Fluoride crystals. Coloured light micrograph of fluoride crystals. Sodium fluoride was first shown to reduce tooth decay in 1949 but how it works is still not known. It is added to many brands of toothpaste and to drinking water supplies, where a concentration of 1 part fluoride per 1 million parts of water can reduce decay by up to 65%. Exposure to too much fluoride causes a condition called fluorosis, in which the teeth become mottled or brown and break off in chips.