While thumb-sucking typically is harmless, it can occasionally cause problems, particularly in children who continue to suck their thumbs for long periods of time. Parents should be concerned if they notice changes in the shape of the mouth or teeth. In particular, they are urged to look for changes in the roof of the mouth (palate) or in the way the child’s teeth line up, a problem known as malocclusion. Such problems are most likely to occur in children who vigorously suck their thumbs, rather than those who simply rest their thumbs passively in their mouths.

Prolonged thumb-sucking also can lead to other physical symptoms, including chapped lips, cracked skin, calluses and fingernail infections. In some cases, teeth that become misaligned due to thumb-sucking can lead to speech defects.