Fungal nail infection (onychomycosis)

Onychomycosis is a common fungal infection of the nails, especially the toenails. It can cause the nail to change color, thicken, break, or detach. Not every altered nail is a fungal infection, so a correct diagnosis is important before treatment.

Fungal nail infections, medically known as onychomycosis , are an infection of the nail plate, nail bed, or both. They can affect fingernails or toenails, although they are much more common on the toes, especially the big toe. Not every yellow, thick, or brittle nail is fungal: trauma, psoriasis, lichen planus, chronic eczema, bacteria, and even tumors can mimic the condition. Therefore, when oral medication is considered or the symptoms are atypical, a specialist evaluation can prevent months of unnecessary treatment (and its associated risks). Lipner 2019

Summary in 60 seconds

• Fungal nail infections often appear as yellow, whitish, thickened, brittle nails or nails that separate from the skin.
• They affect toenails more often than fingernails.
• They can appear after athlete’s foot, persistent moisture, closed footwear, microtrauma, diabetes, poor circulation, or immunosuppression.
• Not all altered nails have fungus; many diseases can mimic onychomycosis.
• Topical treatments can be helpful in mild cases, but they usually require consistent use for months.
• Oral treatments are usually more effective, but they require medical advice and a risk assessment.
• The nail may take many months to look normal even after the fungus is under control.
• Relapses are common, especially if athlete’s foot or moisture in the footwear persists.
• Significant pain, pus, progressive redness, diabetes, or a new dark band on the nail require evaluation by a dermatologist.
• The key is to combine a correct diagnosis, realistic treatment, and prevention of reinfection.

What is it?

Onychomycosis is a fungal infection that invades nail structures. The most common causative agents are dermatophytes , a group of fungi that also causes athlete’s foot. Less frequently, yeasts, such as Candida , or non-dermatophyte molds are involved. DermNet 2022

In practice, many people use the term “nail fungus” for any yellow or thickened nail. This association is understandable, but not always accurate. A nail repeatedly bumped by footwear, nail psoriasis, or a bacterial infection can all resemble a fungal infection. This distinction is important because antifungal medications do not treat problems not caused by fungi.

A yellow or thickened nail does not always mean a fungal infection; confirming the diagnosis can avoid unnecessary prolonged treatments.

Why does it appear?

Fungal infections often thrive in a combination of moisture, warmth, microtrauma, and a compromised nail barrier. This is why they are more common on the feet: closed shoes, sweat, sports, shared showers, and athlete’s foot create an environment where dermatophytes flourish. BAD 2017

There are also personal factors. Age, diabetes, circulatory problems, immunosuppression, repeated trauma, hyperhidrosis, previously damaged nails, and a history of tinea pedis increase the risk. This does not mean the infection is the patient’s fault: onychomycosis is common and can occur even with good hygiene.

What does it look like and what symptoms does it cause?

The most common presentation is a nail that changes color—yellow, white, grayish, or greenish—thickens, becomes brittle, or partially detaches from the nail bed. Sometimes there is whitish or yellowish material under the nail, irregular edges, discomfort with footwear, or difficulty cutting it. DermNet 2022

In its early stages, it may not hurt. Over time, a very thick toenail can press against the shoe, causing pain, leading to ingrown toenails, or limiting activities. If there is also peeling, cracking, or itching between the toes, athlete’s foot may be present.

There are signs that shouldn’t be automatically attributed to a fungal infection: a new brown or black band, pigment spreading to the skin around the nail, bleeding without a clear injury, a growing lesion, intense pain, or rapid destruction of a single nail. In these cases, it’s advisable to rule out other causes.

What can you do today?

The most prudent course of action is to monitor the condition and reduce factors that promote moisture and reinfection. Keep your feet dry, change socks if you sweat, alternate your footwear, wear ventilated shoes whenever possible, and thoroughly dry between your toes. In public showers, swimming pools, and locker rooms, wearing sandals can reduce exposure.

