Nail Fungus: What Causes It, How a Dermatologist Tells It Apart From Nail Psoriasis and Subungual Melanoma, and Why Topical Treatment Alone Often Fails
Nail fungus (onychomycosis) is a fungal infection of the nail bed and the underside of the nail plate. The nail slowly thickens and turns cloudy white, yellow or brown, with chalky debris underneath and lifting away from the skin. It affects roughly 5–10% of people and becomes much more common after age 60. It usually involves the toenails, especially the big toe, and almost never clears on its own without treatment.
What most people do not realise is that about half of thick, discoloured nails are not caused by fungus. The cause may be nail psoriasis, a bruised nail, a bacterial infection that turns the nail green, or nail lifting from gel manicures. Buying a medicine without knowing whether it is really fungus is the main reason people say “I have been applying it for a year and it has not gone away.” In this article a dermatologist explains, one pattern at a time, which nails look like fungus, how to confirm it, which medicines are actually used, how long they take, and why manicures can lead to it.
What causes nail fungus
There are three groups of culprits, and the group determines which medicine will work.
Dermatophytes cause about 60–90% of fungal toenail infections. The main one is Trichophyton rubrum, the same organism behind athlete's foot (tinea pedis) and ringworm on the body, so the infection often spreads from between the toes into the nail.
Candida yeasts are found more often in fingernails than toenails, especially in people whose hands are wet all day, such as home cooks, kitchen staff, dishwashers, and people whose cuticles are often pushed back or cut.
Non-dermatophyte molds such as Fusarium, Scopulariopsis and Neoscytalidium are seen more often in tropical countries like Thailand and respond less well to standard medicines, which is one reason a culture should be done before treating stubborn cases.
Factors that open the door to infection include:
Chronic moisture: wearing closed shoes all day, heavy foot sweating, or frequent hand soaking.
Existing athlete's foot: the foot is a reservoir for the fungus, and unless it is treated at the same time the nail will keep relapsing.
Previous nail injury or lifting: the gap under a lifted nail is the easiest entry point for fungus, whether from knocks, tight shoes or manicures.
Older age: nails grow more slowly and cannot push the infection out in time.
Medical conditions: diabetes, weakened immunity, peripheral vascular disease and smoking.
Family history: in some families nearly everyone has athlete's foot and toenail fungus.
How to tell if a nail has fungus: common signs
Nail fungus develops gradually over months to years and is usually painless at first. The signs that often appear together are:
Colour change: cloudy white, yellow or brown spots or streaks.
Thickening with white-yellow chalky debris packed under the nail (subungual hyperkeratosis), which lifts the nail off the bed.
Nail lifting: the tip separates from the skin (onycholysis), leaving a white gap underneath.
Brittle, crumbling, layered nails with rough edges that are hard to trim.
Uneven involvement: usually one to three nails first, especially the big toe, often with peeling or itchy skin on the sole or between the toes.
Patterns that are “less likely to be fungus” are all ten nails changing within a short time, small pits on the nail surface, a single black line, or a bright green colour. These are covered separately below.
Fungus starting at the tip or side of the nail (the most common type)
What it looks like: the tip or side edge turns cloudy yellow, then spreads toward the base in a band or wedge, with thickening and debris under the nail. The medical name is distal lateral subungual onychomycosis (DLSO), and it makes up about 3 in 4 of all nail fungus cases.
Cause: almost always T. rubrum spreading from athlete's foot.
Severity and urgency: not an emergency, but the closer it spreads to the nail root (matrix), the less well topical treatment works. If more than half the nail is affected, the chance of clearing with topical treatment alone is very low.
When to see a doctor: when more than a third of the nail is involved, several nails are affected, or after 3 months of self-treatment the newly grown part near the base is still not clear.
Chalky white fungus on the surface of the nail
What it looks like: chalk-like white spots or patches on the top of the nail plate that crumble to powder when scraped, without much thickening. It is called white superficial onychomycosis (WSO).
Cause: T. interdigitale or certain molds that feed only on the top layer of the nail. It is common in people who leave nail polish on for a long time, or in damp toenails.
