OBJECTIVE: To evaluate the ability of efinaconazole vehicle to reach the site of toenail onychomycosis by spreading through the subungual space between the nail plate and nail bed. Lacquer-based vehicles are primarily limited to application on the nail plate and dependent on nail plate permeation. METHODS: 11 patients (mean age 48.5 years) were entered with clinically determined onychomycosis. Presence of fungal infection was confirmed by KOH testing in eight patients. Two separate applications of vehicle (with fluorescein incorporated for better visualization) were applied at the hyponychium, avoiding application to the exterior nail plate surface. Affected nails were later clipped to allow examination of the nail bed and further examination of the underside of the nail. Spread of formulation was assessed under visible and UV light conditions by photographing target toenails after vehicle application, and after nail clipping. RESULTS: Assessments under both visible and UV light indicated that the vehicle had spread into the subungual space, with deposition of flourescein wherever vehicle had reached, including in the nail bed. Nail clippings also indicated deposition to the underside of the nail plate. LIMITATIONS: The relative contributions of spreading into the subungual space, or permeation through the nail plate to the efficacy of efinaconazole topical solution, 10% in treating onychomycosis were not assessed. CONCLUSIONS: This study suggests that the vehicle developed for efinaconazole topical solution, 10%, when applied at the hyponychium, spreads into the subungual space between the nail plate and nail bed, reaching the site of infection.