Reviewed by Evan Lewis, PhD. Last updated 10 August 2026.
Black toenail fungus is a fungal infection under the nail plate, clinically called onychomycosis, that turns the nail dark brown to deep black. It is common, persistent, and easy to confuse with a bruise. Here are the symptoms, the causes, and the treatment options that actually apply.
Understanding the symptoms, underlying causes, and available treatment options is the first step toward restoring healthy nails.
Is Every Black Toenail a Fungus?
No. Trauma is at least as common a cause. Black toenail fungus, also called a fungal nail infection or onychomycosis, develops when fungi penetrate the nail bed and nail plate, triggering discoloration, thickening, and sometimes brittleness. What begins as a small dark spot can gradually spread across the entire nail if left unaddressed.
Trauma, underlying medical conditions, and certain skin disorders can also produce similar discoloration, which is why understanding the distinctions matters.
Black Toenail Fungus vs. Other Nail Disorders
Black Toenail Fungus vs. Other Fungal Infections: While black toenail fungus is characterized by dark coloration, other fungal infections (such as athlete's foot) may cause yellowing, white spots, or crumbling nails. Dermatophytes, the fungi most responsible for toenail fungus, can also cause ringworm and other skin conditions.
Black Toenail vs. Trauma-Induced Discoloration: Repetitive pressure or impact from tight footwear can cause a subungual hematoma (blood pooling under the nail), producing a black appearance. Unlike a fungal infection, trauma-related blackening typically is not accompanied by itching, odor, or thickening.
Black Toenail vs. Skin Conditions: Certain skin diseases may discolor nails, but these are usually accompanied by inflammation or rash on the surrounding skin. Fungal infections, by contrast, thrive in warm, damp environments and primarily target the nail plate itself.
Black Toenail vs. Underlying Medical Conditions: Persistent black toenails can sometimes signal underlying health issues, including autoimmune diseases, kidney conditions, or nutritional deficiencies, that should be considered when other causes have been ruled out.
What Does a Black Line on the Toenail Mean?
A black line on the toenail is usually pigment or a bruise rather than fungus. A thin dark stripe running the length of the nail from cuticle to tip is called melanonychia, and in most people it is harmless pigment that has been there for years. Fungal discolouration behaves differently. It tends to start as a patch at the edge and spread sideways, and it comes with thickening.
A black toenail from running is a third thing again. Runners get it when the toe hits the front of the shoe on downhills and blood pools under the nail, so it appears as a solid dark area after a long run or a race and grows out over several months. If it keeps returning, the shoe is usually half a size too small or laced too loosely at the top.
Two situations are worth a professional opinion: a single dark band that is widening, has blurred edges, or spreads onto the skin at the base of the nail, and any dark nail change with no injury you can remember.
What Are the Symptoms of Black Toenail Fungus?
Dark discolouration first, then thickening, brittleness, a faint odour, and sometimes pain when the nail presses against a shoe.
Visual Signs
The most obvious symptom of black toenail fungus is dramatic nail discoloration. As the fungal infection (onychomycosis) spreads beneath the nail, it causes visible changes in color and structure that are difficult to miss.
Common visual signs include:
- Black or Dark Brown Nails: The entire toenail, or portions of it, may darken due to fungal buildup beneath the nail plate.
- Spreading Dark Patches: The discoloration often begins as small spots and expands as the infection progresses.
- Nail Plate Changes: The nail plate may thicken, distort, or become brittle as the fungal infection advances.
- Surrounding Skin Redness: Irritation and redness around the infected toe can indicate the fungal infection is affecting nearby tissue.
Where it starts is the useful part of how to recognize toenail fungus early. Fungus under the toenail usually shows first at the free edge or along one border, so a wedge of discolouration on the side of the toenail is a more common opening move than a whole dark nail. Big toenail fungus is the most frequent presentation, because that nail is the thickest, the slowest growing, and the one that takes the most impact inside a shoe. Advanced toenail fungus looks different again: the plate is hard to cut, it crumbles at the edge, and it can lift away from the bed.
Physical Discomfort
Beyond visible changes, toenail fungus can cause a range of physical symptoms:
- Pain and Tenderness: Pressure on the affected nail, from walking or wearing shoes, can be painful, especially as the nail thickens.
- Itching or Burning: Some people experience itching or burning around the infected nail, signaling the fungal infection may be spreading to nearby skin.
- General Discomfort: A deformed or thickened nail can make everyday movement uncomfortable.
Left untreated, toenail fungus can affect overall foot health in several ways:
- Mobility Issues: Pain and discomfort can interfere with walking and physical activity.
- Spread to Other Nails: Fungal infections are contagious. They can migrate to adjacent nails or other parts of the foot.
