Condition

Nail Fungus (Onychomycosis)

Nail fungus is a slow infection of the nail itself, usually a toenail. It makes the nail thick, yellow-brown, crumbly and lifted away from the skin underneath. It is treatable, but treatment is measured in months, because the damaged nail has to grow out before anything looks better.
Dr. Schwarz, MD · Board-certified dermatologistLast reviewed September 8, 2026
16:9 hero for Nail Fungus (Onychomycosis). Never cropped: the tone strip and the corner logo depend on the full frame.

Start here

First I check that a thick, discolored toenail is actually fungus, and often it is not - old shoe trauma, nail psoriasis and the shape a nail takes with age all look the same. That matters because the treatment that works is a tablet taken for months. Treatment does not improve the nail you can see, it protects new nail, and a toenail takes roughly a year to a year and a half to replace itself, so judge it by the clear band coming in at the base. And treat the athlete's foot, because it is the same organism, and leaving it is how people end up doing this twice.

— Dr. Schwarz, Board Certified Dermatologist

Key Facts

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How commonCommon, and it becomes much more common with age. It is the most frequent nail problem doctors see
Who gets itAdults far more than children, and toenails far more than fingernails. Very common over the age of 60
Curable or managedCurable, but slowly, and it comes back in a meaningful number of people
Prescription neededUsually. Over-the-counter products rarely reach the fungus. Prescription tablets have the best results
Time to improveA toenail takes roughly 12 to 18 months to grow out fully. A fingernail takes about 4 to 6 months

What It Is

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Nail fungus, or onychomycosis, is an infection of the nail plate and the skin under it. Most cases come from dermatophytes, the family behind athlete's foot and ringworm, usually Trichophyton rubrum. Yeasts such as Candida cause fewer cases, mostly in fingernails and hands that are wet all day. Non-dermatophyte molds cause a few and matter because they respond differently to treatment.

The nail plate is dead keratin with no blood supply. That is why this is so stubborn - no medicine reaches it through the bloodstream, so treatment either soaks in from the surface, which is hard, or comes from the nail bed, which takes a long course.

A few patterns turn up. The usual one starts at the free edge or side and works back toward the cuticle, lifting the nail and packing chalky debris underneath. Another sits on the surface as powdery white patches you can scrape off. A third starts at the cuticle and moves forward, which is less common and may prompt a look for a reason the immune system is not keeping up.

The nail ends up thickened, discolored white, yellow or brown, crumbly at the edge and lifted off the bed. It is usually painless. When it hurts, it is the thick nail pressing inside a shoe.

Symptoms

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The usual pattern

Yellow Thickened Nail

Yellow-brown discoloration starting at the free edge and spreading back, with the nail thickening and crumbling underneath. Big toenails are affected most. It is painless at first and often ignored for years.
Chalky white marks

White Surface Patches

Powdery white patches on the surface of the nail plate that can be scraped away, leaving a rough nail. This form sits shallower than the yellow type. It can respond to topical treatment more readily.
Separating from the bed

Nail Lifting Off

The nail separates from the nail bed, leaving a hollow space that collects debris and looks white or dark. Trimming back the loose nail helps treatment reach further. A single dark or damaged nail with no clear cause should be looked at rather than assumed to be fungus.

Where It Shows Up

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Front view of a whole body with red marks on both feet and toes. The rest of the body is clear.
Toenails, and the big toe first
Toenails are affected far more often than fingernails, and the big toe is usually the first and the worst. It normally starts at the far edge or the side of the nail and works back toward the base. Look at the skin between the toes at the same time — athlete's foot is usually there too, and it is where the fungus came from.
Front view of a whole body with red marks on both hands and fingers. The rest of the body is clear.
Fingernails, less often
Fingernails are affected less often, and usually on one hand only, spread there from the feet. A fingernail problem on both hands at once is more likely to be something else, such as psoriasis or damage from wet work.

Causes

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Four things have to line up. First, the fungus has to reach the nail, almost always from your own feet - athlete's foot between the toes or on the sole, less often shared floors, damp shoes, nail tools or a household member. Second, it needs a way in. Intact nail is a good barrier, so it enters under the free edge, through a split at the side, or through a nail already loosened by shoes, sport or an old injury.