Severity and urgency: it responds to topical treatment better than other types because the fungus is shallow. Filing off the powdery surface plus a medicated nail lacquer is often enough.
When to see a doctor: if it occurs in a child or someone with weakened immunity, or in several nails at once, because the white surface pattern can also be a sign of other conditions.
Fungus starting at the nail base
What it looks like: a cloudy white band or patch starts at the base of the nail under the cuticle and spreads toward the tip, the opposite direction to the common type. It is called proximal subungual onychomycosis (PSO).
Cause: dermatophytes or yeasts entering at the nail base. This type is uncommon in healthy people and is seen more often with weakened immunity, such as poorly controlled diabetes, immunosuppressive medicines or HIV infection.
Severity and urgency: the nail itself is not dangerous, but this pattern is a cue to check for conditions that lower immunity, and it usually needs oral medicine.
When to see a doctor: as soon as you notice it, without trying self-treatment first.
Fungus in the fingernails with swollen skin around the nail (yeast)
What it looks like: the skin around the nail is swollen, red and tender, the cuticle is gone, and the nail grows out with horizontal waves, a yellow-brown colour or dark green edges. It often affects several fingernails.
Cause: Candida entering through the crack between the nail fold and the nail plate. It occurs in people whose hands are often wet and in people who regularly push back or cut the cuticle. The condition is chronic paronychia that has spread into the nail.
Severity and urgency: chronic and disruptive to daily life. If there is pus or sudden throbbing pain, bacteria are usually involved as well and treatment is more urgent.
When to see a doctor: when swelling lasts more than 6 weeks, or there is pus and severe pain. For more on treating nail fold infections, see Paronychia: acute vs chronic.
Thick, crumbling nail throughout (late stage)
What it looks like: the whole nail is thick, crumbling, twisted and yellow-brown. Some become so thick that pressing on a shoe is painful, or the nail falls off leaving only debris. This is called total dystrophic onychomycosis.
Cause: the end point of any type of nail fungus left untreated for years.
Severity and urgency: the hardest to treat, needing nail thinning along with oral medicine. In people with diabetes, a thick crumbling nail can press into the skin and cause an ulcer under the nail, which lets bacteria in and can lead to cellulitis.
When to see a doctor: always, especially for older adults and people with diabetes.
Green nails: usually not fungus
What it looks like: the nail is bright green, blue-green or green-black, often starting at an edge where the nail has lifted, sometimes with a musty smell. This is called green nail syndrome or chloronychia.
Cause: almost always the bacterium Pseudomonas aeruginosa, which makes a green pigment (pyocyanin), not a fungus. It likes moisture and settles in the space under a lifted nail. It is common after gel or acrylic nails are removed, or in people whose hands are wet all day. Some people also have a fungus at the same time.
Severity and urgency: not dangerous in most people, but antifungal nail lacquer will not clear the green colour. The right treatment is to trim off the lifted part, keep the nail dry, soak in diluted vinegar (about 1 part vinegar to 4 parts water) for 10 minutes once or twice a day, or use antibiotic drops prescribed by a doctor. The green usually fades within 1–4 weeks, but you still have to wait for new nail to replace the lifted part.
When to see a doctor: when there is swelling, redness or pain around the nail, pus, it occurs in someone with diabetes or weakened immunity, or the green keeps coming back after you stop manicures.
Conditions that look like nail fungus but are treated differently
Because a nail can only go wrong in a few ways, many conditions look alike. These are the ones a dermatologist has to rule out every time.
Nail psoriasis causes small pits on the nail surface (pitting) and orange-brown patches under the nail that look like oil drops (oil-drop sign). It usually affects several nails, often with a rash on the scalp, elbows or knees. About 50% of people with psoriasis have nail changes. It is treated with psoriasis medicines, not antifungals. See Nail psoriasis: signs and treatment.
Thickening from repeated trauma is seen in runners and in people who wear tight shoes or heels. The big toenail is thick and yellow, but fungal tests are negative.
Blood under the nail (subungual haematoma) is dark red to black, follows an injury, and the coloured area moves forward as the nail grows out.
