After examining more than 900 middle school children in the mid-1990s, David Pendrys, a dentist and researcher at the University of Connecticut, found that kids who brushed with more than the recommended amount of toothpaste had a higher risk of enamel fluorosis (snowflaking of tooth enamel). Fluorosis happens when young children are exposed to too much fluoride while teeth are still forming.
"Preschoolers, by and large, tend to swallow whatever you put in their mouths, so if you cover the whole brush, they're going to swallow that much toothpaste," Dr. Pendrys says. "The fluoride (in the toothpaste) is absorbed, so they're eating more than they should."
Kids under six should brush with a pea-sized amount of toothpaste. Those over six can use any amount, because they won't swallow it.