Toenail fungus is not dangerous for most healthy people, but it’s not purely cosmetic either. Left untreated, it can cause pain, permanently damage the nail, and create openings in the skin that lead to bacterial infections. For people with diabetes, weakened immune systems, or poor circulation, the risks are significantly higher.
What Happens if You Leave It Alone
Toenail fungus is a slow-moving infection. It typically starts as a white or yellow spot under the tip of the nail and gradually works its way deeper, thickening and discoloring the nail over months or years. In mild cases, it’s painless and easy to ignore. But the fungus doesn’t resolve on its own. Over time, the nail can become so thick and misshapen that it presses against the inside of your shoe, causing discomfort or pain when you walk.
The bigger concern is what’s happening beneath the nail. A severe, long-standing infection can damage the nail matrix, the tissue at the base of your nail that produces new nail growth. Once the matrix is damaged, the nail may grow back permanently thickened, ridged, or deformed, even after the fungus is successfully treated. According to the Mayo Clinic, severe nail fungus can cause permanent nail damage.
The Bacterial Infection Risk
This is where toenail fungus crosses from nuisance to genuinely dangerous. Fungal infections break down the skin around and beneath the nail, creating tiny cracks and fissures that bacteria can enter. A case-control study published in dermatology research found that people with toenail fungus had roughly 2.2 times the odds of developing cellulitis, a bacterial skin infection that spreads through deeper tissue layers. Athlete’s foot between the toes (which often accompanies nail fungus) raised that risk even higher, to about 3.2 times.
Cellulitis causes redness, swelling, warmth, and pain, usually in the lower leg. It requires antibiotics, and in serious cases, hospitalization. For older adults or people with circulation problems, a skin infection that starts from a cracked toenail can escalate quickly.
Why Diabetes Makes It More Serious
If you have diabetes, toenail fungus deserves more attention than you might think. Diabetes reduces blood flow to the feet and dulls nerve sensation, which means infections can worsen without you noticing pain. A large Dutch study followed over 48,000 diabetes patients for a median of about 10 years and found that those with toenail fungus had a 23% higher risk of developing foot ulcers, even after adjusting for other health factors. They also had a 32% higher risk of needing surgical interventions and a 27% higher risk of secondary skin infections.
Foot ulcers in diabetic patients are a leading cause of hospitalization and amputation. What makes this finding especially notable is that antifungal treatment didn’t significantly reduce the ulcer risk in the study, suggesting the fungus may be a marker of broader foot vulnerability in diabetic patients. That said, treating it still matters because reducing the fungal load helps protect the skin barrier from bacterial entry.
Risks for People With Weakened Immune Systems
For organ transplant recipients, people on immunosuppressive medications, or those with other conditions that weaken immune function, fungal infections carry a different kind of risk. The immune system normally keeps fungi confined to superficial tissues like skin and nails. When that defense is suppressed, fungal organisms can potentially spread beyond the skin.
Kidney transplant recipients, for example, experience fungal infections at rates of 5% to 20%, and invasive fungal infections in this group carry a mortality rate of 25% to 50%. The most common culprits in transplant patients are Candida and Aspergillus species rather than the dermatophytes that cause typical nail fungus, but any fungal colonization in an immunosuppressed person is taken seriously. If you’re on medications that suppress your immune system, even a seemingly minor nail infection warrants a conversation with your care team.
It Can Spread to Other Parts of Your Body
Toenail fungus acts as a reservoir. The same fungal organisms living in your nail can spread to the skin of your feet (athlete’s foot), your groin (jock itch), and your hands. This happens through autoinoculation: you touch or scratch your infected nail, then touch another part of your body. Sharing towels, sheets, or walking barefoot in shared spaces can also spread the infection to other people.
Warm, moist environments accelerate this process. Tight shoes, sweaty socks, and damp skin between the toes all create ideal conditions for the fungus to colonize new territory. Treating the nail infection, not just the skin symptoms, is important because the nail acts as a persistent source that reseeds the surrounding skin.
When a Dark Nail Isn’t Fungus
One reason to take nail changes seriously is that not every discolored nail is fungal. Subungual melanoma, a type of skin cancer that develops under the nail, can initially look like a dark streak or discoloration. Research comparing fungal nail infections to melanoma under a dermatoscope found distinct differences. Fungal infections tend to produce yellow or white streaks, scaling on the nail surface, and a pattern of discoloration that’s widest at the tip and narrows toward the base (a reverse triangle shape). Melanoma, by contrast, tends to produce a dark longitudinal band that’s widest at the base, and it may be accompanied by pigment spreading onto the surrounding skin, a feature called Hutchinson’s sign.
If you notice a dark brown or black streak under your nail that wasn’t caused by an obvious injury, or if the pigment appears to be spreading beyond the nail itself, get it evaluated. These features were never seen in fungal infections in the study and were strong indicators of melanoma.
What Treatment Looks Like
Toenail fungus is treatable, but it requires patience. Toenails grow at roughly 1.6 millimeters per month, meaning a full toenail takes 12 to 18 months to completely grow out. Even after the fungus is eliminated, you’re waiting for a healthy nail to slowly replace the damaged one.
Topical antifungal treatments (applied directly to the nail) work best for mild infections that haven’t reached the base of the nail. For moderate to severe cases, oral antifungal medication taken for about three months is more effective because it reaches the fungus through the bloodstream. The most commonly prescribed oral option carries a small risk of liver irritation. Clinically significant liver injury is rare, occurring in roughly 1 in 2,500 to 1 in 25,000 people who take it. Less than 1% of patients experience any elevation in liver enzymes, and most of those are temporary. Your doctor will typically check liver function before and during treatment.
Who Should Take It Most Seriously
The risk profile of toenail fungus depends heavily on who you are. For a healthy 30-year-old, it’s a cosmetic annoyance that’s worth treating mainly to prevent spread and avoid long-term nail damage. For people over 60 (where prevalence reaches 20% or higher, and exceeds 50% in those over 70), the combination of slower healing, reduced circulation, and thinner skin makes complications more likely. For anyone with diabetes, peripheral vascular disease, or a suppressed immune system, toenail fungus is a genuine medical concern that increases the risk of infections, ulcers, and hospital visits.
The bottom line: toenail fungus won’t kill a healthy person, but calling it harmless undersells the real risks it creates, especially over time and especially in vulnerable populations. Early treatment is easier, cheaper, and far more effective than trying to address a nail that’s been infected for years.