Subungual melanoma appears as a single brown-black streak in one nail that gets wider, darker or blurry at the edges, or with pigment spreading onto the skin around the nail (Hutchinson's sign). See a doctor promptly. Read how to tell the difference in Black nails: causes and when to worry.
Yellow nails from other causes such as nail polish, smoking or yellow nail syndrome. See the table of 7 causes in Yellow nails: causes.
Nail lifting or loss without infection from manicures, certain medicines or thyroid disease. See Nail loss: causes.
Why manicures can lead to nail fungus: gel, extensions and nail salons
A manicure does not usually “put fungus in” directly. It creates the two conditions fungus needs: a gap under the nail and trapped moisture.
Scraping or prying off gel or acrylic thins the nail surface and makes the tip lift. The lifted gap holds water after hand washing and becomes a home for yeast and Pseudomonas.
Extensions applied over a nail that has started to lift trap water under the artificial nail for weeks, which is why nails look green or cloudy white when the extension comes off.
Pushing back or cutting the cuticle removes the waterproof seal against moisture and germs, leading to paronychia and yeast infection.
Shared tools such as files, buffers and cuticle nippers that are not sterilised can pass infection from one client to the next. Foot baths that are not cleaned thoroughly do the same.
Checklist before sitting down at a nail salon
Metal tools (scissors, cuticle nippers) are heat-sterilised and the pouch is opened in front of you.
Paper files and buffers are single-use, or you bring your own.
Ask them not to cut the cuticle, only to push it back gently after soaking.
Take a break from gel or extensions for 1–2 weeks every 2–3 rounds, and remove gel by soaking in remover rather than prying.
If the tip has already lifted or turned green, stop manicures until the lifted part has grown out and a test shows no infection, because polish or extensions trap moisture and hide the nail colour you need to monitor.
How to know it really is nail fungus (testing before treatment)
Treating nail fungus effectively usually means taking medicine for weeks to months. Dermatology guidelines in Europe, the UK and the US therefore recommend confirming the fungus before starting oral medicine every time. The doctor takes a sample from the debris under the nail next to healthy tissue (not from a tip that has already come off) and sends it for one or more of these tests.
KOH microscopy for fungal threads can be done in the clinic with a result the same day, but a negative result does not always rule out fungus. If the sample is not taken from the right spot it can miss about one in three cases.
Fungal culture gives a result in 2–4 weeks and shows whether it is a dermatophyte, yeast or mold, which changes the choice of medicine.
Nail clipping with PAS stain is highly sensitive, at about 80–90%, and is used when KOH is negative but suspicion remains.
Fungal DNA testing (PCR) is available at some facilities, gives a quick result and can identify the species.
The test helps in both directions. If it is fungus, the medicine can be matched to the organism. If it is not, you avoid months of medicine for a disease you do not have, and you can look for the real cause.
Why antifungal products applied to the nail often fail
The nail plate is about 0.5–1 mm thick and hard like horn, and the fungus lives underneath, so topical medicine struggles to reach it. There are five main reasons treatment fails.
Using skin cream on the nail. Antifungal creams or solutions for skin sold at pharmacies, such as clotrimazole or miconazole, penetrate the nail plate very poorly. They are fine for the skin around the nail and the foot, but not enough for the nail itself.
The infection is too deep for a topical product. Medicated nail lacquers work when no more than about half the nail is involved, the infection has not reached the nail base, and no more than 3–4 nails are affected.
A dense ball of fungus under the nail (dermatophytoma). It appears as a yellow-white streak or lump running lengthways under the nail. Both topical and oral medicines struggle to reach it, so that part needs to be trimmed or filed away as well.
Not treating the feet at the same time. Fungus between the toes keeps re-entering the new nail.
It was never fungus. See the lookalike conditions above.
Topical antifungal medicine for nails: when to use it, how, and for how many months
Medicated nail lacquer suits mild cases, the chalky white surface type, children, and people who cannot take oral medicine, and it is used alongside oral medicine to improve the chance of clearing. The medicines actually used are:
Amorolfine 5% nail lacquer is applied once or twice a week. Before each application, gently file off the affected part of the nail surface and wipe with an alcohol pad. Use it continuously for about 6 months for fingernails and 9–12 months for toenails.
