Middle and high school students who take medication to treat attention deficit hyperactivity disorder are twice as likely to be bullied than those who don't have ADHD, a University of Michigan study finds.

As of 2011, 6.4 million children between the ages of 4 and 17 (nearly 11 percent) have been diagnosed with ADHD, reports the Centers for Disease Control and Prevention. Between the years 2003 and 2011, ADHD diagnoses increased by 42 percent, while between 2007 and 2011 ADHD patients treated with stimulants increased by 27 percent, noted the researchers in their study. Stimulants, which have a calming effect for children with ADHD, are the most common medications used to treat the condition. Medication reduces hyperactivity and impulsivity and improves an affected child’s ability to focus, work, and learn, according to the National Institute of Mental Health. In some cases, pills also improve physical coordination.

For the study, Dr. Quyen Epstein-Ngo, first author and a research assistant professor, and her colleagues surveyed nearly 5,000 middle and high school students over four years. About 15 percent had been diagnosed with ADHD while nearly 4 percent had been prescribed Ritalin or another stimulant within the past year. Of those who took pills, 20 percent said they’d been asked to sell or share them, and about half of them did.

After analyzing the data, the research team discovered those taking medication were twice as likely to be bullied as students without ADHD while those who sold or shared their medications were four-and-a-half times likelier to be victimized. This was true for both sexes, the researchers said.

According to the CDC, boys are more likely than girls to receive a diagnosis of ADHD: 13.2 percent compared to 5.6 percent. The average age at diagnosis is 7 years old, though usually children suffering from a more severe condition are diagnosed earlier. The CDC also reports one in five children with a history of attentional problems have peer problems, which is three times as many as children without a history of ADHD. About the same percentage also find it difficult to make and maintain friendships. Generally, children with this condition are more likely to injure themselves and also be admitted to the emergency room.

The University of Michigan researchers recommend a closer collaboration between parents and doctors to ensure effective treatment of ADHD children without increasing their risk for victimization.

Source: Epstein-Ngo Q, McCabe SE, Veliz PT, et al. Diversion of ADHD stimulants and peer victimization among adolescents. Journal of Pediatric Psychology. 2015.