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Am Fam Physician. 2021;104(4):359-367

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Onychomycosis is a chronic fungal infection of the fingernail or toenail bed leading to brittle, discolored, and thickened nails. Onychomycosis is not just a cosmetic problem. Untreated onychomycosis can cause pain, discomfort, and physical impairment, negatively impacting quality of life. Onychomycosis should be suspected in patients with discolored nails, nail plate thickening, nail separation, and foul-smelling nails. Accurate diagnosis is important before initiating treatment because therapy is lengthy and can cause adverse effects. A potassium hydroxide preparation with confirmatory fungal culture, periodic acid–Schiff stain, or polymerase chain reaction is the preferred diagnostic approach if confirmative testing is cost prohibitive or not available. Treatment decisions should be based on severity, comorbidities, and patient preference. Oral terbinafine is preferred over topical therapy because of better effectiveness and shorter treatment duration. Patients taking terbinafine in combination with tricyclic antidepressants, selective serotonin reuptake inhibitors, atypical antipsychotics, beta blockers, or tamoxifen should be monitored for drug-drug interactions. Topical therapy, including ciclopirox 8%, efinaconazole 10%, and tavaborole 5%, is less effective than oral agents but can be used to treat mild to moderate onychomycosis, with fewer adverse effects and drug-drug interactions. Nail trimming and debridement used concurrently with pharmacologic therapy improve treatment response. Although photodynamic and plasma therapies are newer treatment options that have been explored for the treatment of onychomycosis, larger randomized trials are needed. Preventive measures such as avoiding walking barefoot in public places and disinfecting shoes and socks are thought to reduce the 25% relapse rate.

Onychomycosis, a chronic fungal infection of the fingernail or toenail bed, is commonly encountered in primary care. Onychomycosis is not just a cosmetic problem. If untreated, it can cause pain, discomfort, and physical impairment, negatively impacting quality of life. This article provides a summary of the best available patient-oriented evidence on the diagnosis and management of this condition.

RecommendationSponsoring organization
Do not prescribe oral antifungal therapy for suspected nail fungus without confirmation of fungal infection.American Academy of Dermatology


  • The estimated point prevalence of onychomycosis in North America is up to 13.8% for adults and 0.44% for children and adolescents younger than 18 years.1,2

  • Age older than 60 years is an important risk factor because of poor peripheral circulation, suboptimal immune function, slower nail growth, and longer exposure to pathogenic fungi.3

  • Other risk factors include recurrent nail trauma, tobacco use, and certain comorbidities (diabetes mellitus, obesity, psoriasis, malignancy, HIV, peripheral vascular disease, immunocompromised state).


  • Dermatophytes cause 70% of onychomycosis infections in the United States, with the remaining 30% caused by nondermatophyte molds and yeasts.4

  • One study showed that 39% of infections were mixed (caused by dermatophytes plus nondermatophyte mold and/or yeast), making diagnosis and treatment challenging.5



  • With fungi causing 50% of nail dystrophies, the differential diagnosis for nail abnormalities is large (Table 1).610

  • Common signs and symptoms of onychomycosis include nails that appear discolored (Figure 1), deformed (Figure 2), hypertrophic, or hyperkeratotic; subungual debris; separation from the nail bed; brittle nails that break easily or crumble (Figure 3); and nails that are foul smelling.

  • Onychomycosis is classified into several subtypes based on the phenotypic pattern of nail invasion (Table 2).8,11,12

  • Severity is classified as mild, moderate, or severe based on the Onychomycosis Severity Index. This index uses three clinical features to assess severity: area of involvement, proximity of disease to the nail matrix, and presence of dermatophytoma or subungual hyperkeratosis thickness greater than 2 mm.13

ConditionFrequency of nail involvement (%)*Nail manifestations
Skin disorders
Psoriasis50% to 90%Nail pitting, onycholysis, subungual hyperkeratosis, brownish discoloration (oil stains) or salmon-colored patches
Lichen planus10% to 16%Longitudinal grooves and fissures, progressive nail thinning, dorsal pterygium
Chronic dermatitis11%Nail pitting, Beau lines (transverse grooves)
Infectious conditions
Paronychia100%Inflammation of the nail bed associated with erythema, edema, and pain at the proximal nail folds
WartsNodules cause onycholysis or longitudinal grooves in the nail plate and splinter hemorrhages
Herpetic whitlowHemorrhagic and purpuric nail lesions
OnychodystrophyOnycholysis, periungual keratosis
Bowen disease8%Paronychia, lack of nail growth, onycholysis
Melanoma0.2% to 7% of cutaneous melanomasBrownish-yellow discoloration of the nail plate, subungual hyperkeratosis, onychorrhexis
FibromaSmooth, firm, flesh-colored lumps that emerge from the nail folds
Trachyonychia100%Longitudinal ridging; thin, brittle nails; nail pitting; usually involves all 20 nails
Yellow nail syndrome100%Yellowish nail discoloration
Subtype*Clinical featuresComments
Distal lateral subungualOnycholysis, keratosisMost common subtype, first and fifth toenails are most commonly involved
SuperficialWhite or black superficial patches or transverse striaeMostly occurs in children
EndonyxDiffuse white discoloration without onycholysis or keratosisRare
Proximal subungualLeukonychia and onycholysisSuggests underlying immunosuppression (e.g., AIDS)
Mixed patternMore than one subtype of nail plate infection within the same nailMost common combination is superficial onychomycosis with distal lateral subungual onychomycosis or proximal subungual onychomycosis
Total dystrophicDiffuse thickening and crumbling of the nail plate, friable and yellowish discolorationEnd-stage distal lateral subungual onychomycosis or proximal subungual onychomycosis, or caused by primary immunodeficiency
SecondaryFeatures depend on the underlying conditionPsoriasis is most common


  • Laboratory confirmation of nail infection is important for accurate diagnosis.14

  • A potassium hydroxide (KOH) preparation with direct microscopy is the preferred diagnostic method because it is highly specific, has rapid results, and is cost-effective.12,15 Diagnosis by KOH preparation alone is sufficient for treatment initiation. However, if KOH results are negative and there is high clinical suspicion for onychomycosis, other testing may be performed to confirm the diagnosis. Table 3 includes the accuracy of diagnostic testing methods.16,17

  • Fungal culture of nail clippings or subungual debris allows for species differentiation but is limited by cost and the time it takes to get results. Biopsy and periodic acid–Schiff stain of nail clippings can help assess the degree of nail plate involvement. Polymerase chain reaction can also confirm the diagnosis but is more expensive than other tests.1517

  • Because samples should be taken from the most proximal area of onycholysis (Figure 4), the nail plate may need to be trimmed to reveal this area.

  • Diagnostic testing is generally recommended before initiating treatment, but empiric treatment with terbinafine can be considered if testing is cost prohibitive.18

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