Third, it needs warmth, moisture and darkness, a fair description of the inside of a shoe. Fourth, it has to survive, and it does, because it lives in dead keratin where the immune system has little reach and blood-borne medicines cannot follow. Over months to years it works back toward the cuticle, digesting keratin and leaving the crumbly debris that lifts the nail.

Two things explain most of the frustration with treatment. Fungus sometimes forms a dense compact mass called a dermatophytoma, which drugs barely penetrate and which usually has to be pared or drilled down first. And because the nail is a reservoir, nothing looks better until the infected nail grows out and is cut off.

Risk Factors

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Nail fungus is about age, exposure and nail damage. Cleanliness has almost nothing to do with it.

Age
Age
The strongest factor by far. Nails grow slower and take more damage with time.
Other health conditions
Athlete's foot
The usual source. Untreated scaling between the toes feeds the nails for years.
Other health conditions
Diabetes
More common, more extensive and more serious - a fungal nail can be where a bad skin infection starts.
Other health conditions
Poor circulation in the legs
Arterial and venous disease both make infection more likely and slower to clear.
Other health conditions
A weakened immune system
HIV, chemotherapy, transplant medicines, long-term steroids. Infection starting at the cuticle is especially linked to this.
Other health conditions
Psoriasis
Nail psoriasis looks like fungus, makes real infection more likely, and both can sit in one nail.
Habits & environment
Nail trauma
Running, hiking, football, tight shoes, one bad crush injury. A damaged nail is an open door.
Habits & environment
Occlusive footwear and sweaty feet
Work boots, trainers worn all day, heavy sweating.
Habits & environment
Communal wet floors
Changing rooms, pool surrounds, gym showers, shared bathrooms.
Habits & environment
Nail salons and shared tools
Clippers, files and foot spas transfer infection when tools are not disinfected.
Habits & environment
Smoking
Consistently linked, probably through circulation.
Inherited
Family history
Susceptibility to Trichophyton rubrum does run in families.

Course

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Nail fungus does not clear on its own. Left alone it takes over more of the nail and, over years, more nails. Once treatment starts, nail growth sets the timeline, so watch the base.

Left untreatedMonths to years
It spreads back, then to other nails

It starts as a painless white or yellow patch under the tip or side of one nail, often a big toe, and gets blamed on a bump or on polish. Over years it works back toward the cuticle, the nail thickens, chalky debris builds underneath, and the nail lifts off the bed. It becomes brittle, hard to cut and uncomfortable in shoes.

One nailOften a big toeThen other nailsSoleBetween the toes
Weeks 1 to 12 of treatmentTaking the course
Nothing looks different yet

Tablets run about 6 weeks for fingernails and about 12 for toenails. The medicine binds into the nail and keeps working after the course ends. Seeing no change here is normal.

Months 3 to 6 of treatmentThe first real sign
Clear nail appears at the base

Clear, normal nail appears at the base and grows forward. That band at the cuticle is the real measure, not the old nail.

Base of the nailAt the cuticle
Months 9 to 18Growing out
The damaged part is cut away

The damaged nail is cut away as it advances toward the tip. A toenail replaces itself in roughly 12 to 18 months, a fingernail in about 4 to 6.

After clearingThe years that follow
Reinfection is common

Some nails never look normal again if the nail-forming tissue was scarred. Reinfection is common, which is why the feet and shoes matter as much as the nail.

What Makes It Better & Worse

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A fungal nail is four problems - picking up the fungus, warmth and moisture in the shoe, a break that lets it in, and fungus growing in dead nail. Most treatment failures are not failures of the medicine - they are an untreated source, a shoe problem, or a course stopped too soon.

1Picking up the fungus from your own feet

What helps

  • Treat the athlete's foot too This is what stops reinfection.
  • Shoes in communal wet areas Where people acquire it.

What makes it worse

  • Leaving athlete's foot untreated The skin between your toes reseeds it.
  • Sharing clippers and files Fungus transfers on tools easily.
  • Walking barefoot in communal wet areas Where most people pick it up.

