New research suggests that medications used to treat attention-deficit hyperactivity disorder (ADHD) could contribute to bullying in schools.

Researchers at the University of Michigan found that kids and teens who take medications like Ritalin to treat are twice as likely to be physically or emotionally bullied by peers than those who don't have ADHD.

It's long been known that kids with ADHD have a harder time making and keeping friends and are bullied and victimized more. This study is believed to be the first known to look at how stimulant medications affect their relationships with peers.

"Many youth with ADHD are prescribed stimulant medications to treat their ADHD and we know that these medications are the most frequently shared or sold among adolescents," researcher Quyen Epstein-Ngo said in a statement.

For the study, researchers collected and analyzed data from nearly 5,000 middle and high school students over four years. About 15 percent were diagnosed with ADHD and roughly 4 percent were prescribed stimulants within the past 12 months.

Of those who took ADHD meds, 20 percent reported being approached to sell or share them, and about half of them did. When looking at the overall figures, relatively few students were asked to divert their medications or did. However, Epstein-Ngo said the numbers don't tell the entire story.

"Having a diagnosis of ADHD has lifelong consequences," she said. "These youth aren't living in isolation. As they transition into adulthood, the social effects of their ADHD diagnosis will impact a broad range of people with whom they come into contact."

From 2003 to 2011, there was a 42 percent increase in ADHD cases diagnosed in the U.S., and between 2007 and 2011, there was a 27 percent increase in stimulant-treated ADHD.

Epstein-Ngo said the findings shouldn't scare parents away from considering a stimulant medication. Rather, the study reinforces why parents must talk to kids about never sharing their medications.

At even higher risk were middle and high school students who sold or shared their medications--those kids were four-and-a-half times likelier to be victimized by peers than kids without ADHD.

The findings are detailed in the Journal of Experimental Child Psychology.