- Secondary Bacterial Infections: The damaged nail plate and surrounding skin become more susceptible to bacterial infections, which can complicate recovery.
Early recognition of these symptoms is key. The sooner a fungal nail infection is identified, the easier it is to manage and the less likely it is to spread.
Not every sore toe is fungal. Ingrown toenail symptoms look different: redness and swelling along one edge of the nail, pain when you press the skin fold, sometimes a bead of pus, and a nail plate that is still a normal colour. If the skin around the nail is what hurts rather than the nail itself, our guide to what an infection in the toe looks like walks through the warning signs and the home routine.
What Causes Black Toenail Fungus?
Dermatophyte fungi, in most cases, growing in the warm damp conditions inside a closed shoe.
Fungal Infections (Onychomycosis)
The primary cause of black toenail fungus is a fungal infection, most commonly caused by dermatophytes, though non-dermatophyte molds and yeasts can also be responsible. These organisms thrive in warm, moist environments, making sweaty feet inside closed-toe shoes an ideal breeding ground.
As the infection develops, fungi penetrate through microscopic cracks in the nail plate or between the nail and nail bed. Discoloration deepens from light brown to black as the fungal colony grows and the nail structure deteriorates.
How Fungal Nail Infections Spread
Toenail fungus spreads easily in environments where bare feet are exposed to moisture: public showers, swimming pools, locker rooms, and nail salons. It can also transfer between nails via infected clippings or shared pedicure tools. People with athlete's foot are at heightened risk of developing a concurrent fungal nail infection, as the same fungi are often responsible for both conditions.
Home is a bigger factor than most people expect. A 2022 scoping review in Journal of Fungi reported transmission of onychomycosis and dermatophytosis between household members in 44 to 47% of cases, with the fungi surviving up to 60 days in nail polish.
Environmental and Lifestyle Risk Factors
- Tight or non-breathable footwear traps heat and moisture, creating the warm, damp conditions fungi require to flourish.
- Walking barefoot in communal areas increases exposure to fungal organisms.
- Excessive sweating (hyperhidrosis) raises the risk. StatPearls notes that the risk of tinea infections is elevated with excessive sweating, and that occlusive shoes worn for long stretches predispose people to dermatophyte infections.
- Prior nail injury creates entry points for fungal organisms.
Underlying Medical Conditions
Autoimmune Diseases: Conditions like rheumatoid arthritis or psoriasis compromise the immune system's ability to fight off fungal infections, leaving nails more vulnerable.
Diabetes: Poor circulation and reduced immune response associated with diabetes make individuals significantly more prone to toenail fungus and slower to recover from it.
Kidney Disease: Impaired kidney function can disrupt the body's ability to maintain proper nutritional balance and waste elimination, creating conditions that support fungal growth.
Nutritional Deficiencies: Insufficient levels of certain vitamins and minerals can weaken immune defenses, reducing resistance to fungal nail infections.
How Is Fingernail Fungus Different From Toenail Fungus?
Same organisms, faster nail. Finger nail fungus is caused by the same dermatophytes plus a larger share of Candida yeasts, and the practical difference is speed. Fingernails grow at about 3.47mm a month against 1.62mm for toenails, so a fingernail replaces itself in roughly six months where a big toenail takes twelve to eighteen.
The entry points differ too. Fingernail vs toenail fungus usually comes down to hands that stay wet, gel or acrylic overlays, and damaged cuticles, rather than a warm shoe. Fingernail fungus treatment follows the same order as the toes, starting with hygiene and topicals and moving to oral options for stubborn cases, but the payoff arrives sooner because the nail turns over twice as fast.
How Is Black Toenail Fungus Treated?
With daily hygiene, topical antifungals, oral antifungals for stubborn cases, and nail removal only when nothing else has worked. Fungal nail treatment is chosen by how much of the nail is involved, how many nails are affected, and whether the matrix at the base has been reached.
At-Home Care and Natural Remedies
For mild cases of black toenail fungus caught early, at-home management can be a helpful starting point. Good foot hygiene is foundational:
- Keep Nails Clean and Trimmed: Regular nail maintenance limits the surface area available for fungal organisms to colonize.
- Dry Feet Thoroughly: Moisture is a fungal infection's best friend. Dry your feet completely after bathing before putting on socks and shoes.
- Wear Breathable Footwear: Natural material shoes allow air circulation, reducing the humid conditions that toenail fungus thrives in.
- Use Antifungal Foot Powder: Dusting feet and shoes can help manage moisture and reduce fungal exposure in daily wear.
Natural remedies such as tea tree oil have antifungal properties and may offer supplementary support for mild toenail fungus, though evidence for their effectiveness against established onychomycosis is limited. They work best as part of a consistent routine rather than a standalone solution.
