How much toothpaste (牙膏) do you usually put on your toothbrush? Using too much toothpaste may actually hurt your teeth, according to a US study. Toothpaste usually has fluoride (氟化物). Using too much fluoride in a long time is bad for your teeth. For young children, toothpaste as big as a piece of rice is enough. For older children and adults, a pea-sized (豌豆大小的) piece of toothpaste is just enough.