2Warmth and moisture inside the shoe

What helps

  • Keep feet dry and rotate shoes Dry between the toes, let each pair dry.

What makes it worse

  • Wearing the same shoes every day They stay damp longer than you think.
  • Damp socks and sweaty feet Synthetic socks, wet trainers, feet never dried.
  • Covering it with nail polish Traps moisture, hides the new nail.

3A break or lift in the nail

What helps

  • Get the diagnosis confirmed first A clipping separates fungus from psoriasis and trauma.
  • Oral terbinafine First choice, best cure rate.
  • Oral itraconazole The alternative, but it interacts with many medicines.
  • Prescription topical treatments Months of painting, low cure rates.
  • Have thick nails reduced Thinning lets topical treatment in.
  • Cut nails straight across Less trauma, fewer splits.

What makes it worse

  • Nail trauma Knocks and short shoes lift the nail.
  • Cutting or filing corners aggressively Digging the sides creates splits.

4Fungus growing inside dead nail

What helps

  • Judge by the new nail at the base A clear band at the cuticle means progress.

What makes it worse

  • Stopping because nothing has changed Tablets work before the nail looks better.
  • Very thick nails left unthinned Medicine cannot get through.

Over-the-Counter Products

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Everything here you can buy without seeing anyone.

16:9 hero for Terbinafine. Never cropped: the tone strip and the corner logo depend on the full frame.
Terbinafine cream for the feet
Moderate evidence
This will not cure the nail, but it clears the athlete's foot on the skin that keeps reseeding it. Use it on the whole sole and between the toes, not just the itchy bit, and carry on for a week or two after the skin looks normal. Treating the skin is the single cheapest thing you can do to stop the nail coming back after a course of tablets.
Five white jars of 40% urea cream lined up, several also listing 2% salicylic acid
Urea 40% nail softener
Limited evidence
A strong urea cream or patch softens a thick nail so more of it can be filed or cut away. It kills nothing on its own. Its job here is to reduce the amount of dense nail sitting between a painted-on treatment and the fungus, and to make an uncuttable nail manageable.
Five dark glass tea tree essential oil bottles with glass droppers and one green box on white16:9 hero for Tea Tree Oil. Never cropped: the tone strip and the corner logo depend on the full frame.
Weak evidence
It does have antifungal activity in the laboratory, and the small studies in nails have shown some improvement in appearance. None have shown reliable cure of an established nail infection. It also cannot get through a thick nail, and it can irritate the skin around it.
Vinegar soaks and vapor rub
Vinegar soaks and vapor rub
Weak evidence
Both are recommended constantly online and the evidence behind them is thin. At best they slow surface fungus a little. The real cost is time: months spent soaking are months the nail could have been growing out clear under treatment that works.

Prescriptions

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These need a prescription.

A white nail lacquer bottle on a white background, with a plain white label reading Jublia and a blue swoosh beneath it.
Jublia
Moderate evidence
A solution painted on the nail once a day for about a year. It penetrates nail better than the older lacquers, and it suits mild infection, one or two nails, or people who cannot take tablets. Complete cure rates are still well below tablets, so it is a reasonable choice rather than the strongest one.
A white nail lacquer bottle on a white background, with a plain white label reading Kerydin and a blue swoosh beneath it.
Kerydin
Limited evidence
Another daily paint-on solution used for about a year, with a small molecule that gets through the nail plate. Results are similar to the other topicals: many nails look better, far fewer clear completely. It works best on thin nails with the infection limited to the tip.
A white nail lacquer bottle on a white background, with a plain white label reading Penlac and a blue swoosh beneath it.
Penlac
Limited evidence
The oldest of the prescription nail paints, applied daily with the built-up layers removed weekly with alcohol. Complete cure rates are low, in single figures in the original studies. It is cheap and safe, and it is mostly used when tablets are not an option.
16:9 hero for Terbinafine. Never cropped: the tone strip and the corner logo depend on the full frame.Diagram: how Terbinafine works in the skin
Strong evidence
The first choice for dermatophyte nail infection and the option with the best cure rate: usually 12 weeks for toenails and 6 for fingernails. It binds into the nail and keeps working there for months after the last tablet. A liver blood test is standard, taste changes are a known side effect, and nothing will look different while you are taking it.
An amber prescription bottle lying on its side on a white background with a plain white label reading Itraconazole, and six small round white tablets spilled out in front of it.
Itraconazole
Strong evidence
The main alternative, and the better option when a yeast or a mould is grown rather than a dermatophyte. It is often given as pulses, a week on and three weeks off. It interacts with a long list of common medicines and is avoided in some heart conditions, so your doctor needs everything you take, including anything bought over the counter.
16:9 hero for Fluconazole. Never cropped: the tone strip and the corner logo depend on the full frame.Diagram: how Fluconazole works in the skin
Moderate evidence
Taken once a week rather than daily, which suits people who struggle with a long daily course. Cure rates sit below terbinafine and it usually has to be continued until the nail has grown out, which can be a year. It also interacts with a number of other medicines.
An amber prescription bottle lying on its side on a white background with a plain white label reading Griseofulvin, and six small round white tablets spilled out in front of it.
Griseofulvin
Weak evidence
The old antifungal tablet, now largely replaced for nails. It needs 12 to 18 months of daily treatment, it clears fewer nails than terbinafine, and relapse afterwards is common. It is still occasionally used in children or when the newer drugs cannot be taken.