Topical Antifungal Medications
Topical antifungal medications, available as creams, ointments, and nail lacquers, are commonly used to address toenail fungus by delivering antifungal agents directly to the infected nail and surrounding skin. Common topical antifungals include terbinafine, clotrimazole, and ciclopirox.
These treatments work by penetrating the nail plate to reach the underlying fungal infection, gradually suppressing fungal growth. Results take months, because toenails grow at an average of 1.62mm a month and healthy nail tissue needs that time to replace the infected portion.
Oral Antifungal Medications
For more severe or stubborn cases of black toenail fungus, particularly when the infection has spread to multiple nails or penetrated deep into the nail bed, oral antifungal medications may be recommended by a healthcare provider.
Oral options such as terbinafine, itraconazole, and fluconazole work systemically, reaching the infection through the bloodstream. They tend to produce faster and more comprehensive results than topical treatments but may carry side effects that require monitoring.
Itraconazole for toenail fungus is often given as pulse dosing, one week on and three weeks off, over several months, while terbinafine is usually taken daily. Both need liver monitoring and both interact with common medications. That is why doctor recommended toenail fungus treatment starts with a confirmed diagnosis rather than a guess at the pharmacy counter.
Severe Cases: Surgical Options
In cases where black toenail fungus persists despite conventional treatments and is causing significant pain or nail damage, surgical removal of the nail may be considered. Under local anesthesia, the nail is removed to allow a healthy nail to grow in its place. This approach is reserved for the most persistent and damaging fungal infections.
Natural Iodine Support: EZ Clear Nails by IodinePure
Alongside conventional options, many people incorporate natural, iodine-based care into their toenail fungus routine. EZ Clear Nails by IodinePure is a natural iodine soak designed to support nail care as part of a consistent daily routine.
The EZ Clear Nails Kit pairs a concentrated iodine soak with a dedicated toe pan for easy, consistent application, designed to work with your nail health routine, not replace medical care.
The IodinePure Protocol: Soak. Spray. Repeat.
- Place toes in the toe pan and pour 10 to 15ml of the EZ Clear Nails liquid per tray section (20 to 30ml total), enough to fully submerge the nails. The same liquid works for both feet.
- Soak for 5 to 7 minutes, 3 to 4 times per week or simply every other day, using the soak tray included in your kit.
- Allow to air dry completely.
- Complement soaks with the EZ Clear Nails spray, applied twice daily to the affected nail and surrounding skin.
Important: Avoid EZ Clear Nails if you are sensitive to iodine, have a seafood allergy, or have a thyroid condition. Do not ingest. IodinePure products are listed with Health Canada as cosmetics.
Also available: EZ Clear Nails 500ml Refill for ongoing use once your kit is established.
How Do You Hide Toenail Fungus While It Grows Out?
Carefully, and not with the polish bottle you already own. Nails move slowly, so most people want a cover option for the months in between. File the surface flat so the nail is smooth rather than ridged, keep it short, and choose a polish you can take off every few days instead of a gel or acrylic that seals the nail under a lid for weeks.
One detail matters more than the shade. The same 2022 review found dermatophytes surviving up to 60 days inside nail polish, so an old bottle and an old brush can put the same organisms straight back onto a nail that was starting to look better. Start a fresh bottle, keep it to yourself, and bin it when the nail has grown out.
Can you go to the beach with toenail fungus? Yes. Sand and salt water do not make it worse, and nobody is inspecting your feet as closely as you are. Wear sandals on pool decks and in changing rooms, rinse the sand off afterwards, and dry between the toes rather than just around them.
How Do You Keep It From Coming Back?
By changing the conditions, not just the nail. Once you have addressed a black toenail fungus infection, preventing recurrence is essential. Fungal nail infections have a high rate of recurrence, particularly in people with ongoing risk factors. Gupta and Versteeg, writing in the Journal of the American Podiatric Medical Association, report that recurrence and relapse rates for superficial fungal infections of the feet run from 10% to 53%.
- Rotate footwear to allow shoes to dry fully between wears.
- Wear moisture-wicking socks and change them daily, or immediately after sweating.
- Wash socks hot. In a 2022 study in Journal of Fungi, dermatophyte spores survived a 40°C wash and were only removed at 60°C.
- Use protective footwear in communal wet areas (pools, showers, gyms).
- Disinfect nail clippers and tools after each use, and keep a separate set of fungal nail clippers for the affected toe so spores are not carried onto healthy nails.
- Maintain a consistent daily foot care routine including drying, inspection, and supportive topical care.
- Keep toenails trimmed short and filed smooth to reduce areas where fungi can take hold.
Frequently Asked Questions About Black Toenail Fungus
What are the most common signs of black toenail fungus?
