more hospitable environment for fungi. Reduced sensation in the feet also means injuries and early infections go unnoticed.
Other groups at heightened risk include people with chronic kidney disease, those who have received organ transplants, people living with HIV, and those with chronic venous disease in the legs. Even conditions that seem unrelated, like knee osteoarthritis, are associated with dramatically higher rates, likely because reduced mobility changes foot mechanics and hygiene habits. Shared showers, tight footwear, and warm, moist environments also increase exposure.
How It’s Diagnosed
Discolored or thickened nails aren’t always fungal. Psoriasis, trauma, and other conditions can mimic the appearance of onychomycosis, so lab confirmation matters before starting treatment.
The simplest test involves dissolving a nail clipping in a potassium hydroxide solution and examining it under a microscope. This can reveal fungal filaments directly, though its accuracy varies widely, with sensitivity reported anywhere from 34% to 93% depending on technique. A more reliable option is a tissue stain called PAS, which outperforms both the potassium hydroxide test and fungal culture in head-to-head comparisons.
Fungal culture, where the sample is placed on growth media to see what develops, has the advantage of identifying the exact species involved. However, it has a high rate of false negatives, meaning fungi fail to grow even when infection is present. For this reason, culture is most useful when combined with one of the other methods. Newer visual tools like dermoscopy can offer clues in the office. Characteristic patterns of streaks and spikes on the nail surface have specificity rates reaching 100% for certain features, helping clinicians decide whether to pursue lab testing.
Treatment Options
Treatment depends on the severity and extent of the infection. Mild cases limited to the nail surface may respond to topical solutions, while deeper or more widespread infections typically require oral medication.
Topical Solutions
Three topical antifungals are commonly prescribed, all requiring daily application for 48 weeks. The most effective option achieves complete cure rates of 15% to 18% in clinical trials. A second option clears nails completely in about 7% to 9% of patients, while an older nail lacquer has a complete cure rate around 7%. These numbers may sound low, but “complete cure” is a strict standard requiring both a visually normal nail and negative lab tests. Rates of significant improvement are considerably higher.
Topicals work best for infections that haven’t reached the nail root and when less than half the nail is affected. Their main advantage is avoiding the side effects of oral drugs. The tradeoff is a lower success rate and a long commitment to daily application.
Oral Antifungals
For moderate to severe infections, oral treatment taken daily for 12 weeks is the standard approach. The two main options have been compared head to head, and the one used most often (terbinafine) consistently produces higher cure rates than the alternative. A continuous 12-week course clears the fungus in roughly 74% to 79% of patients, as measured by lab tests. The results are durable, though some people do experience recurrence months or years later.
Even after the medication successfully kills the fungus, you won’t see a fully clear nail right away. You’re waiting for the damaged nail to grow out and be replaced by new, healthy nail. Fingernails typically take 4 to 5 months to fully regrow, while toenails require 10 to 18 months. This long growth cycle is why patience is essential, and why follow-up assessments usually happen 9 to 12 months after starting treatment.
Why Treatment Matters
In otherwise healthy people, nail mycosis is mainly a cosmetic nuisance and a source of discomfort. But in people with diabetes or compromised circulation, the stakes are much higher. Thickened, brittle nails can dig into surrounding skin and create small wounds that go unnoticed, especially when foot sensation is diminished. These tiny breaks in the skin become entry points for bacterial infections.
Onychomycosis and the athlete’s foot that often accompanies it are independent predictors of foot ulceration in diabetic patients. One study found that diabetic patients with nail mycosis had roughly 4.5 times the odds of having undergone a minor amputation compared to those without the infection. Chronic fungal infection impairs wound healing and creates conditions ripe for secondary infections that can escalate quickly in people whose immune response is already compromised.
Even in the general population, untreated nail mycosis tends to spread. It can move from one nail to others, infect the surrounding skin, and serve as a reservoir that reinfects your feet after you’ve cleared athlete’s foot. Addressing it early, when the infection is limited and the nail is only partially involved, gives treatment the best chance of success.