Trim your nails carefully, without pulling them off or cutting too deeply into the edges. If they are very thick, gently filing them can relieve pressure, but avoid harsh tools. Don’t share nail clippers, files, or shoes. If there is peeling or itching on the soles of your feet or between your toes, treating athlete’s foot can be an important part of preventing recurrence.

Based on the evidence, topical treatments may be more helpful in mild cases and require consistent use; some are used for approximately 48 weeks. Oral treatments, such as terbinafine or itraconazole, may be more effective, but are not suitable for everyone and require specialist medical evaluation due to potential interactions, pregnancy, breastfeeding, or liver disease. AAD 2025

A practical measure, although it doesn’t replace a diagnosis, is to take a monthly photograph in good lighting. Nails grow slowly; comparing images avoids judging the treatment day by day, when changes are usually minimal.

In nail fungus, patience is part of the treatment: the infection may improve before the nail looks normal.

What NOT to do?

It is not advisable to start taking antifungal pills without a doctor’s prescription. Although they can be helpful, they are not cosmetic treatments and are not harmless: they can interact with other medications and in some cases require testing or monitoring.

It is also not recommended to apply chlorine, acids, garlic, concentrated essential oils, irritating remedies, or abrasive homemade mixtures. These can cause dermatitis, burns, pain, or worsen nail fragility without resolving the infection.

Avoid indefinitely covering the nail with gel polish or artificial nails if there is worsening, pain, odor, or progressive separation. This can hinder assessment, retain moisture, and delay diagnosis.

Do not pull off the nail or cut deeply to «get rid of the fungus.» This can open the door to bacterial infections, especially in people with diabetes, poor circulation, or weakened immune systems.

When should I consult?

Consult a dermatology specialist if any of these situations appear:

• Significant pain, redness, heat, pus, or swelling around the nail.
• Diabetes, poor circulation, immunosuppression, or a history of foot ulcers.
• A new brown, black, or irregular band on a nail.
• Pigment that spreads to the skin around the nail.
• Bleeding, injury, or rapid destruction of a single nail.
• Several affected nails, progressive thickening, or difficulty walking.
• Lack of response after well-performed topical treatments.
• Pregnancy, breastfeeding, or use of multiple medications should be considered before taking pills.

The consultation is especially important if the treatment will be oral, if the diagnosis is not clear, or if the altered nail is a single one and persistently changes shape.

What does a dermatologist usually do?

The dermatologist examines all the nails, the skin of the feet, and, when appropriate, other areas of the skin. They can assess whether the distribution appears consistent with onychomycosis or if there are signs of psoriasis, lichen planus, trauma, bacterial infection, or another condition. Dermatoscopy can help differentiate patterns of fungal infection, trauma, or pigmentation. DermNet 2022

If confirmation of the diagnosis is needed, nail material can be taken: clippings, scrapings from under the nail, or a sample from the most affected area. Tests may include direct examination with potassium hydroxide, culture, PAS staining of nail clippings, or molecular testing, depending on availability. Culture can take weeks but helps identify the type of fungus.

Treatment is determined based on the extent and number of affected nails, matrix involvement, thickness, pain, associated medical conditions, current medications, and patient preferences. Topical treatment may be indicated in mild cases. Oral treatment may be considered in moderate or extensive cases, sometimes combined with medical filing or debridement to reduce thickness. It may take a year or more for the toenail to regain a clearer appearance because a new nail must grow. AAD 2025

Regarding lasers and devices, the results should be interpreted with caution. Some technologies may improve appearance or be used as an adjunct, but comparative evidence against systemic therapies is limited and should not be presented as a guaranteed cure. AAD 2025

Frequently Asked Questions

Does nail fungus heal on its own?

• In general, an established infection does not usually clear up quickly on its own. Some people with minimal symptoms may choose to monitor the situation if there are no discomforts or risk factors, but if it progresses, becomes painful, or if there is diabetes/immunosuppression, it is advisable to consult a doctor. BAD 2017

What is the most effective treatment?

• It depends on the case. Generally speaking, oral antifungals have better cure rates than many topical treatments, especially when there are multiple nails, significant thickening, or widespread involvement. However, they require medical evaluation. Terbinafine is often a common choice for dermatophytes, but it is not suitable for everyone. Kreijkamp-Kaspers 2017

Are over-the-counter creams effective for toenail fungus?

