The most common clinical condition in which toenails turn yellow and hollow is gray nails. Gray nails are a type of onychomycosis caused by a fungal infection. Patients need to visit the dermatology department of a regular hospital to confirm the diagnosis by doing fungal culture and mycelial examination. The nail is not easy to get into the nail plate, and the nail grows slowly, so it is important to remove the nail first and then use the medication, which is commonly used clinically, such as glacial acetic acid solution, tincture of iodine, ciclopirox, and amorolfine hydrochloride. The nail also needs to be combined with oral antifungal drugs, commonly used are itraconazole and terbinafine. The treatment of grey nails is slow and the efficacy is sometimes not particularly good, but patients must adhere to the medication.