Efinaconazole is triazole used as a 10% topical solution for the treatment of onychomycosis, a fungal infection of the nails. It was approved for use in Canada and the USA in 2014 and is marketed by Valeant Pharmaceuticals North America LLC under the name Jublia. Like other antifungal triazoles, efinaconazole inhibits the fungal cytochrome P450 enzyme lanosterol 14α demethylase (CYP51), thereby disrupting ergosterol synthesis and, consequently, membrane integrity and growth in fungi. CYP51 is evolutionarily conserved and, in mammals, mediates conversion of lanosterol to meiosis-activating sterols (MAS); MAS are intermediates in the biosynthesis of cholesterol and may have a signaling role in initiating meiosis and oocyte maturation. Azoles have higher affinity for fungal CYP51 compared to the mammalian enzyme and such selectivity contributes to the safety of this therapeutic class. Azoles have been reported to produce reproductive and developmental toxicity in both humans and laboratory animals. The mechanism is unknown but inhibition of mammalian CYP51 as well as other CYPs, e.g. CYP17, CYP19 and CYP26, have been postulated to play a role.