Children with chronic or recurring stomach pain without a clear medical explanation were also more likely to have an anxiety disorder than those without stomach problems, in a new study.

By the time kids with stomach pain reached age 20, just over half had had symptoms of an anxiety disorder at some point, most often social anxiety, researchers found.

Anxiety tended to start in early childhood, around the same time as the chronic stomach problems.

Past studies suggest between eight and 25 percent of all youth have chronic stomach pain, researchers noted. When there's no clear medical cause for the pain - such as inflammatory bowel disease or celiac disease - it's known as functional abdominal pain.

"It's very prevalent, and it's one of the most common reasons that children and adolescents end up in their pediatrician's office. It's one of the most common reasons kids are missing school," said Dr. Eva Szigethy, head of the Medical Coping Clinic at the Children's Hospital of Pittsburgh Inflammatory Bowel Disease Center.

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