Ciclopirox 8% nail lacquer is applied once a day over the previous layer, removed once a week with alcohol, and then started again. It can be used for up to 48 weeks.
Efinaconazole 10% solution is applied once a day for 48 weeks. It penetrates the nail better than older products but is sold in only some countries.
A realistic expectation based on research is that nail lacquer alone completely clears the nail in about 5–18% of people with moderate disease. The figure is much higher when only truly mild cases are selected. For details on other antifungal topicals and where they can be used, see Antifungal creams: which to choose and for how many weeks.
About antifungal nail liquids sold at pharmacies and online: before buying, check the generic name of the active ingredient on the box. If it is clotrimazole, miconazole, tolnaftate or a natural extract, it is a skin product not designed to penetrate the nail, and on a heavily affected nail it usually does not work. Products that soften or whiten the nail may make it look better for a while while the fungus is still there.
Oral medicine for nail fungus: names, doses and how many months
For moderate to severe cases, oral medicine is the main treatment, because it reaches the nail from the nail bed and stays in the nail for months after you stop.
Terbinafine 250 mg once daily is the first choice when the cause is a dermatophyte. It is taken continuously for 6 weeks for fingernails and 12 weeks for toenails. In studies the fungus cleared in about 70% and the nail returned fully to normal in about 35–50%. Side effects include bloating, altered taste, rash and, rarely, hepatitis.
Itraconazole is given either continuously at 200 mg once daily for 12 weeks for toenails, or as pulses of 200 mg twice daily for 1 week followed by 3 weeks off, which counts as 1 pulse. Use 2 pulses for fingernails and 3–4 pulses for toenails. It covers dermatophytes, yeasts and some molds. It must be taken with food. It interacts with many medicines, such as some cholesterol-lowering drugs, some sleeping pills and heart medicines, and it is not used in people with heart failure.
Fluconazole 150–450 mg once a week for 6–12 months, or until new nail has fully replaced the old, is used when the first two cannot be used or when the cause is yeast.
Griseofulvin is an older medicine that has to be taken for more than a year for toenails and works less well, so it is now rarely used for nails.
Before starting oral medicine, the doctor will take a full history of your regular medicines and will usually check liver function before starting. In people with liver disease, or when medicine is taken for more than 6 weeks, it may be rechecked during treatment. If you develop yellow eyes, dark urine, severe nausea or a peeling rash, stop the medicine and see a doctor immediately. Pregnant and breastfeeding women usually postpone oral treatment. The choice of medicine and dose is a decision for the doctor who examines you, and you should not buy oral medicine yourself.
Do you need the nail removed? Nail thinning methods actually used
Most people do not need the whole nail removed. Traditional surgical removal is painful, has a long recovery, and the regrown nail may be misshapen. Doctors now use gentler methods to reduce the amount of fungus and help medicine reach it.
Trimming and filing down the thick part (debridement) is repeated every 4–8 weeks during treatment and immediately reduces pain when wearing shoes.
Covering the nail with high-strength 40% urea softens the crumbling part so it comes off by itself within 1–2 weeks, without surgery.
Partial nail removal is reserved for nails that are very thick, painful or contain a dense ball of fungus, and is combined with oral or topical medicine.
About lasers: lasers are used for nail fungus in some places, but current evidence suggests they may increase the clear part of the nail somewhat and has not shown killing of the fungus comparable to oral medicine. Clinical guidelines therefore do not list them as a main treatment. They are only an optional add-on for people who cannot take oral medicine.
How many months until it clears: the timeline to expect
The biggest reason people give up midway is the mistaken belief that the nail should be clear as soon as the medicine stops. In fact, the medicine kills the fungus in the newly growing nail, while the old cloudy nail has to grow out and be trimmed away.
Fingernails grow about 3 mm a month and take 4–6 months to grow out completely.
Toenails grow about 1–1.5 mm a month and the big toenail takes 12–18 months to clear completely.
Months 2–3 after starting oral medicine: you should see a clear band of nail growing out from the base with a sharp dividing line. This is a sign the medicine is working, even if the tip is still cloudy.
The real assessment is at 12–18 months from the start of treatment, for toenails.