Procedures

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These are done in the office, usually over several visits.

Thinning and clipping the nail
Moderate evidence
Cutting the nail back and filing or drilling down the thickened part, usually by a podiatrist. It is painless and it does two things: it takes pressure off a sore toe, and it removes dense nail and any packed clump of fungus that medicine cannot get through. Done alongside tablets or a nail paint, it makes them work better; done alone, it does not treat the infection.
A gloved hand guiding a laser handpiece along a patient's lower leg16:9 hero for Nd:YAG Laser. Never cropped: the tone strip and the corner logo depend on the full frame.
Limited evidence
Laser devices are cleared to temporarily increase the amount of clear nail, which is not the same thing as curing the infection. Studies are small, short and mostly measure appearance rather than clearance, and the fungus commonly returns. It is usually several sessions, paid for out of pocket, and it is a reasonable thing to consider only when tablets are genuinely not an option.
Removing the nail
Limited evidence
Taking off part or all of the nail, either surgically or with a strong urea paste. It is not a cure on its own, because the fungus is also in the nail bed and the new nail grows back into it. It is kept for a nail that is painful, hopelessly thick, or holding a dense clump of fungus, and it is done alongside tablets rather than instead of them.

When to See a Dermatologist

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There is no rush with a discolored toenail in a healthy adult, but do not treat it blind - the main treatment is months of tablets, and a large proportion of nails called fungal by eye have no fungus in them. Book if you want it treated, since effective treatment is prescription-only and starts with a clipping sent for testing, and also if more than one or two nails are involved, the nail hurts in shoes, it is too thick to cut, or months of an over-the-counter product have produced no clear nail at the base. Go sooner with diabetes, poor circulation, a suppressed immune system or numb feet, and get same-day care if redness, swelling, heat and pain spread into the toe or foot, which is bacterial, not fungal. Do not wait on a single new dark band down a nail, especially one widening or spreading pigment onto the cuticle skin, and stop any antifungal for a couple of weeks before the clipping, because it makes the test less reliable.

— Dr. Schwarz, Board Certified Dermatologist

Complications

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Cellulitis

A thickened, cracked nail and the split skin around it give bacteria a way into the leg. The result is a spreading area of hot, swollen, tender skin, usually with feeling unwell and a fever, and it needs antibiotics quickly. It matters most for people with diabetes, poor circulation in the legs or a weakened immune system, which is why fungal nails are treated more actively in those situations.

Spread to other nails and the skin

Left alone, one infected nail slowly seeds the others, and the same fungus keeps the skin of the sole and the gaps between the toes scaly and itchy. This is athlete's foot and nail fungus feeding each other. Treating the nail while ignoring the skin is the usual reason people end up doing the whole course twice.

Permanent nail damage

If the infection has been there for years, the tissue that makes the nail can be scarred by the time it clears. The nail then grows back ridged, short or misshapen even though the fungus is gone. It is not common, and it is a reason to treat sooner rather than later rather than a reason to expect it.