The most recognizable signs of black toenail fungus (onychomycosis) include dark brown or black discoloration of the nail, thickening of the nail plate, brittleness, a slight odor, and in some cases itching or pain. Discoloration that begins at the nail tip and spreads toward the base is a classic indicator of a progressing fungal infection.
Can black toenail fungus go away on its own?
Fungal nail infections rarely resolve without intervention. Because fungi embed themselves in the nail bed and plate, they tend to persist and spread if left unaddressed. Early treatment with topical antifungal options or natural iodine-based care, combined with consistent foot hygiene, offers the best chance of improvement.
How do I tell the difference between black toenail fungus and a bruised nail?
A bruised nail (subungual hematoma) typically results from trauma, like dropping something on the toe or repetitive impact in tight shoes, and usually does not cause itching, thickening, or odor. Black toenail fungus tends to develop gradually, is often accompanied by texture changes and a faint smell, and may spread to adjacent nails. When in doubt, consult a healthcare professional for an accurate diagnosis.
How long does it take to treat a fungal nail infection?
Toenail fungus treatment is measured in months, not days. Topical antifungal medications may require 6 to 12 months to show full results, as nails grow slowly. Oral antifungal medications can work faster, typically 3 to 6 months, but require medical supervision. Consistency with any chosen treatment is the single biggest factor in achieving results.
Are natural remedies effective against toenail fungus?
Some natural remedies, including tea tree oil and iodine-based soaks, have demonstrated antifungal properties and may help support nail health as part of a broader care routine. They tend to work best for mild or early-stage infections and as a complement to other approaches. For established or spreading black toenail fungus infections, natural options are most effective when used consistently alongside good foot hygiene practices.
Can toenail fungus be cured?
The research uses two different definitions, which is why the answers people find online disagree. Mycologic cure means the lab can no longer grow fungus from the nail. Complete cure means that plus a normal-looking nail. The gap is wide: CADTH's review of efinaconazole reported mycologic cure of 55.2% and 53.4% at 52 weeks but complete cure of only 17.8% and 15.2%. So how to cure nail fungus is mostly a question about how long you keep the routine going, because the nail still has to grow out clean afterwards.
Is toenail fungus contagious?
Yes, between people and between nails. The 2022 scoping review in Journal of Fungi put household transmission of onychomycosis and dermatophytosis at 44 to 47% of cases. The same organisms cause athlete's foot, so a fungal nail can reseed the skin and the skin can reseed the nail. Shared towels, shared bath mats, shared clippers and communal showers are the usual routes.
Is toenail fungus hereditary?
The infection itself is not inherited, but susceptibility does appear to run in families. Households share floors, bathrooms and footwear habits, which explains a lot of the clustering, and nail shape, sweat rate and circulation are all partly inherited. If a parent has had persistent nail fungus, take it as a reason to be careful with shared spaces at home rather than as a verdict.
Does Betadine work for toenail fungus?
Betadine is povidone-iodine, an antiseptic sold for skin and wounds rather than nails, and it stains everything it touches. People still reach for Betadine for toenail fungus because iodine is broadly active against fungi, but the strong brown formulation is hard to live with daily and the label was never written for nail use. If iodine is what interests you, the labels are worth reading side by side, which we did in this breakdown of what the Betadine alternative labels actually say.
Does garlic work for toenail fungus?
Garlic contains allicin, which has measurable antifungal activity in a dish, and that is the whole basis of the home remedy. Getting it through a nail plate is the problem. Crushed garlic held against skin under a bandage also causes chemical burns, which is a documented reason people end up at a clinic. Interesting chemistry, poor daily routine.
What is a fungal nail culture?
A fungal nail culture is a lab test. A clinician clips or scrapes debris from under the nail and sends it away to be grown on a plate, which takes two to four weeks, or tested by PCR, which is faster. It matters because a good share of thick, discoloured nails turn out to be something other than fungus, and the answer changes what is worth doing. Ask for one before starting oral medication.
Is toenail fungus treatment covered by insurance?
It depends on the plan and on what is being addressed. Many private plans class nail fungus as cosmetic and cover little, while prescription orals are often covered under drug benefits when a doctor documents pain, diabetes, or a recurring infection. Laser sessions are usually not covered at all. In Canada, provincial health plans generally do not pay for routine nail care. Get the diagnosis coded properly and ask your insurer before booking anything expensive.
Evan Lewis, PhD, specializes in natural and nutritional therapies for the prevention and management of chronic diseases and their complications. His research spans chronic disease, diabetes complications, clinical nutrition, and nerve health, including the development of topical iodine approaches for foot care and wound health. IodinePure products are listed with Health Canada as cosmetics.
