Nail Fungus Guide: What Onychomycosis Is — and What It Isn't
Onychomycosis is a fungal infection of the nail unit. It is common, but not every yellow, thick or brittle nail is fungal, which is why the diagnosis matters before you spend months on a treatment.
Common signs
The American Academy of Dermatology lists color change, thickening, brittleness, cracking, crumbling, lifting and buildup beneath the nail among common signs. Toenails are affected more often than fingernails.
Why appearance alone is not enough
Nail psoriasis, repeated trauma and other nail disorders can look similar. Dermatologists may examine nail scrapings or clippings under a microscope or use other testing to confirm a fungal cause.
Treatment categories
Prescription topical medicines
Examples include efinaconazole, tavaborole and ciclopirox. They require long courses and work best in selected cases.
Oral antifungals
Oral terbinafine and itraconazole may be used when appropriate, with clinician consideration of interactions and monitoring.
Cosmetic / wellness products
Oils and nail-renewal products may support appearance or surrounding-skin care but should not be equated with proven infection treatment.
Why treatment takes time
Even when the infection is controlled, the abnormal nail has to grow out. A toenail may take 12 to 18 months to fully replace itself.
Medical references
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Last reviewed: September 28, 2026