Signs that treatment is not working include no clear nail growing out from the base after 3–6 months, the cloudy band spreading toward the base, or a streak of fungus under the nail. In that case the doctor will repeat the tests, change medicine according to the organism, or file the nail down further.
Factors that make clearing harder include more than half the nail affected, spread to the nail base, nail thicker than 2 mm, a dense ball of fungus, diabetes or narrowed peripheral arteries, and a non-dermatophyte mold infection.
Recurrence after clearing: why you need to retest before a second course
Nail fungus comes back in about 20–25% of people within 2–3 years of clearing, most often because the fungus on the feet is still there or because they go back to the same damp conditions. But a nail that looks like it has relapsed is not always fungus. A nail that has been ill for a long time often stays partly thick and cloudy even after the fungus is gone.
So the dermatologist's rule is this: if the nail becomes abnormal again after a full course, always confirm the fungus with a new test before starting a second oral course. If it is confirmed as fungus, the doctor will use the culture to see the species and choose a matching medicine, not just repeat the same one. In some people, after clearing, the doctor may advise a nail lacquer once a week, or an antifungal for the feet, from time to time to prevent recurrence.
Preventing nail fungus: checklist for shoes and socks
Prevention matters as much as treatment, because a cleared nail still lives in the same shoes.
Rotate at least 2 pairs of shoes. Wear the same pair again only after it has rested and dried for 24–48 hours. Shoes worn every day never dry completely.
Use antifungal powder or spray in your shoes once or twice a week, especially trainers and work shoes.
Change socks every day and as soon as they get wet. Choose fabric that wicks sweat. If you have athlete's foot, wash socks in hot water at 60 degrees or dry them in strong sunlight.
Dry between the toes after every shower and treat athlete's foot until it clears at the same time. To tell athlete's foot from other conditions, see Peeling feet: causes and Foot fungus between the toes: causes.
Wear sandals in shared showers, swimming pools and locker rooms.
Keep nail clippers separate for the infected nails, away from others in the household, and trim the infected nails last.
Cut nails straight and not too short, not so short that the nail lifts, and choose shoes with a toe box that does not squeeze the toes.
Heavy foot sweating should be managed at its source. See Smelly feet and excessive foot sweating.
Is nail fungus contagious?
Yes, but not as easily as a cold. It spreads through shared wet floors walked on barefoot, and through shared nail clippers, files, shoes and towels. People in the same household can therefore have athlete's foot and nail fungus together. Keeping personal items separate and treating everyone in the home who is affected helps break the cycle. The fungus from a nail can also spread to other parts of your own body, such as the groin (jock itch) or the palms, through scratching or using the same towel.
Do vinegar, garlic and home remedies really help?
The remedies people search for most are vinegar soaks, garlic, tea tree oil and menthol-camphor ointment. What we know at present is:
Diluted vinegar really helps with green nails caused by bacteria and helps keep the feet dry and clean, but there is not yet good research showing it treats fungus under the nail plate. Strong vinegar soaked for long periods makes the skin around the nail soggy and peeling.
Fresh garlic kills germs in the laboratory, but packing it on the skin can cause burns and blisters.
Tea tree oil and menthol-camphor ointment have small studies showing some improvement in some people, but cure rates are low and not equal to medicated nail lacquer.
In short, they can be used as an extra if they do not irritate, but they should not replace real treatment, because every month of delay lets the fungus spread closer to the nail root and makes it harder to treat.
When to see a doctor urgently (warning signs)
See a dermatologist without waiting if any of the following applies.
A single brown-black streak in one nail that is getting wider or darker, or colour spreading onto the skin around the nail.
Swelling, redness, warmth and throbbing pain, pus, or redness spreading up the toe and over the top of the foot.
You have diabetes, weakened immunity or peripheral vascular disease, because a crumbling nail and foot sores can progress quickly to a serious infection.
Abnormal nails in several fingers within a few weeks, or a rash on other parts of the skin as well.
It occurs in a child, which is uncommon, and should be confirmed by testing before any medicine is used.
You have treated it yourself for 3 months and there is still no clear nail growing out from the base.
