A: Lamisil (terbinafine) is a prescription medication that is used to treat fingernail or toenail fungus. Nail fungus, onychomycosis, is caused by certain types of fungi that cause nails to become brittle, discolored or thick. The medication comes in oral tablets or cream.: Lamisil (terbinafine) is a prescription medication that is used to treat fingernail or toenail fungus. Nail fungus, onychomycosis, is caused by certain types of fungi that cause nails to become brittle, discolored or thick. The medication comes in oral tablets or cream.