Lookalikes

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Nail Psoriasis

The most frequent impostor, and the two look almost identical once a nail is thick, yellow-brown and lifted. Psoriasis usually gives pinprick dents across several nails, a salmon-pink oil-drop patch under the nail, and psoriasis on the scalp, elbows or knees to go with it. Fungus more often starts in one big toenail and works back from the tip. A nail clipping sent for testing is what settles it, and both can be present in the same nail.

Nail damage from shoes and sport

Years of running, hiking, football or short shoes produce a thick, ridged, discolored big toenail that looks exactly like an infection. The clues are that it is usually one or both big toenails and nothing else, the skin of the feet is clear, and it dates from a period of hard use or one crush injury. Old bleeding under the nail adds a red-brown to black patch that grows out with the nail rather than spreading back toward the cuticle.

Melanoma

Melanoma can start under a nail, and because it discolors and damages the nail it is sometimes treated as fungus for months. The pattern is different: one nail only, a brown or black band running the length of the nail that is widening, uneven in color, or spreading pigment onto the skin of the cuticle or fingertip. It is rare, it is diagnosed late more often on deeper skin tones, and it is worth one appointment rather than waiting to see.

Myths

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  • "Thick, yellow toenails are always fungus." Often they are not. Shoe pressure, sport, nail psoriasis, lichen planus, poor circulation and aging all do this. Testing a clipping is the only way to be sure, and a large share come back negative.
  • "Nail polish caused it." Polish does not cause it. It traps moisture, hides a problem, and damages the plate when gels are ripped off, but the infection comes from fungus.
  • "Vinegar, tea tree oil, bleach or vapor rub will cure it." The evidence is weak. Some may slightly slow surface fungus, but none clear an established infection in the nail plate. Months on them are months the nail could have been growing out.
  • "If it is not better in two months, the treatment failed." Nothing visible happens in two months. The medicine works on the new nail at the base, and a toenail takes roughly a year to replace itself. Judging early is the commonest reason people quit treatment that was working.
  • "Laser treatment cures nail fungus." Lasers are cleared to temporarily increase clear nail, which is not curing it. The evidence is limited and short-term, and it is expensive and not covered by insurance.
  • "Once it is treated, it is gone for good." Reinfection is common in the years afterward, especially if the feet and the inside of the shoes were never dealt with. Prevention is more about feet and footwear than nails.
  • "It is very contagious to other people." It spreads within a household through shared floors, towels and clippers, but far more readily from your own feet to your own nails. Not from a handshake.

Questions Patients Ask

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How long before my nail looks normal?

Longer than almost anyone expects. The medicine protects new nail as it forms, and a toenail takes roughly 12 to 18 months to replace itself, a fingernail about 4 to 6. Progress is a widening band of clear nail at the cuticle, not the tip.

Do I really need tablets, or can I use something on the nail?

It depends how much nail is involved. Prescription topicals suit mild infection, one or two nails, or when tablets are not suitable, and they avoid blood tests. Cure rates are distinctly lower and they need daily use for close to a year. For thick nails, several nails, or infection at the cuticle, tablets are what work.

Are the tablets dangerous?

Most healthy adults tolerate them well and serious problems are uncommon. Terbinafine can affect the liver, so a blood test before or during the course is standard, and taste changes are common. Itraconazole interacts with a long list of medicines and is avoided in some heart conditions. Your doctor needs a full list of what else you take.

Can I still paint my nails?

While you are being tested, no - polish makes sampling harder and hides what is happening. During treatment it is not forbidden, but it hides the new nail you are meant to be watching, and gel removal damages the plate. Going bare is more useful.

Will it come back?

It can, and in a meaningful number of people it does. The strongest protection is not another course of tablets - it is treating the athlete's foot, keeping feet dry, rotating shoes, and wearing something in communal wet areas.

Can I give it to my family?

It spreads within a household more than between strangers - shared floors, towels, bath mats and clippers. Use your own clippers and file, keep a separate foot towel, and treat any athlete's foot in the house. Ordinary contact is very unlikely.

References

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