Onychomycosis is caused by dermatophytes — the same fungi responsible for athlete's foot — entering the nail through small cuts or gaps between nail and bed. Risk factors include older age, diabetes, peripheral vascular disease, nail trauma, communal shower use, wearing occlusive footwear, and prior athlete's foot.

OTC topical treatments (clotrimazole, tolnaftate) have very low cure rates for onychomycosis because they cannot penetrate the nail plate effectively. Prescription topical ciclopirox and efinaconazole have modestly better penetration but still lower cure rates than oral therapy. Oral terbinafine has the highest cure rate (60–80%) and is generally first-line.

Oral terbinafine is typically prescribed for 6 weeks for fingernails and 12 weeks for toenails. However, visible nail clearance takes much longer — 6–12 months for fingernails and 12–18 months for toenails — as healthy nail slowly grows in to replace the infected nail. Patience is essential; treatment success is assessed by nail appearance at the end of the growth cycle.

Yes — recurrence rates are significant, approximately 20–25% within 3 years. Continuing to apply antifungal spray or powder in shoes, treating concurrent athlete's foot, keeping nails short and dry, and wearing moisture-wicking socks reduce recurrence risk significantly after successful treatment.

Oral terbinafine requires liver function monitoring — liver toxicity is rare but possible. GI side effects and taste disturbance are occasionally reported. Topical treatments are very well tolerated with minimal systemic absorption. Oral itraconazole — an alternative — has a greater drug interaction profile and requires cardiac caution. Fingernail or toenail fungus (onychomycosis) visit What to expect from a Fingernail or toenail fungus (onychomycosis) visit