Fungal nail infection: causes and treatment
A nail that has turned yellow-brown, thickened and started to crumble at the edge is usually a fungal infection, and it almost never clears up by itself. This page explains how it is recognised, why a nail sample is worth taking before months of treatment, what lacquers and tablets can realistically achieve, how long it takes, and which nail changes are not fungal at all and need to be examined.
How does a fungal nail infection look?
It usually begins at the free edge or the side of a toenail, most often the big toe. The nail loses its shine, turns white, yellow or brown, and the discolouration slowly spreads towards the base. Underneath, debris builds up so the nail lifts away from the nail bed.
As it progresses, the nail thickens, becomes brittle and crumbles when cut. Shoes may become uncomfortable, and a very thick nail can press on the skin and be genuinely painful. There is usually no redness or throbbing; a hot, red, painful nail fold with pus is a bacterial problem, not this.
Fingernails are affected less often, and when they are, the possibility of another skin condition should be considered more seriously. Many people have had athlete's foot for years beforehand, and the skin between the toes often still shows scaling. Fungal nail infection tends to be painless in the early stages, which is exactly why it is usually well advanced by the time anyone deals with it.
What causes it and who gets it?
The commonest cause is a group of fungi called dermatophytes, particularly Trichophyton rubrum, the same organism that causes athlete's foot. It spreads from the skin into the nail, which is why treating the skin matters as much as treating the nail. Yeasts and moulds account for a smaller share, mainly in fingernails.
The conditions that favour it are warm, damp and enclosed: occlusive footwear, feet that sweat, shared floors in changing rooms, pools and gyms. Repeated minor trauma from sport or from tight shoes damages the nail and gives the fungus a way in.
Risk rises with age, because nails grow more slowly and the circulation to the feet is less good. Diabetes, peripheral arterial disease, psoriasis, immunosuppressive medicines and being a smoker all increase the likelihood. It is mildly contagious within a household through shared floors, towels and nail clippers, so keeping a separate set of clippers for an affected nail is sensible.
Why should the nail be sampled first?
Roughly half of all abnormal nails turn out not to be fungal, and treatment lasting many months is a poor thing to commit to on appearance alone. A clipping of the affected nail with some of the debris from underneath can be sent to a laboratory for microscopy and culture, which confirms whether a fungus is present and often which type.
This matters for two reasons. First, oral antifungal treatment is long and is not free of risk, so confirmation before starting is proportionate. Second, the treatment differs: an infection caused by a mould or a yeast may not respond to the medicine used for dermatophytes.
Send the sample before starting any treatment where possible, because antifungal lacquer applied for a few weeks can make the laboratory result falsely negative. A negative result in a nail that clearly looks abnormal usually means either the sample missed the affected part or the diagnosis is something else, and both possibilities deserve a second look rather than a repeat prescription.
What treatments exist?
For an infection limited to the edge of one or two nails, without involvement of the base, a topical antifungal nail lacquer bought at a pharmacy is a reasonable first step. Amorolfine is the usual choice. Treatment is applied for many months and works better if the thickened surface is filed down before each application.
Where several nails are involved, where the base of the nail is affected, or where the nail is very thickened, topical treatment alone rarely succeeds and an oral antifungal is considered. Terbinafine is the standard choice for dermatophyte infection and itraconazole is used in some situations. Both are prescription-only medicines in Ireland.
Oral treatment is not started casually. Terbinafine can affect the liver and taste, and itraconazole interacts with a long list of common medicines, including some statins and heart medicines, so a doctor reviews everything you take before prescribing and may arrange blood tests. Laser and device treatments are marketed widely; the evidence for lasting cure is weak, and the cost is not.
How long does it take, and will the nail look normal again?
The honest answer is: slowly, and not always. The medicine kills the fungus, but the damaged part of the nail does not repair itself. It has to grow out and be replaced, and a toenail replaces itself over the better part of a year, more slowly in older people and in those with poor circulation.
That is why the nail can look unchanged for months while treatment is working. The sign to look for is healthy new nail at the base, not improvement in the damaged part. If new growth at the base still looks abnormal after several months of treatment, the plan needs reviewing rather than extending.
Even after successful treatment a nail can remain permanently misshapen if the nail matrix was damaged, and recurrence is common. Reinfection is reduced by treating the skin at the same time, keeping feet dry, changing socks daily, rotating shoes so they dry out, treating the inside of footwear with an antifungal powder, and not walking barefoot in shared changing areas.
When do I need medical help without delay?
Call 112 or 999 or go to an Emergency Department if redness and swelling are spreading up the foot or leg with fever, shivering or feeling very unwell, or if there is severe pain with a cold, pale or dusky foot. Skin broken by fungal infection is a common entry point for bacterial cellulitis, which is a medical emergency when it spreads.
Arrange an assessment on the same day if you have diabetes, poor circulation or a weakened immune system and you develop a red, hot, painful or discharging nail or toe, or an ulcer anywhere on the foot. In those groups a foot infection is never treated as a cosmetic matter, and self-treatment is not appropriate.
See a doctor rather than starting antifungal treatment if a nail shows a new dark brown or black streak running along it, if pigment spreads onto the surrounding skin, if a single nail becomes distorted with bleeding or a growth beneath it, or if a nail changes rapidly without any history of athlete's foot. A melanoma can appear under a nail, and it is missed exactly when it is assumed to be fungus.
What else looks like a fungal nail?
Psoriasis of the nail produces pitting, oil-drop discolouration, thickening and separation from the nail bed, and it often accompanies psoriasis elsewhere on the skin or scalp. Antifungal treatment does nothing for it, and it is one of the commonest reasons a course of tablets appears to fail.
Repeated trauma, typically from running or from shoes that are too short, causes thickening and bruising that resembles infection. Lichen planus, eczema around the nail, and chronic irritation from water and detergents all change nail shape and texture. Yellow nail syndrome and some medicines discolour nails without any infection at all.
Green-black discolouration in a nail that has lifted often indicates bacterial colonisation, and a painful, red, swollen nail fold is paronychia, which is treated quite differently. This is the practical argument for sampling the nail before committing months to treatment: appearance alone is unreliable even to experienced eyes.
Will a fungal nail infection clear up on its own?
It rarely does. Left alone it usually spreads slowly to more of the nail and often to other nails, and it can act as a reservoir that keeps reinfecting the skin of the foot. Treatment is a choice rather than an obligation for a mild, painless case, but improvement without treatment should not be expected.
Do over-the-counter nail lacquers work?
They can work for early infection affecting the edge of one or two nails, provided they are applied consistently for many months and the nail surface is filed first. Where the base of the nail is involved or several nails are affected, topical treatment alone is usually not enough.
Do I need a blood test before taking antifungal tablets?
A doctor decides that individually. Oral antifungals can affect the liver and interact with other medicines, so your medication list, alcohol intake and any liver problems are reviewed beforehand, and blood tests are arranged where indicated. This is one reason the medicines are prescription-only.
Can I paint over an infected nail?
Nail varnish and false nails trap moisture and make it harder to judge whether new growth is healthy, so they are best avoided during treatment. They also make laboratory sampling less reliable. Once the nail has grown out clear, there is no medical objection.
How do I stop it coming back?
Treat the skin as well as the nail, dry between the toes after washing, change socks daily, alternate shoes so they dry out fully, use an antifungal powder in footwear, wear something on your feet in changing rooms and pools, and keep separate nail clippers for the affected nail.