Quick summary
A thick, cloudy, yellow nail with debris underneath, starting at the tip or side of a toenail, is the pattern that best fits nail fungus.
About half of abnormal nails are not fungus. They can be nail psoriasis, thickening from trauma, green nails from bacteria, or subungual melanoma, so test before taking oral medicine.
For mild disease use amorolfine or ciclopirox nail lacquer for 6–12 months. For moderate to severe disease use terbinafine for 12 weeks for toenails, or itraconazole in 3–4 pulses.
A toenail takes 12–18 months to clear completely. A sign that the medicine is working is a clear band growing out from the base within 3 months.
Gel or extension manicures make the nail lift and trap moisture, opening the way for yeast and bacteria. Stop manicures during treatment.
If it comes back after clearing, confirm the fungus with a new test before a second course, and always treat athlete's foot at the same time.
Frequently asked questions about nail fungus
How can you tell if a nail has fungus?
Look for a nail that gradually thickens over months, turns cloudy white, yellow or brown, has chalky debris packed underneath, and has a tip that lifts from the skin, usually starting at the tip or side of one to three toenails, especially in people who also have peeling or itchy feet. But these features overlap with nail psoriasis and nail thickening from trauma. The only way to confirm it is for a doctor to scrape the debris under the nail for KOH microscopy, a culture, or a nail clipping sent for pathology, all of which can be done in a single visit.
How do you get rid of nail fungus?
The first step is to confirm it really is fungus, then choose treatment by severity. If it is mild, no more than half the nail and not reaching the base, use amorolfine nail lacquer once or twice a week, or ciclopirox daily, for 6–12 months. If it is extensive or affects several nails, doctors usually give terbinafine 250 mg once daily for 12 weeks for toenails, together with filing down the thick part, treating athlete's foot at the same time, and rotating shoes so they dry. Then wait for new nail to grow in, which takes 12–18 months for toenails.
Does nail fungus mean the nail has to be removed?
Most people do not need the whole nail removed, because surgical removal is painful, has a long recovery and the regrown nail may be misshapen. Doctors now trim and file down the thick part every 4–8 weeks, or cover the nail with 40% urea so the crumbling part softens and comes off by itself in 1–2 weeks, together with oral or topical medicine. Partial nail removal is reserved for nails that are very thick, too painful to wear shoes, or contain a dense ball of fungus that medicine cannot reach.
Why do nails get fungus after a manicure?
A manicure creates gaps under the nail and traps moisture. Prying or scraping off gel thins the nail and makes the tip lift. Extensions applied over a nail that has started to lift leave water trapped underneath for weeks, and cutting the cuticle destroys the natural waterproof seal. These gaps are where yeast and the bacteria that turn nails green settle in, while tools that are not sterilised can pass fungus between clients. If the nail has lifted or changed colour, stop manicures and get it checked, because covering it with polish hides how treatment is going.
Does nail fungus go away on its own?
Generally it does not. Unlike some skin fungal infections, the fungus lives under the nail plate, which the immune system and topical medicine struggle to reach. Left alone it usually spreads slowly toward the nail base and to other nails, the nail thickens until it is hard to trim and painful in shoes, and the longer it goes on the harder it is to treat. In people with diabetes a crumbling nail can also let bacteria in. Cases that seem to go away on their own are usually green nails from bacteria or nail lifting from manicures that clear once the nail grows out, not true fungus.
Does vinegar help nail fungus?
Diluted vinegar, about 1 part to 4 parts water, really does help with green nails caused by bacteria and helps keep the feet clean and dry, but there is no good evidence that it kills fungus under the nail plate, especially when it is extensive or involves several nails. Soaking in strong vinegar for long periods makes the skin around the nail soggy and irritated. It can be used as an extra if your skin does not sting, but it should not replace nail lacquer or oral medicine that has evidence, because every month wasted lets the fungus spread deeper.
Written by Dr. Patara Voravutinon, Dermatologist · Last reviewed September 30, 2026
The information in this article is for general understanding and does not replace diagnosis and treatment by a doctor. The choice of medicine and dose must be decided by the doctor who examines you. If symptoms do not improve or you have warning signs, consult a dermatologist.
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