Surveying parents and guardians of about 5,157 children, the CDC found that nearly 40% of children ages three to six years old used too much toothpaste.

Children ages three or younger should be using a “smear—the CDC’s recommended amount—which is about the size of a grain of rice. Children ages three to six years old should be using a pea-sized amount of toothpaste.

The concern of overusing toothpaste is the intake of fluoride. Although fluoride has many benefits like preventing cavities, the CDC warns “ingestion of too much fluoride while teeth are developing can result in visibly detectable changes in enamel structure such as discoloration.”

The CDC wants parents to take charge of the amount of toothpaste their child has on their toothbrush, as “supervision is emphasized as a critical role for the parent or caregiver as the child first begins using a toothbrush and toothpaste.” The CDC also noted that nearly 80% of the children studied started brushing their teeth later than recommended.

The CDC says children’s teeth should be brushed as soon as the first tooth erupts using a clean, damp cloth. Toothpaste with fluoride can be introduced when the child is two years old, the agency says.