• Over-the-counter antifungal creams can help on the skin, for example, with athlete’s foot, but they often don’t penetrate the nail well. Specific formulations, such as lacquers or solutions, are used for the nail, and even these require months of application. AAD 2025

How long does it take for a nail to look better?

• Even if the fungus is controlled, the appearance improves slowly because the nail needs to grow. Toenails can take many months, even a year or more, to look renewed. This does not necessarily mean that the treatment has failed. AAD 2025

Can I paint my nails if I have a fungal infection?

• It can occasionally be done if there is no pain, discharge, or worsening of the condition, but covering the nail for extended periods can make it difficult to observe changes and can trap moisture. If the nail lifts, hurts, or changes rapidly, it is best to avoid nail polish and consult a doctor.

Are toenail fungi contagious?

• They can be transmitted through contact with infected scales, damp surfaces, or shared instruments. It’s not usually an emergency, but it’s advisable not to share nail clippers, treat athlete’s foot if present, and use protection in public showers.

When can a dark nail be something more serious?

• A new, irregular, growing dark band that affects a single nail or extends onto the surrounding skin should be evaluated by a dermatologist. Not all pigmentation is dangerous, but some pigmented lesions require ruling out subungual melanoma or other diagnoses.

Bibliography and sources

• Lipner SR, Scher RK. Onychomycosis: Clinical overview and diagnosis . J Am Acad Dermatol. 2019;80(4):835-851. doi:10.1016/j.jaad.2018.03.062.
• Lipner SR, Scher RK. Onychomycosis: Treatment and prevention of recurrence . J Am Acad Dermatol. 2019;80(4):853-867. doi:10.1016/j.jaad.2018.05.1260.
• Leung AKC, Lam JM, Leong KF, Hon KL, Barankin B, Leung AAM, et al. Onychomycosis: An Updated Review . Recent Pat Inflamm Allergy Drug Discov. 2020;14(1):32-45. doi:10.2174/1872213X13666191026090713.
• Kreijkamp-Kaspers S, Hawke K, Guo L, Kerin G, Bell-Syer SEM, Magin P, et al. Oral antifungal medication for toenail onychomycosis . Cochrane Database Syst Rev. 2017;7(7):CD010031. doi:10.1002/14651858.CD010031.pub2.
• Foley K, Gupta AK, Versteeg SG, Mays R, Villanueva E, John D. Topical and device-based treatments for fungal infections of the toenails . Cochrane Database Syst Rev. 2020;1(1):CD012093. doi:10.1002/14651858.CD012093.pub2.
• Ameen M, Lear JT, Madan V, Mohd Mustapa MF, Richardson M. British Association of Dermatologists’ guidelines for the management of onychomycosis 2014 . Br J Dermatol. 2014;171(5):937-958. doi:10.1111/bjd.13358.
• British Association of Dermatologists. Fungal infections of the nails [Internet] . London: British Association of Dermatologists; 2017 [cited 2026 May 8] .
• American Academy of Dermatology Association. Nail fungus: diagnosis and treatment [Internet] . Rosemont (IL): American Academy of Dermatology Association; 2025 [cited 2026 May 8] .
• DermNet. Fungal nail infections [Internet] . New Zealand: DermNet; 2022 [cited 2026 May 8] .
• Frazier WT, Santiago-Delgado ZM, Stupka KC. Onychomycosis: Rapid Evidence Review . Am Fam Physician. 2021;104(4):359-367.
• Nenoff P, Reinel D, Mayser P, Abeck D, Bezold G, Bosshard PP, et al. S1 Guideline onychomycosis . J Dtsch Dermatol Ges. 2023;21(6):678-692. doi:10.1111/ddg.14988.
• Falotico JM, Lipner SR. Updated Perspectives on the Diagnosis and Management of Onychomycosis . Clin Cosmet Investig Dermatol. 2022;15:1933-1957. doi:10.2147/CCID.S362635.