Bullied kids are twice as likely to report feeling sick than other children, according to a new study. The study findings show that certain physical conditions such as headache, backache, dizziness, Diarrhea and nausea are common complaints in kids who are bullied at school.

The data used in the study was based on 220,000 school-aged children from 14 countries. Researchers at the University of Padua, Italy, and their colleagues analyzed 30 studies on the subject.

"The results of this study suggest that any recurrent and unexplained physical symptom can be a warning sign of bullying," said Gianluca Gini, an assistant professor of developmental psychology at the University of Padua, in Italy, study author of the study, HealthDay reported.

But not all headaches or stomach aches are related with bullying. The experts said that parents need to monitor their children and ask the right kind of questions to know whether their child is dealing with bullies.

"Children do not easily talk about their bullying experiences," Gini told Healthday. Parents can open up the subject of bullying with their kids by asking indirect questions such as whether he/she feels safe at school.

Previous research has shown that bullying alters a person's genetic structure and makes him/her vulnerable to depression and mood swings later in life. According to Centers for Disease control and Prevention (CDC), about 30 percent of students had been bullied on school property in 2011. 

According to the scientists, parents must talk to their children about bullying. Role playing, planning strategies that can help a kid become more confident and avoid being bullied and intervention programs can help children who are bullied. Parents are advised to be careful when intervening as direct confrontation with parents of the bullies could only add to the problem.

"I think the first thing is really finding time to really observe, watch, and talk with kids so you know when something is off," said Marlene Snyder, a faculty member in the Institute on Family and Neighborhood Life at Clemson University, in South Carolina, Healthday reported.  Ask them questions: 'How was your day? Who did you sit with at lunch?'" Snyder wasn't part of the current study.

The study is published in the journal Pediatrics.