People with attention-deficit/hyperactivity disorder are at increased risk of motor-vehicle accidents, but it is significantly reduced when they are taking ADHD medication, a 10-year study finds.

The researchers estimate that 1 in 5 of the accidents among more than 2 million people with ADHD during the study period could have been avoided if these individuals had been receiving medication the entire time.

“The patients should be aware of the potential risk of [crashes], and seek specific treatment advice from their doctors if they experience difficulties in driving from their condition,” said lead author Zheng Chang, of the Karolinska Institute in Stockholm.

Chang said that motor-vehicle crashes kill more than 1.25 million people around the world each year.

ADHD is a common disorder with symptoms that include poor sustained attention, impaired impulse control and hyperactivity, he added.

Past studies have found that people with ADHD are at an increased risk for crashes and that medication may reduce symptoms and ultimately improve driving skills.

To examine the risk of crashes with ADHD and how it is influenced by medication, the researchers analyzed U.S. commercial health insurance claims between 2005 and 2014.

They identified 2,319,450 adults with an ADHD diagnosis, half of whom were older than 33. About 1.9 million of them received at least one prescription to treat their ADHD during the study period.

Overall, 11,224 people in the study had at least one emergency department visit for a motor-vehicle crash. Compared with people without ADHD, the researchers found that men with the condition had a 49 percent increased risk of being in a crash and women had a 44 percent increased risk.

When the researchers looked at data for each individual with ADHD, they found the men’s risk of crashes dropped by 38 percent during months when they received prescriptions to treat their ADHD. Similarly, women’s risk fell by 42 percent during months when they received a prescription.

Chang said the results can’t explain why people treated for ADHD have a reduced risk of crashes, but there are a few possible explanations.

“It could be due to alleviation of the core symptoms of ADHD (inattention, hyperactivity, and impulsivity), as well as to control of problems that frequently co-occur with ADHD, such as excessive risk taking, poor control of aggression, and substance use,” Chang said by email.

“I think with psycho-stimulant medications, we’re oftentimes concerned about side effects and the potential for abuse, but the focus on those risks can sometimes distract from the benefits people get from the medication,” said Jonathan Posner, an associate professor of psychiatry at Columbia University Medical Center.

Other studies show the benefits of treating ADHD with medications, he added. For example, one found that people treated for ADHD had a lower risk of incarceration.